1
@node Sockets, Low-Level Terminal Interface, Pipes and FIFOs, Top
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@c %MENU% A more complicated IPC mechanism, with networking support
5
This chapter describes the GNU facilities for interprocess
6
communication using sockets.
9
@cindex interprocess communication, with sockets
10
A @dfn{socket} is a generalized interprocess communication channel.
11
Like a pipe, a socket is represented as a file descriptor. Unlike pipes
12
sockets support communication between unrelated processes, and even
13
between processes running on different machines that communicate over a
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network. Sockets are the primary means of communicating with other
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machines; @code{telnet}, @code{rlogin}, @code{ftp}, @code{talk} and the
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other familiar network programs use sockets.
18
Not all operating systems support sockets. In the GNU library, the
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header file @file{sys/socket.h} exists regardless of the operating
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system, and the socket functions always exist, but if the system does
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not really support sockets these functions always fail.
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@strong{Incomplete:} We do not currently document the facilities for
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broadcast messages or for configuring Internet interfaces. The
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reentrant functions and some newer functions that are related to IPv6
26
aren't documented either so far.
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* Socket Concepts:: Basic concepts you need to know about.
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* Communication Styles::Stream communication, datagrams and other styles.
31
* Socket Addresses:: How socket names (``addresses'') work.
32
* Interface Naming:: Identifying specific network interfaces.
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* Local Namespace:: Details about the local namespace.
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* Internet Namespace:: Details about the Internet namespace.
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* Misc Namespaces:: Other namespaces not documented fully here.
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* Open/Close Sockets:: Creating sockets and destroying them.
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* Connections:: Operations on sockets with connection state.
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* Datagrams:: Operations on datagram sockets.
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* Inetd:: Inetd is a daemon that starts servers on request.
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The most convenient way to write a server
41
is to make it work with Inetd.
42
* Socket Options:: Miscellaneous low-level socket options.
43
* Networks Database:: Accessing the database of network names.
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@section Socket Concepts
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@cindex communication style (of a socket)
50
@cindex style of communication (of a socket)
51
When you create a socket, you must specify the style of communication
52
you want to use and the type of protocol that should implement it.
53
The @dfn{communication style} of a socket defines the user-level
54
semantics of sending and receiving data on the socket. Choosing a
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communication style specifies the answers to questions such as these:
61
@cindex stream (sockets)
62
@strong{What are the units of data transmission?} Some communication
63
styles regard the data as a sequence of bytes with no larger
64
structure; others group the bytes into records (which are known in
65
this context as @dfn{packets}).
68
@cindex loss of data on sockets
69
@cindex data loss on sockets
70
@strong{Can data be lost during normal operation?} Some communication
71
styles guarantee that all the data sent arrives in the order it was
72
sent (barring system or network crashes); other styles occasionally
73
lose data as a normal part of operation, and may sometimes deliver
74
packets more than once or in the wrong order.
76
Designing a program to use unreliable communication styles usually
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involves taking precautions to detect lost or misordered packets and
78
to retransmit data as needed.
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@strong{Is communication entirely with one partner?} Some
82
communication styles are like a telephone call---you make a
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@dfn{connection} with one remote socket and then exchange data
84
freely. Other styles are like mailing letters---you specify a
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destination address for each message you send.
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@cindex namespace (of socket)
89
@cindex domain (of socket)
90
@cindex socket namespace
92
You must also choose a @dfn{namespace} for naming the socket. A socket
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name (``address'') is meaningful only in the context of a particular
94
namespace. In fact, even the data type to use for a socket name may
95
depend on the namespace. Namespaces are also called ``domains'', but we
96
avoid that word as it can be confused with other usage of the same
97
term. Each namespace has a symbolic name that starts with @samp{PF_}.
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A corresponding symbolic name starting with @samp{AF_} designates the
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address format for that namespace.
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@cindex network protocol
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@cindex protocol (of socket)
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@cindex socket protocol
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@cindex protocol family
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Finally you must choose the @dfn{protocol} to carry out the
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communication. The protocol determines what low-level mechanism is used
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to transmit and receive data. Each protocol is valid for a particular
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namespace and communication style; a namespace is sometimes called a
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@dfn{protocol family} because of this, which is why the namespace names
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start with @samp{PF_}.
112
The rules of a protocol apply to the data passing between two programs,
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perhaps on different computers; most of these rules are handled by the
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operating system and you need not know about them. What you do need to
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know about protocols is this:
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In order to have communication between two sockets, they must specify
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the @emph{same} protocol.
123
Each protocol is meaningful with particular style/namespace
124
combinations and cannot be used with inappropriate combinations. For
125
example, the TCP protocol fits only the byte stream style of
126
communication and the Internet namespace.
129
For each combination of style and namespace there is a @dfn{default
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protocol}, which you can request by specifying 0 as the protocol
131
number. And that's what you should normally do---use the default.
134
Throughout the following description at various places
135
variables/parameters to denote sizes are required. And here the trouble
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starts. In the first implementations the type of these variables was
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simply @code{int}. On most machines at that time an @code{int} was 32
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bits wide, which created a @emph{de facto} standard requiring 32-bit
139
variables. This is important since references to variables of this type
140
are passed to the kernel.
142
Then the POSIX people came and unified the interface with the words "all
143
size values are of type @code{size_t}". On 64-bit machines
144
@code{size_t} is 64 bits wide, so pointers to variables were no longer
147
The Unix98 specification provides a solution by introducing a type
148
@code{socklen_t}. This type is used in all of the cases that POSIX
149
changed to use @code{size_t}. The only requirement of this type is that
150
it be an unsigned type of at least 32 bits. Therefore, implementations
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which require that references to 32-bit variables be passed can be as
152
happy as implementations which use 64-bit values.
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@node Communication Styles
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@section Communication Styles
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The GNU library includes support for several different kinds of sockets,
159
each with different characteristics. This section describes the
160
supported socket types. The symbolic constants listed here are
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defined in @file{sys/socket.h}.
164
@comment sys/socket.h
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@deftypevr Macro int SOCK_STREAM
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The @code{SOCK_STREAM} style is like a pipe (@pxref{Pipes and FIFOs}).
168
It operates over a connection with a particular remote socket and
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transmits data reliably as a stream of bytes.
171
Use of this style is covered in detail in @ref{Connections}.
174
@comment sys/socket.h
176
@deftypevr Macro int SOCK_DGRAM
177
The @code{SOCK_DGRAM} style is used for sending
178
individually-addressed packets unreliably.
179
It is the diametrical opposite of @code{SOCK_STREAM}.
181
Each time you write data to a socket of this kind, that data becomes
182
one packet. Since @code{SOCK_DGRAM} sockets do not have connections,
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you must specify the recipient address with each packet.
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The only guarantee that the system makes about your requests to
186
transmit data is that it will try its best to deliver each packet you
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send. It may succeed with the sixth packet after failing with the
188
fourth and fifth packets; the seventh packet may arrive before the
189
sixth, and may arrive a second time after the sixth.
191
The typical use for @code{SOCK_DGRAM} is in situations where it is
192
acceptable to simply re-send a packet if no response is seen in a
193
reasonable amount of time.
195
@xref{Datagrams}, for detailed information about how to use datagram
200
@c This appears to be only for the NS domain, which we aren't
201
@c discussing and probably won't support either.
202
@comment sys/socket.h
204
@deftypevr Macro int SOCK_SEQPACKET
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This style is like @code{SOCK_STREAM} except that the data are
206
structured into packets.
208
A program that receives data over a @code{SOCK_SEQPACKET} socket
209
should be prepared to read the entire message packet in a single call
210
to @code{read}; if it only reads part of the message, the remainder of
211
the message is simply discarded instead of being available for
212
subsequent calls to @code{read}.
214
Many protocols do not support this communication style.
219
@comment sys/socket.h
221
@deftypevr Macro int SOCK_RDM
222
This style is a reliable version of @code{SOCK_DGRAM}: it sends
223
individually addressed packets, but guarantees that each packet sent
224
arrives exactly once.
226
@strong{Warning:} It is not clear this is actually supported
227
by any operating system.
231
@comment sys/socket.h
233
@deftypevr Macro int SOCK_RAW
234
This style provides access to low-level network protocols and
235
interfaces. Ordinary user programs usually have no need to use this
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@node Socket Addresses
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@section Socket Addresses
242
@cindex address of socket
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@cindex name of socket
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@cindex binding a socket address
245
@cindex socket address (name) binding
246
The name of a socket is normally called an @dfn{address}. The
247
functions and symbols for dealing with socket addresses were named
248
inconsistently, sometimes using the term ``name'' and sometimes using
249
``address''. You can regard these terms as synonymous where sockets
252
A socket newly created with the @code{socket} function has no
253
address. Other processes can find it for communication only if you
254
give it an address. We call this @dfn{binding} the address to the
255
socket, and the way to do it is with the @code{bind} function.
257
You need be concerned with the address of a socket if other processes
258
are to find it and start communicating with it. You can specify an
259
address for other sockets, but this is usually pointless; the first time
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you send data from a socket, or use it to initiate a connection, the
261
system assigns an address automatically if you have not specified one.
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Occasionally a client needs to specify an address because the server
264
discriminates based on address; for example, the rsh and rlogin
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protocols look at the client's socket address and only bypass password
266
checking if it is less than @code{IPPORT_RESERVED} (@pxref{Ports}).
268
The details of socket addresses vary depending on what namespace you are
269
using. @xref{Local Namespace}, or @ref{Internet Namespace}, for specific
272
Regardless of the namespace, you use the same functions @code{bind} and
273
@code{getsockname} to set and examine a socket's address. These
274
functions use a phony data type, @code{struct sockaddr *}, to accept the
275
address. In practice, the address lives in a structure of some other
276
data type appropriate to the address format you are using, but you cast
277
its address to @code{struct sockaddr *} when you pass it to
281
* Address Formats:: About @code{struct sockaddr}.
282
* Setting Address:: Binding an address to a socket.
283
* Reading Address:: Reading the address of a socket.
286
@node Address Formats
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@subsection Address Formats
289
The functions @code{bind} and @code{getsockname} use the generic data
290
type @code{struct sockaddr *} to represent a pointer to a socket
291
address. You can't use this data type effectively to interpret an
292
address or construct one; for that, you must use the proper data type
293
for the socket's namespace.
295
Thus, the usual practice is to construct an address of the proper
296
namespace-specific type, then cast a pointer to @code{struct sockaddr *}
297
when you call @code{bind} or @code{getsockname}.
299
The one piece of information that you can get from the @code{struct
300
sockaddr} data type is the @dfn{address format designator}. This tells
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you which data type to use to understand the address fully.
304
The symbols in this section are defined in the header file
307
@comment sys/socket.h
309
@deftp {Data Type} {struct sockaddr}
310
The @code{struct sockaddr} type itself has the following members:
313
@item short int sa_family
314
This is the code for the address format of this address. It
315
identifies the format of the data which follows.
317
@item char sa_data[14]
318
This is the actual socket address data, which is format-dependent. Its
319
length also depends on the format, and may well be more than 14. The
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length 14 of @code{sa_data} is essentially arbitrary.
324
Each address format has a symbolic name which starts with @samp{AF_}.
325
Each of them corresponds to a @samp{PF_} symbol which designates the
326
corresponding namespace. Here is a list of address format names:
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@comment sys/socket.h
333
This designates the address format that goes with the local namespace.
334
(@code{PF_LOCAL} is the name of that namespace.) @xref{Local Namespace
335
Details}, for information about this address format.
337
@comment sys/socket.h
341
This is a synonym for @code{AF_LOCAL}. Although @code{AF_LOCAL} is
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mandated by POSIX.1g, @code{AF_UNIX} is portable to more systems.
343
@code{AF_UNIX} was the traditional name stemming from BSD, so even most
344
POSIX systems support it. It is also the name of choice in the Unix98
345
specification. (The same is true for @code{PF_UNIX}
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vs. @code{PF_LOCAL}).
348
@comment sys/socket.h
352
This is another synonym for @code{AF_LOCAL}, for compatibility.
353
(@code{PF_FILE} is likewise a synonym for @code{PF_LOCAL}.)
355
@comment sys/socket.h
359
This designates the address format that goes with the Internet
360
namespace. (@code{PF_INET} is the name of that namespace.)
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@xref{Internet Address Formats}.
363
@comment sys/socket.h
364
@comment IPv6 Basic API
366
This is similar to @code{AF_INET}, but refers to the IPv6 protocol.
367
(@code{PF_INET6} is the name of the corresponding namespace.)
369
@comment sys/socket.h
373
This designates no particular address format. It is used only in rare
374
cases, such as to clear out the default destination address of a
375
``connected'' datagram socket. @xref{Sending Datagrams}.
377
The corresponding namespace designator symbol @code{PF_UNSPEC} exists
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for completeness, but there is no reason to use it in a program.
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@file{sys/socket.h} defines symbols starting with @samp{AF_} for many
382
different kinds of networks, most or all of which are not actually
383
implemented. We will document those that really work as we receive
384
information about how to use them.
386
@node Setting Address
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@subsection Setting the Address of a Socket
390
Use the @code{bind} function to assign an address to a socket. The
391
prototype for @code{bind} is in the header file @file{sys/socket.h}.
392
For examples of use, see @ref{Local Socket Example}, or see @ref{Inet Example}.
394
@comment sys/socket.h
396
@deftypefun int bind (int @var{socket}, struct sockaddr *@var{addr}, socklen_t @var{length})
397
The @code{bind} function assigns an address to the socket
398
@var{socket}. The @var{addr} and @var{length} arguments specify the
399
address; the detailed format of the address depends on the namespace.
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The first part of the address is always the format designator, which
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specifies a namespace, and says that the address is in the format of
404
The return value is @code{0} on success and @code{-1} on failure. The
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following @code{errno} error conditions are defined for this function:
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The @var{socket} argument is not a valid file descriptor.
412
The descriptor @var{socket} is not a socket.
415
The specified address is not available on this machine.
418
Some other socket is already using the specified address.
421
The socket @var{socket} already has an address.
424
You do not have permission to access the requested address. (In the
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Internet domain, only the super-user is allowed to specify a port number
426
in the range 0 through @code{IPPORT_RESERVED} minus one; see
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Additional conditions may be possible depending on the particular namespace
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@node Reading Address
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@subsection Reading the Address of a Socket
438
Use the function @code{getsockname} to examine the address of an
439
Internet socket. The prototype for this function is in the header file
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@comment sys/socket.h
444
@deftypefun int getsockname (int @var{socket}, struct sockaddr *@var{addr}, socklen_t *@var{length-ptr})
445
The @code{getsockname} function returns information about the
446
address of the socket @var{socket} in the locations specified by the
447
@var{addr} and @var{length-ptr} arguments. Note that the
448
@var{length-ptr} is a pointer; you should initialize it to be the
449
allocation size of @var{addr}, and on return it contains the actual
450
size of the address data.
452
The format of the address data depends on the socket namespace. The
453
length of the information is usually fixed for a given namespace, so
454
normally you can know exactly how much space is needed and can provide
455
that much. The usual practice is to allocate a place for the value
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using the proper data type for the socket's namespace, then cast its
457
address to @code{struct sockaddr *} to pass it to @code{getsockname}.
459
The return value is @code{0} on success and @code{-1} on error. The
460
following @code{errno} error conditions are defined for this function:
464
The @var{socket} argument is not a valid file descriptor.
467
The descriptor @var{socket} is not a socket.
470
There are not enough internal buffers available for the operation.
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You can't read the address of a socket in the file namespace. This is
475
consistent with the rest of the system; in general, there's no way to
476
find a file's name from a descriptor for that file.
478
@node Interface Naming
479
@section Interface Naming
481
Each network interface has a name. This usually consists of a few
482
letters that relate to the type of interface, which may be followed by a
483
number if there is more than one interface of that type. Examples
484
might be @code{lo} (the loopback interface) and @code{eth0} (the first
487
Although such names are convenient for humans, it would be clumsy to
488
have to use them whenever a program needs to refer to an interface. In
489
such situations an interface is referred to by its @dfn{index}, which is
490
an arbitrarily-assigned small positive integer.
492
The following functions, constants and data types are declared in the
493
header file @file{net/if.h}.
496
@deftypevr Constant size_t IFNAMSIZ
497
This constant defines the maximum buffer size needed to hold an
498
interface name, including its terminating zero byte.
502
@comment IPv6 basic API
503
@deftypefun {unsigned int} if_nametoindex (const char *ifname)
504
This function yields the interface index corresponding to a particular
505
name. If no interface exists with the name given, it returns 0.
509
@comment IPv6 basic API
510
@deftypefun {char *} if_indextoname (unsigned int ifindex, char *ifname)
511
This function maps an interface index to its corresponding name. The
512
returned name is placed in the buffer pointed to by @code{ifname}, which
513
must be at least @code{IFNAMSIZ} bytes in length. If the index was
514
invalid, the function's return value is a null pointer, otherwise it is
519
@comment IPv6 basic API
520
@deftp {Data Type} {struct if_nameindex}
521
This data type is used to hold the information about a single
522
interface. It has the following members:
525
@item unsigned int if_index;
526
This is the interface index.
529
This is the null-terminated index name.
535
@comment IPv6 basic API
536
@deftypefun {struct if_nameindex *} if_nameindex (void)
537
This function returns an array of @code{if_nameindex} structures, one
538
for every interface that is present. The end of the list is indicated
539
by a structure with an interface of 0 and a null name pointer. If an
540
error occurs, this function returns a null pointer.
542
The returned structure must be freed with @code{if_freenameindex} after
547
@comment IPv6 basic API
548
@deftypefun void if_freenameindex (struct if_nameindex *ptr)
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This function frees the structure returned by an earlier call to
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@node Local Namespace
554
@section The Local Namespace
555
@cindex local namespace, for sockets
557
This section describes the details of the local namespace, whose
558
symbolic name (required when you create a socket) is @code{PF_LOCAL}.
559
The local namespace is also known as ``Unix domain sockets''. Another
560
name is file namespace since socket addresses are normally implemented
564
* Concepts: Local Namespace Concepts. What you need to understand.
565
* Details: Local Namespace Details. Address format, symbolic names, etc.
566
* Example: Local Socket Example. Example of creating a socket.
569
@node Local Namespace Concepts
570
@subsection Local Namespace Concepts
572
In the local namespace socket addresses are file names. You can specify
573
any file name you want as the address of the socket, but you must have
574
write permission on the directory containing it.
575
@c XXX The following was said to be wrong.
576
@c In order to connect to a socket you must have read permission for it.
577
It's common to put these files in the @file{/tmp} directory.
579
One peculiarity of the local namespace is that the name is only used
580
when opening the connection; once open the address is not meaningful and
583
Another peculiarity is that you cannot connect to such a socket from
584
another machine--not even if the other machine shares the file system
585
which contains the name of the socket. You can see the socket in a
586
directory listing, but connecting to it never succeeds. Some programs
587
take advantage of this, such as by asking the client to send its own
588
process ID, and using the process IDs to distinguish between clients.
589
However, we recommend you not use this method in protocols you design,
590
as we might someday permit connections from other machines that mount
591
the same file systems. Instead, send each new client an identifying
592
number if you want it to have one.
594
After you close a socket in the local namespace, you should delete the
595
file name from the file system. Use @code{unlink} or @code{remove} to
596
do this; see @ref{Deleting Files}.
598
The local namespace supports just one protocol for any communication
599
style; it is protocol number @code{0}.
601
@node Local Namespace Details
602
@subsection Details of Local Namespace
605
To create a socket in the local namespace, use the constant
606
@code{PF_LOCAL} as the @var{namespace} argument to @code{socket} or
607
@code{socketpair}. This constant is defined in @file{sys/socket.h}.
609
@comment sys/socket.h
611
@deftypevr Macro int PF_LOCAL
612
This designates the local namespace, in which socket addresses are local
613
names, and its associated family of protocols. @code{PF_Local} is the
614
macro used by Posix.1g.
617
@comment sys/socket.h
619
@deftypevr Macro int PF_UNIX
620
This is a synonym for @code{PF_LOCAL}, for compatibility's sake.
623
@comment sys/socket.h
625
@deftypevr Macro int PF_FILE
626
This is a synonym for @code{PF_LOCAL}, for compatibility's sake.
629
The structure for specifying socket names in the local namespace is
630
defined in the header file @file{sys/un.h}:
635
@deftp {Data Type} {struct sockaddr_un}
636
This structure is used to specify local namespace socket addresses. It has
637
the following members:
640
@item short int sun_family
641
This identifies the address family or format of the socket address.
642
You should store the value @code{AF_LOCAL} to designate the local
643
namespace. @xref{Socket Addresses}.
645
@item char sun_path[108]
646
This is the file name to use.
648
@strong{Incomplete:} Why is 108 a magic number? RMS suggests making
649
this a zero-length array and tweaking the following example to use
650
@code{alloca} to allocate an appropriate amount of storage based on
651
the length of the filename.
655
You should compute the @var{length} parameter for a socket address in
656
the local namespace as the sum of the size of the @code{sun_family}
657
component and the string length (@emph{not} the allocation size!) of
658
the file name string. This can be done using the macro @code{SUN_LEN}:
662
@deftypefn {Macro} int SUN_LEN (@emph{struct sockaddr_un *} @var{ptr})
663
The macro computes the length of socket address in the local namespace.
666
@node Local Socket Example
667
@subsection Example of Local-Namespace Sockets
669
Here is an example showing how to create and name a socket in the local
673
@include mkfsock.c.texi
676
@node Internet Namespace
677
@section The Internet Namespace
678
@cindex Internet namespace, for sockets
680
This section describes the details of the protocols and socket naming
681
conventions used in the Internet namespace.
683
Originally the Internet namespace used only IP version 4 (IPv4). With
684
the growing number of hosts on the Internet, a new protocol with a
685
larger address space was necessary: IP version 6 (IPv6). IPv6
686
introduces 128-bit addresses (IPv4 has 32-bit addresses) and other
687
features, and will eventually replace IPv4.
689
To create a socket in the IPv4 Internet namespace, use the symbolic name
690
@code{PF_INET} of this namespace as the @var{namespace} argument to
691
@code{socket} or @code{socketpair}. For IPv6 addresses you need the
692
macro @code{PF_INET6}. These macros are defined in @file{sys/socket.h}.
695
@comment sys/socket.h
697
@deftypevr Macro int PF_INET
698
This designates the IPv4 Internet namespace and associated family of
702
@comment sys/socket.h
704
@deftypevr Macro int PF_INET6
705
This designates the IPv6 Internet namespace and associated family of
709
A socket address for the Internet namespace includes the following components:
713
The address of the machine you want to connect to. Internet addresses
714
can be specified in several ways; these are discussed in @ref{Internet
715
Address Formats}, @ref{Host Addresses} and @ref{Host Names}.
718
A port number for that machine. @xref{Ports}.
721
You must ensure that the address and port number are represented in a
722
canonical format called @dfn{network byte order}. @xref{Byte Order},
723
for information about this.
726
* Internet Address Formats:: How socket addresses are specified in the
728
* Host Addresses:: All about host addresses of Internet host.
729
* Protocols Database:: Referring to protocols by name.
730
* Ports:: Internet port numbers.
731
* Services Database:: Ports may have symbolic names.
732
* Byte Order:: Different hosts may use different byte
733
ordering conventions; you need to
734
canonicalize host address and port number.
735
* Inet Example:: Putting it all together.
738
@node Internet Address Formats
739
@subsection Internet Socket Address Formats
741
In the Internet namespace, for both IPv4 (@code{AF_INET}) and IPv6
742
(@code{AF_INET6}), a socket address consists of a host address
743
and a port on that host. In addition, the protocol you choose serves
744
effectively as a part of the address because local port numbers are
745
meaningful only within a particular protocol.
747
The data types for representing socket addresses in the Internet namespace
748
are defined in the header file @file{netinet/in.h}.
751
@comment netinet/in.h
753
@deftp {Data Type} {struct sockaddr_in}
754
This is the data type used to represent socket addresses in the
755
Internet namespace. It has the following members:
758
@item sa_family_t sin_family
759
This identifies the address family or format of the socket address.
760
You should store the value @code{AF_INET} in this member.
761
@xref{Socket Addresses}.
763
@item struct in_addr sin_addr
764
This is the Internet address of the host machine. @xref{Host
765
Addresses}, and @ref{Host Names}, for how to get a value to store
768
@item unsigned short int sin_port
769
This is the port number. @xref{Ports}.
773
When you call @code{bind} or @code{getsockname}, you should specify
774
@code{sizeof (struct sockaddr_in)} as the @var{length} parameter if
775
you are using an IPv4 Internet namespace socket address.
777
@deftp {Data Type} {struct sockaddr_in6}
778
This is the data type used to represent socket addresses in the IPv6
779
namespace. It has the following members:
782
@item sa_family_t sin6_family
783
This identifies the address family or format of the socket address.
784
You should store the value of @code{AF_INET6} in this member.
785
@xref{Socket Addresses}.
787
@item struct in6_addr sin6_addr
788
This is the IPv6 address of the host machine. @xref{Host
789
Addresses}, and @ref{Host Names}, for how to get a value to store
792
@item uint32_t sin6_flowinfo
793
This is a currently unimplemented field.
795
@item uint16_t sin6_port
796
This is the port number. @xref{Ports}.
802
@subsection Host Addresses
804
Each computer on the Internet has one or more @dfn{Internet addresses},
805
numbers which identify that computer among all those on the Internet.
806
Users typically write IPv4 numeric host addresses as sequences of four
807
numbers, separated by periods, as in @samp{128.52.46.32}, and IPv6
808
numeric host addresses as sequences of up to eight numbers separated by
809
colons, as in @samp{5f03:1200:836f:c100::1}.
811
Each computer also has one or more @dfn{host names}, which are strings
812
of words separated by periods, as in @samp{mescaline.gnu.org}.
814
Programs that let the user specify a host typically accept both numeric
815
addresses and host names. To open a connection a program needs a
816
numeric address, and so must convert a host name to the numeric address
820
* Abstract Host Addresses:: What a host number consists of.
821
* Data type: Host Address Data Type. Data type for a host number.
822
* Functions: Host Address Functions. Functions to operate on them.
823
* Names: Host Names. Translating host names to host numbers.
826
@node Abstract Host Addresses
827
@subsubsection Internet Host Addresses
828
@cindex host address, Internet
829
@cindex Internet host address
832
Each computer on the Internet has one or more Internet addresses,
833
numbers which identify that computer among all those on the Internet.
836
@cindex network number
837
@cindex local network address number
838
An IPv4 Internet host address is a number containing four bytes of data.
839
Historically these are divided into two parts, a @dfn{network number} and a
840
@dfn{local network address number} within that network. In the
841
mid-1990s classless addresses were introduced which changed this
842
behavior. Since some functions implicitly expect the old definitions,
843
we first describe the class-based network and will then describe
844
classless addresses. IPv6 uses only classless addresses and therefore
845
the following paragraphs don't apply.
847
The class-based IPv4 network number consists of the first one, two or
848
three bytes; the rest of the bytes are the local address.
850
IPv4 network numbers are registered with the Network Information Center
851
(NIC), and are divided into three classes---A, B and C. The local
852
network address numbers of individual machines are registered with the
853
administrator of the particular network.
855
Class A networks have single-byte numbers in the range 0 to 127. There
856
are only a small number of Class A networks, but they can each support a
857
very large number of hosts. Medium-sized Class B networks have two-byte
858
network numbers, with the first byte in the range 128 to 191. Class C
859
networks are the smallest; they have three-byte network numbers, with
860
the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes
861
of an Internet address specify a network. The remaining bytes of the
862
Internet address specify the address within that network.
864
The Class A network 0 is reserved for broadcast to all networks. In
865
addition, the host number 0 within each network is reserved for broadcast
866
to all hosts in that network. These uses are obsolete now but for
867
compatibility reasons you shouldn't use network 0 and host number 0.
869
The Class A network 127 is reserved for loopback; you can always use
870
the Internet address @samp{127.0.0.1} to refer to the host machine.
872
Since a single machine can be a member of multiple networks, it can
873
have multiple Internet host addresses. However, there is never
874
supposed to be more than one machine with the same host address.
876
@c !!! this section could document the IN_CLASS* macros in <netinet/in.h>.
877
@c No, it shouldn't since they're obsolete.
879
@cindex standard dot notation, for Internet addresses
880
@cindex dot notation, for Internet addresses
881
There are four forms of the @dfn{standard numbers-and-dots notation}
882
for Internet addresses:
885
@item @var{a}.@var{b}.@var{c}.@var{d}
886
This specifies all four bytes of the address individually and is the
887
commonly used representation.
889
@item @var{a}.@var{b}.@var{c}
890
The last part of the address, @var{c}, is interpreted as a 2-byte quantity.
891
This is useful for specifying host addresses in a Class B network with
892
network address number @code{@var{a}.@var{b}}.
894
@item @var{a}.@var{b}
895
The last part of the address, @var{b}, is interpreted as a 3-byte quantity.
896
This is useful for specifying host addresses in a Class A network with
897
network address number @var{a}.
900
If only one part is given, this corresponds directly to the host address
904
Within each part of the address, the usual C conventions for specifying
905
the radix apply. In other words, a leading @samp{0x} or @samp{0X} implies
906
hexadecimal radix; a leading @samp{0} implies octal; and otherwise decimal
909
@subsubheading Classless Addresses
911
IPv4 addresses (and IPv6 addresses also) are now considered classless;
912
the distinction between classes A, B and C can be ignored. Instead an
913
IPv4 host address consists of a 32-bit address and a 32-bit mask. The
914
mask contains set bits for the network part and cleared bits for the
915
host part. The network part is contiguous from the left, with the
916
remaining bits representing the host. As a consequence, the netmask can
917
simply be specified as the number of set bits. Classes A, B and C are
918
just special cases of this general rule. For example, class A addresses
919
have a netmask of @samp{255.0.0.0} or a prefix length of 8.
921
Classless IPv4 network addresses are written in numbers-and-dots
922
notation with the prefix length appended and a slash as separator. For
923
example the class A network 10 is written as @samp{10.0.0.0/8}.
925
@subsubheading IPv6 Addresses
927
IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data. A host
928
address is usually written as eight 16-bit hexadecimal numbers that are
929
separated by colons. Two colons are used to abbreviate strings of
930
consecutive zeros. For example, the IPv6 loopback address
931
@samp{0:0:0:0:0:0:0:1} can just be written as @samp{::1}.
933
@node Host Address Data Type
934
@subsubsection Host Address Data Type
936
IPv4 Internet host addresses are represented in some contexts as integers
937
(type @code{uint32_t}). In other contexts, the integer is
938
packaged inside a structure of type @code{struct in_addr}. It would
939
be better if the usage were made consistent, but it is not hard to extract
940
the integer from the structure or put the integer into a structure.
942
You will find older code that uses @code{unsigned long int} for
943
IPv4 Internet host addresses instead of @code{uint32_t} or @code{struct
944
in_addr}. Historically @code{unsigned long int} was a 32-bit number but
945
with 64-bit machines this has changed. Using @code{unsigned long int}
946
might break the code if it is used on machines where this type doesn't
947
have 32 bits. @code{uint32_t} is specified by Unix98 and guaranteed to have
950
IPv6 Internet host addresses have 128 bits and are packaged inside a
951
structure of type @code{struct in6_addr}.
953
The following basic definitions for Internet addresses are declared in
954
the header file @file{netinet/in.h}:
957
@comment netinet/in.h
959
@deftp {Data Type} {struct in_addr}
960
This data type is used in certain contexts to contain an IPv4 Internet
961
host address. It has just one field, named @code{s_addr}, which records
962
the host address number as an @code{uint32_t}.
965
@comment netinet/in.h
967
@deftypevr Macro {uint32_t} INADDR_LOOPBACK
968
You can use this constant to stand for ``the address of this machine,''
969
instead of finding its actual address. It is the IPv4 Internet address
970
@samp{127.0.0.1}, which is usually called @samp{localhost}. This
971
special constant saves you the trouble of looking up the address of your
972
own machine. Also, the system usually implements @code{INADDR_LOOPBACK}
973
specially, avoiding any network traffic for the case of one machine
977
@comment netinet/in.h
979
@deftypevr Macro {uint32_t} INADDR_ANY
980
You can use this constant to stand for ``any incoming address'' when
981
binding to an address. @xref{Setting Address}. This is the usual
982
address to give in the @code{sin_addr} member of @w{@code{struct
983
sockaddr_in}} when you want to accept Internet connections.
986
@comment netinet/in.h
988
@deftypevr Macro {uint32_t} INADDR_BROADCAST
989
This constant is the address you use to send a broadcast message.
990
@c !!! broadcast needs further documented
993
@comment netinet/in.h
995
@deftypevr Macro {uint32_t} INADDR_NONE
996
This constant is returned by some functions to indicate an error.
999
@comment netinet/in.h
1000
@comment IPv6 basic API
1001
@deftp {Data Type} {struct in6_addr}
1002
This data type is used to store an IPv6 address. It stores 128 bits of
1003
data, which can be accessed (via a union) in a variety of ways.
1006
@comment netinet/in.h
1007
@comment IPv6 basic API
1008
@deftypevr Constant {struct in6_addr} in6addr_loopback
1009
This constant is the IPv6 address @samp{::1}, the loopback address. See
1010
above for a description of what this means. The macro
1011
@code{IN6ADDR_LOOPBACK_INIT} is provided to allow you to initialize your
1012
own variables to this value.
1015
@comment netinet/in.h
1016
@comment IPv6 basic API
1017
@deftypevr Constant {struct in6_addr} in6addr_any
1018
This constant is the IPv6 address @samp{::}, the unspecified address. See
1019
above for a description of what this means. The macro
1020
@code{IN6ADDR_ANY_INIT} is provided to allow you to initialize your
1021
own variables to this value.
1024
@node Host Address Functions
1025
@subsubsection Host Address Functions
1029
These additional functions for manipulating Internet addresses are
1030
declared in the header file @file{arpa/inet.h}. They represent Internet
1031
addresses in network byte order, and network numbers and
1032
local-address-within-network numbers in host byte order. @xref{Byte
1033
Order}, for an explanation of network and host byte order.
1035
@comment arpa/inet.h
1037
@deftypefun int inet_aton (const char *@var{name}, struct in_addr *@var{addr})
1038
This function converts the IPv4 Internet host address @var{name}
1039
from the standard numbers-and-dots notation into binary data and stores
1040
it in the @code{struct in_addr} that @var{addr} points to.
1041
@code{inet_aton} returns nonzero if the address is valid, zero if not.
1044
@comment arpa/inet.h
1046
@deftypefun {uint32_t} inet_addr (const char *@var{name})
1047
This function converts the IPv4 Internet host address @var{name} from the
1048
standard numbers-and-dots notation into binary data. If the input is
1049
not valid, @code{inet_addr} returns @code{INADDR_NONE}. This is an
1050
obsolete interface to @code{inet_aton}, described immediately above. It
1051
is obsolete because @code{INADDR_NONE} is a valid address
1052
(255.255.255.255), and @code{inet_aton} provides a cleaner way to
1053
indicate error return.
1056
@comment arpa/inet.h
1058
@deftypefun {uint32_t} inet_network (const char *@var{name})
1059
This function extracts the network number from the address @var{name},
1060
given in the standard numbers-and-dots notation. The returned address is
1061
in host order. If the input is not valid, @code{inet_network} returns
1064
The function works only with traditional IPv4 class A, B and C network
1065
types. It doesn't work with classless addresses and shouldn't be used
1069
@comment arpa/inet.h
1071
@deftypefun {char *} inet_ntoa (struct in_addr @var{addr})
1072
This function converts the IPv4 Internet host address @var{addr} to a
1073
string in the standard numbers-and-dots notation. The return value is
1074
a pointer into a statically-allocated buffer. Subsequent calls will
1075
overwrite the same buffer, so you should copy the string if you need
1078
In multi-threaded programs each thread has an own statically-allocated
1079
buffer. But still subsequent calls of @code{inet_ntoa} in the same
1080
thread will overwrite the result of the last call.
1082
Instead of @code{inet_ntoa} the newer function @code{inet_ntop} which is
1083
described below should be used since it handles both IPv4 and IPv6
1087
@comment arpa/inet.h
1089
@deftypefun {struct in_addr} inet_makeaddr (uint32_t @var{net}, uint32_t @var{local})
1090
This function makes an IPv4 Internet host address by combining the network
1091
number @var{net} with the local-address-within-network number
1095
@comment arpa/inet.h
1097
@deftypefun uint32_t inet_lnaof (struct in_addr @var{addr})
1098
This function returns the local-address-within-network part of the
1099
Internet host address @var{addr}.
1101
The function works only with traditional IPv4 class A, B and C network
1102
types. It doesn't work with classless addresses and shouldn't be used
1106
@comment arpa/inet.h
1108
@deftypefun uint32_t inet_netof (struct in_addr @var{addr})
1109
This function returns the network number part of the Internet host
1112
The function works only with traditional IPv4 class A, B and C network
1113
types. It doesn't work with classless addresses and shouldn't be used
1117
@comment arpa/inet.h
1118
@comment IPv6 basic API
1119
@deftypefun int inet_pton (int @var{af}, const char *@var{cp}, void *@var{buf})
1120
This function converts an Internet address (either IPv4 or IPv6) from
1121
presentation (textual) to network (binary) format. @var{af} should be
1122
either @code{AF_INET} or @code{AF_INET6}, as appropriate for the type of
1123
address being converted. @var{cp} is a pointer to the input string, and
1124
@var{buf} is a pointer to a buffer for the result. It is the caller's
1125
responsibility to make sure the buffer is large enough.
1128
@comment arpa/inet.h
1129
@comment IPv6 basic API
1130
@deftypefun {const char *} inet_ntop (int @var{af}, const void *@var{cp}, char *@var{buf}, size_t @var{len})
1131
This function converts an Internet address (either IPv4 or IPv6) from
1132
network (binary) to presentation (textual) form. @var{af} should be
1133
either @code{AF_INET} or @code{AF_INET6}, as appropriate. @var{cp} is a
1134
pointer to the address to be converted. @var{buf} should be a pointer
1135
to a buffer to hold the result, and @var{len} is the length of this
1136
buffer. The return value from the function will be this buffer address.
1140
@subsubsection Host Names
1141
@cindex hosts database
1142
@cindex converting host name to address
1143
@cindex converting host address to name
1145
Besides the standard numbers-and-dots notation for Internet addresses,
1146
you can also refer to a host by a symbolic name. The advantage of a
1147
symbolic name is that it is usually easier to remember. For example,
1148
the machine with Internet address @samp{158.121.106.19} is also known as
1149
@samp{alpha.gnu.org}; and other machines in the @samp{gnu.org}
1150
domain can refer to it simply as @samp{alpha}.
1154
Internally, the system uses a database to keep track of the mapping
1155
between host names and host numbers. This database is usually either
1156
the file @file{/etc/hosts} or an equivalent provided by a name server.
1157
The functions and other symbols for accessing this database are declared
1158
in @file{netdb.h}. They are BSD features, defined unconditionally if
1159
you include @file{netdb.h}.
1163
@deftp {Data Type} {struct hostent}
1164
This data type is used to represent an entry in the hosts database. It
1165
has the following members:
1169
This is the ``official'' name of the host.
1171
@item char **h_aliases
1172
These are alternative names for the host, represented as a null-terminated
1175
@item int h_addrtype
1176
This is the host address type; in practice, its value is always either
1177
@code{AF_INET} or @code{AF_INET6}, with the latter being used for IPv6
1178
hosts. In principle other kinds of addresses could be represented in
1179
the database as well as Internet addresses; if this were done, you
1180
might find a value in this field other than @code{AF_INET} or
1181
@code{AF_INET6}. @xref{Socket Addresses}.
1184
This is the length, in bytes, of each address.
1186
@item char **h_addr_list
1187
This is the vector of addresses for the host. (Recall that the host
1188
might be connected to multiple networks and have different addresses on
1189
each one.) The vector is terminated by a null pointer.
1192
This is a synonym for @code{h_addr_list[0]}; in other words, it is the
1197
As far as the host database is concerned, each address is just a block
1198
of memory @code{h_length} bytes long. But in other contexts there is an
1199
implicit assumption that you can convert IPv4 addresses to a
1200
@code{struct in_addr} or an @code{uint32_t}. Host addresses in
1201
a @code{struct hostent} structure are always given in network byte
1202
order; see @ref{Byte Order}.
1204
You can use @code{gethostbyname}, @code{gethostbyname2} or
1205
@code{gethostbyaddr} to search the hosts database for information about
1206
a particular host. The information is returned in a
1207
statically-allocated structure; you must copy the information if you
1208
need to save it across calls. You can also use @code{getaddrinfo} and
1209
@code{getnameinfo} to obtain this information.
1213
@deftypefun {struct hostent *} gethostbyname (const char *@var{name})
1214
The @code{gethostbyname} function returns information about the host
1215
named @var{name}. If the lookup fails, it returns a null pointer.
1219
@comment IPv6 Basic API
1220
@deftypefun {struct hostent *} gethostbyname2 (const char *@var{name}, int @var{af})
1221
The @code{gethostbyname2} function is like @code{gethostbyname}, but
1222
allows the caller to specify the desired address family (e.g.@:
1223
@code{AF_INET} or @code{AF_INET6}) of the result.
1228
@deftypefun {struct hostent *} gethostbyaddr (const char *@var{addr}, size_t @var{length}, int @var{format})
1229
The @code{gethostbyaddr} function returns information about the host
1230
with Internet address @var{addr}. The parameter @var{addr} is not
1231
really a pointer to char - it can be a pointer to an IPv4 or an IPv6
1232
address. The @var{length} argument is the size (in bytes) of the address
1233
at @var{addr}. @var{format} specifies the address format; for an IPv4
1234
Internet address, specify a value of @code{AF_INET}; for an IPv6
1235
Internet address, use @code{AF_INET6}.
1237
If the lookup fails, @code{gethostbyaddr} returns a null pointer.
1241
If the name lookup by @code{gethostbyname} or @code{gethostbyaddr}
1242
fails, you can find out the reason by looking at the value of the
1243
variable @code{h_errno}. (It would be cleaner design for these
1244
functions to set @code{errno}, but use of @code{h_errno} is compatible
1245
with other systems.)
1247
Here are the error codes that you may find in @code{h_errno}:
1252
@item HOST_NOT_FOUND
1253
@vindex HOST_NOT_FOUND
1254
No such host is known in the database.
1260
This condition happens when the name server could not be contacted. If
1261
you try again later, you may succeed then.
1267
A non-recoverable error occurred.
1273
The host database contains an entry for the name, but it doesn't have an
1274
associated Internet address.
1277
The lookup functions above all have one in common: they are not
1278
reentrant and therefore unusable in multi-threaded applications.
1279
Therefore provides the GNU C library a new set of functions which can be
1280
used in this context.
1284
@deftypefun int gethostbyname_r (const char *restrict @var{name}, struct hostent *restrict @var{result_buf}, char *restrict @var{buf}, size_t @var{buflen}, struct hostent **restrict @var{result}, int *restrict @var{h_errnop})
1285
The @code{gethostbyname_r} function returns information about the host
1286
named @var{name}. The caller must pass a pointer to an object of type
1287
@code{struct hostent} in the @var{result_buf} parameter. In addition
1288
the function may need extra buffer space and the caller must pass an
1289
pointer and the size of the buffer in the @var{buf} and @var{buflen}
1292
A pointer to the buffer, in which the result is stored, is available in
1293
@code{*@var{result}} after the function call successfully returned. If
1294
an error occurs or if no entry is found, the pointer @code{*@var{result}}
1295
is a null pointer. Success is signalled by a zero return value. If the
1296
function failed the return value is an error number. In addition to the
1297
errors defined for @code{gethostbyname} it can also be @code{ERANGE}.
1298
In this case the call should be repeated with a larger buffer.
1299
Additional error information is not stored in the global variable
1300
@code{h_errno} but instead in the object pointed to by @var{h_errnop}.
1302
Here's a small example:
1305
gethostname (char *host)
1307
struct hostent hostbuf, *hp;
1314
/* Allocate buffer, remember to free it to avoid memory leakage. */
1315
tmphstbuf = malloc (hstbuflen);
1317
while ((res = gethostbyname_r (host, &hostbuf, tmphstbuf, hstbuflen,
1318
&hp, &herr)) == ERANGE)
1320
/* Enlarge the buffer. */
1322
tmphstbuf = realloc (tmphstbuf, hstbuflen);
1324
/* Check for errors. */
1325
if (res || hp == NULL)
1334
@deftypefun int gethostbyname2_r (const char *@var{name}, int @var{af}, struct hostent *restrict @var{result_buf}, char *restrict @var{buf}, size_t @var{buflen}, struct hostent **restrict @var{result}, int *restrict @var{h_errnop})
1335
The @code{gethostbyname2_r} function is like @code{gethostbyname_r}, but
1336
allows the caller to specify the desired address family (e.g.@:
1337
@code{AF_INET} or @code{AF_INET6}) for the result.
1342
@deftypefun int gethostbyaddr_r (const char *@var{addr}, size_t @var{length}, int @var{format}, struct hostent *restrict @var{result_buf}, char *restrict @var{buf}, size_t @var{buflen}, struct hostent **restrict @var{result}, int *restrict @var{h_errnop})
1343
The @code{gethostbyaddr_r} function returns information about the host
1344
with Internet address @var{addr}. The parameter @var{addr} is not
1345
really a pointer to char - it can be a pointer to an IPv4 or an IPv6
1346
address. The @var{length} argument is the size (in bytes) of the address
1347
at @var{addr}. @var{format} specifies the address format; for an IPv4
1348
Internet address, specify a value of @code{AF_INET}; for an IPv6
1349
Internet address, use @code{AF_INET6}.
1351
Similar to the @code{gethostbyname_r} function, the caller must provide
1352
buffers for the result and memory used internally. In case of success
1353
the function returns zero. Otherwise the value is an error number where
1354
@code{ERANGE} has the special meaning that the caller-provided buffer is
1358
You can also scan the entire hosts database one entry at a time using
1359
@code{sethostent}, @code{gethostent} and @code{endhostent}. Be careful
1360
when using these functions because they are not reentrant.
1364
@deftypefun void sethostent (int @var{stayopen})
1365
This function opens the hosts database to begin scanning it. You can
1366
then call @code{gethostent} to read the entries.
1368
@c There was a rumor that this flag has different meaning if using the DNS,
1369
@c but it appears this description is accurate in that case also.
1370
If the @var{stayopen} argument is nonzero, this sets a flag so that
1371
subsequent calls to @code{gethostbyname} or @code{gethostbyaddr} will
1372
not close the database (as they usually would). This makes for more
1373
efficiency if you call those functions several times, by avoiding
1374
reopening the database for each call.
1379
@deftypefun {struct hostent *} gethostent (void)
1380
This function returns the next entry in the hosts database. It
1381
returns a null pointer if there are no more entries.
1386
@deftypefun void endhostent (void)
1387
This function closes the hosts database.
1391
@subsection Internet Ports
1394
A socket address in the Internet namespace consists of a machine's
1395
Internet address plus a @dfn{port number} which distinguishes the
1396
sockets on a given machine (for a given protocol). Port numbers range
1399
Port numbers less than @code{IPPORT_RESERVED} are reserved for standard
1400
servers, such as @code{finger} and @code{telnet}. There is a database
1401
that keeps track of these, and you can use the @code{getservbyname}
1402
function to map a service name onto a port number; see @ref{Services
1405
If you write a server that is not one of the standard ones defined in
1406
the database, you must choose a port number for it. Use a number
1407
greater than @code{IPPORT_USERRESERVED}; such numbers are reserved for
1408
servers and won't ever be generated automatically by the system.
1409
Avoiding conflicts with servers being run by other users is up to you.
1411
When you use a socket without specifying its address, the system
1412
generates a port number for it. This number is between
1413
@code{IPPORT_RESERVED} and @code{IPPORT_USERRESERVED}.
1415
On the Internet, it is actually legitimate to have two different
1416
sockets with the same port number, as long as they never both try to
1417
communicate with the same socket address (host address plus port
1418
number). You shouldn't duplicate a port number except in special
1419
circumstances where a higher-level protocol requires it. Normally,
1420
the system won't let you do it; @code{bind} normally insists on
1421
distinct port numbers. To reuse a port number, you must set the
1422
socket option @code{SO_REUSEADDR}. @xref{Socket-Level Options}.
1424
@pindex netinet/in.h
1425
These macros are defined in the header file @file{netinet/in.h}.
1427
@comment netinet/in.h
1429
@deftypevr Macro int IPPORT_RESERVED
1430
Port numbers less than @code{IPPORT_RESERVED} are reserved for
1434
@comment netinet/in.h
1436
@deftypevr Macro int IPPORT_USERRESERVED
1437
Port numbers greater than or equal to @code{IPPORT_USERRESERVED} are
1438
reserved for explicit use; they will never be allocated automatically.
1441
@node Services Database
1442
@subsection The Services Database
1443
@cindex services database
1444
@cindex converting service name to port number
1445
@cindex converting port number to service name
1447
@pindex /etc/services
1448
The database that keeps track of ``well-known'' services is usually
1449
either the file @file{/etc/services} or an equivalent from a name server.
1450
You can use these utilities, declared in @file{netdb.h}, to access
1451
the services database.
1456
@deftp {Data Type} {struct servent}
1457
This data type holds information about entries from the services database.
1458
It has the following members:
1462
This is the ``official'' name of the service.
1464
@item char **s_aliases
1465
These are alternate names for the service, represented as an array of
1466
strings. A null pointer terminates the array.
1469
This is the port number for the service. Port numbers are given in
1470
network byte order; see @ref{Byte Order}.
1473
This is the name of the protocol to use with this service.
1474
@xref{Protocols Database}.
1478
To get information about a particular service, use the
1479
@code{getservbyname} or @code{getservbyport} functions. The information
1480
is returned in a statically-allocated structure; you must copy the
1481
information if you need to save it across calls.
1485
@deftypefun {struct servent *} getservbyname (const char *@var{name}, const char *@var{proto})
1486
The @code{getservbyname} function returns information about the
1487
service named @var{name} using protocol @var{proto}. If it can't find
1488
such a service, it returns a null pointer.
1490
This function is useful for servers as well as for clients; servers
1491
use it to determine which port they should listen on (@pxref{Listening}).
1496
@deftypefun {struct servent *} getservbyport (int @var{port}, const char *@var{proto})
1497
The @code{getservbyport} function returns information about the
1498
service at port @var{port} using protocol @var{proto}. If it can't
1499
find such a service, it returns a null pointer.
1503
You can also scan the services database using @code{setservent},
1504
@code{getservent} and @code{endservent}. Be careful when using these
1505
functions because they are not reentrant.
1509
@deftypefun void setservent (int @var{stayopen})
1510
This function opens the services database to begin scanning it.
1512
If the @var{stayopen} argument is nonzero, this sets a flag so that
1513
subsequent calls to @code{getservbyname} or @code{getservbyport} will
1514
not close the database (as they usually would). This makes for more
1515
efficiency if you call those functions several times, by avoiding
1516
reopening the database for each call.
1521
@deftypefun {struct servent *} getservent (void)
1522
This function returns the next entry in the services database. If
1523
there are no more entries, it returns a null pointer.
1528
@deftypefun void endservent (void)
1529
This function closes the services database.
1533
@subsection Byte Order Conversion
1534
@cindex byte order conversion, for socket
1535
@cindex converting byte order
1538
@cindex little-endian
1539
Different kinds of computers use different conventions for the
1540
ordering of bytes within a word. Some computers put the most
1541
significant byte within a word first (this is called ``big-endian''
1542
order), and others put it last (``little-endian'' order).
1544
@cindex network byte order
1545
So that machines with different byte order conventions can
1546
communicate, the Internet protocols specify a canonical byte order
1547
convention for data transmitted over the network. This is known
1548
as @dfn{network byte order}.
1550
When establishing an Internet socket connection, you must make sure that
1551
the data in the @code{sin_port} and @code{sin_addr} members of the
1552
@code{sockaddr_in} structure are represented in network byte order.
1553
If you are encoding integer data in the messages sent through the
1554
socket, you should convert this to network byte order too. If you don't
1555
do this, your program may fail when running on or talking to other kinds
1558
If you use @code{getservbyname} and @code{gethostbyname} or
1559
@code{inet_addr} to get the port number and host address, the values are
1560
already in network byte order, and you can copy them directly into
1561
the @code{sockaddr_in} structure.
1563
Otherwise, you have to convert the values explicitly. Use @code{htons}
1564
and @code{ntohs} to convert values for the @code{sin_port} member. Use
1565
@code{htonl} and @code{ntohl} to convert IPv4 addresses for the
1566
@code{sin_addr} member. (Remember, @code{struct in_addr} is equivalent
1567
to @code{uint32_t}.) These functions are declared in
1568
@file{netinet/in.h}.
1569
@pindex netinet/in.h
1571
@comment netinet/in.h
1573
@deftypefun {uint16_t} htons (uint16_t @var{hostshort})
1574
This function converts the @code{uint16_t} integer @var{hostshort} from
1575
host byte order to network byte order.
1578
@comment netinet/in.h
1580
@deftypefun {uint16_t} ntohs (uint16_t @var{netshort})
1581
This function converts the @code{uint16_t} integer @var{netshort} from
1582
network byte order to host byte order.
1585
@comment netinet/in.h
1587
@deftypefun {uint32_t} htonl (uint32_t @var{hostlong})
1588
This function converts the @code{uint32_t} integer @var{hostlong} from
1589
host byte order to network byte order.
1591
This is used for IPv4 Internet addresses.
1594
@comment netinet/in.h
1596
@deftypefun {uint32_t} ntohl (uint32_t @var{netlong})
1597
This function converts the @code{uint32_t} integer @var{netlong} from
1598
network byte order to host byte order.
1600
This is used for IPv4 Internet addresses.
1603
@node Protocols Database
1604
@subsection Protocols Database
1605
@cindex protocols database
1607
The communications protocol used with a socket controls low-level
1608
details of how data are exchanged. For example, the protocol implements
1609
things like checksums to detect errors in transmissions, and routing
1610
instructions for messages. Normal user programs have little reason to
1611
mess with these details directly.
1613
@cindex TCP (Internet protocol)
1614
The default communications protocol for the Internet namespace depends on
1615
the communication style. For stream communication, the default is TCP
1616
(``transmission control protocol''). For datagram communication, the
1617
default is UDP (``user datagram protocol''). For reliable datagram
1618
communication, the default is RDP (``reliable datagram protocol'').
1619
You should nearly always use the default.
1621
@pindex /etc/protocols
1622
Internet protocols are generally specified by a name instead of a
1623
number. The network protocols that a host knows about are stored in a
1624
database. This is usually either derived from the file
1625
@file{/etc/protocols}, or it may be an equivalent provided by a name
1626
server. You look up the protocol number associated with a named
1627
protocol in the database using the @code{getprotobyname} function.
1629
Here are detailed descriptions of the utilities for accessing the
1630
protocols database. These are declared in @file{netdb.h}.
1635
@deftp {Data Type} {struct protoent}
1636
This data type is used to represent entries in the network protocols
1637
database. It has the following members:
1641
This is the official name of the protocol.
1643
@item char **p_aliases
1644
These are alternate names for the protocol, specified as an array of
1645
strings. The last element of the array is a null pointer.
1648
This is the protocol number (in host byte order); use this member as the
1649
@var{protocol} argument to @code{socket}.
1653
You can use @code{getprotobyname} and @code{getprotobynumber} to search
1654
the protocols database for a specific protocol. The information is
1655
returned in a statically-allocated structure; you must copy the
1656
information if you need to save it across calls.
1660
@deftypefun {struct protoent *} getprotobyname (const char *@var{name})
1661
The @code{getprotobyname} function returns information about the
1662
network protocol named @var{name}. If there is no such protocol, it
1663
returns a null pointer.
1668
@deftypefun {struct protoent *} getprotobynumber (int @var{protocol})
1669
The @code{getprotobynumber} function returns information about the
1670
network protocol with number @var{protocol}. If there is no such
1671
protocol, it returns a null pointer.
1674
You can also scan the whole protocols database one protocol at a time by
1675
using @code{setprotoent}, @code{getprotoent} and @code{endprotoent}.
1676
Be careful when using these functions because they are not reentrant.
1680
@deftypefun void setprotoent (int @var{stayopen})
1681
This function opens the protocols database to begin scanning it.
1683
If the @var{stayopen} argument is nonzero, this sets a flag so that
1684
subsequent calls to @code{getprotobyname} or @code{getprotobynumber} will
1685
not close the database (as they usually would). This makes for more
1686
efficiency if you call those functions several times, by avoiding
1687
reopening the database for each call.
1692
@deftypefun {struct protoent *} getprotoent (void)
1693
This function returns the next entry in the protocols database. It
1694
returns a null pointer if there are no more entries.
1699
@deftypefun void endprotoent (void)
1700
This function closes the protocols database.
1704
@subsection Internet Socket Example
1706
Here is an example showing how to create and name a socket in the
1707
Internet namespace. The newly created socket exists on the machine that
1708
the program is running on. Rather than finding and using the machine's
1709
Internet address, this example specifies @code{INADDR_ANY} as the host
1710
address; the system replaces that with the machine's actual address.
1713
@include mkisock.c.texi
1716
Here is another example, showing how you can fill in a @code{sockaddr_in}
1717
structure, given a host name string and a port number:
1720
@include isockad.c.texi
1723
@node Misc Namespaces
1724
@section Other Namespaces
1731
Certain other namespaces and associated protocol families are supported
1732
but not documented yet because they are not often used. @code{PF_NS}
1733
refers to the Xerox Network Software protocols. @code{PF_ISO} stands
1734
for Open Systems Interconnect. @code{PF_CCITT} refers to protocols from
1735
CCITT. @file{socket.h} defines these symbols and others naming protocols
1736
not actually implemented.
1738
@code{PF_IMPLINK} is used for communicating between hosts and Internet
1739
Message Processors. For information on this and @code{PF_ROUTE}, an
1740
occasionally-used local area routing protocol, see the GNU Hurd Manual
1741
(to appear in the future).
1743
@node Open/Close Sockets
1744
@section Opening and Closing Sockets
1746
This section describes the actual library functions for opening and
1747
closing sockets. The same functions work for all namespaces and
1751
* Creating a Socket:: How to open a socket.
1752
* Closing a Socket:: How to close a socket.
1753
* Socket Pairs:: These are created like pipes.
1756
@node Creating a Socket
1757
@subsection Creating a Socket
1758
@cindex creating a socket
1759
@cindex socket, creating
1760
@cindex opening a socket
1762
The primitive for creating a socket is the @code{socket} function,
1763
declared in @file{sys/socket.h}.
1764
@pindex sys/socket.h
1766
@comment sys/socket.h
1768
@deftypefun int socket (int @var{namespace}, int @var{style}, int @var{protocol})
1769
This function creates a socket and specifies communication style
1770
@var{style}, which should be one of the socket styles listed in
1771
@ref{Communication Styles}. The @var{namespace} argument specifies
1772
the namespace; it must be @code{PF_LOCAL} (@pxref{Local Namespace}) or
1773
@code{PF_INET} (@pxref{Internet Namespace}). @var{protocol}
1774
designates the specific protocol (@pxref{Socket Concepts}); zero is
1775
usually right for @var{protocol}.
1777
The return value from @code{socket} is the file descriptor for the new
1778
socket, or @code{-1} in case of error. The following @code{errno} error
1779
conditions are defined for this function:
1782
@item EPROTONOSUPPORT
1783
The @var{protocol} or @var{style} is not supported by the
1784
@var{namespace} specified.
1787
The process already has too many file descriptors open.
1790
The system already has too many file descriptors open.
1793
The process does not have the privilege to create a socket of the specified
1794
@var{style} or @var{protocol}.
1797
The system ran out of internal buffer space.
1800
The file descriptor returned by the @code{socket} function supports both
1801
read and write operations. However, like pipes, sockets do not support file
1802
positioning operations.
1805
For examples of how to call the @code{socket} function,
1806
see @ref{Local Socket Example}, or @ref{Inet Example}.
1809
@node Closing a Socket
1810
@subsection Closing a Socket
1811
@cindex socket, closing
1812
@cindex closing a socket
1813
@cindex shutting down a socket
1814
@cindex socket shutdown
1816
When you have finished using a socket, you can simply close its
1817
file descriptor with @code{close}; see @ref{Opening and Closing Files}.
1818
If there is still data waiting to be transmitted over the connection,
1819
normally @code{close} tries to complete this transmission. You
1820
can control this behavior using the @code{SO_LINGER} socket option to
1821
specify a timeout period; see @ref{Socket Options}.
1823
@pindex sys/socket.h
1824
You can also shut down only reception or transmission on a
1825
connection by calling @code{shutdown}, which is declared in
1826
@file{sys/socket.h}.
1828
@comment sys/socket.h
1830
@deftypefun int shutdown (int @var{socket}, int @var{how})
1831
The @code{shutdown} function shuts down the connection of socket
1832
@var{socket}. The argument @var{how} specifies what action to
1837
Stop receiving data for this socket. If further data arrives,
1841
Stop trying to transmit data from this socket. Discard any data
1842
waiting to be sent. Stop looking for acknowledgement of data already
1843
sent; don't retransmit it if it is lost.
1846
Stop both reception and transmission.
1849
The return value is @code{0} on success and @code{-1} on failure. The
1850
following @code{errno} error conditions are defined for this function:
1854
@var{socket} is not a valid file descriptor.
1857
@var{socket} is not a socket.
1860
@var{socket} is not connected.
1865
@subsection Socket Pairs
1866
@cindex creating a socket pair
1868
@cindex opening a socket pair
1870
@pindex sys/socket.h
1871
A @dfn{socket pair} consists of a pair of connected (but unnamed)
1872
sockets. It is very similar to a pipe and is used in much the same
1873
way. Socket pairs are created with the @code{socketpair} function,
1874
declared in @file{sys/socket.h}. A socket pair is much like a pipe; the
1875
main difference is that the socket pair is bidirectional, whereas the
1876
pipe has one input-only end and one output-only end (@pxref{Pipes and
1879
@comment sys/socket.h
1881
@deftypefun int socketpair (int @var{namespace}, int @var{style}, int @var{protocol}, int @var{filedes}@t{[2]})
1882
This function creates a socket pair, returning the file descriptors in
1883
@code{@var{filedes}[0]} and @code{@var{filedes}[1]}. The socket pair
1884
is a full-duplex communications channel, so that both reading and writing
1885
may be performed at either end.
1887
The @var{namespace}, @var{style} and @var{protocol} arguments are
1888
interpreted as for the @code{socket} function. @var{style} should be
1889
one of the communication styles listed in @ref{Communication Styles}.
1890
The @var{namespace} argument specifies the namespace, which must be
1891
@code{AF_LOCAL} (@pxref{Local Namespace}); @var{protocol} specifies the
1892
communications protocol, but zero is the only meaningful value.
1894
If @var{style} specifies a connectionless communication style, then
1895
the two sockets you get are not @emph{connected}, strictly speaking,
1896
but each of them knows the other as the default destination address,
1897
so they can send packets to each other.
1899
The @code{socketpair} function returns @code{0} on success and @code{-1}
1900
on failure. The following @code{errno} error conditions are defined
1905
The process has too many file descriptors open.
1908
The specified namespace is not supported.
1910
@item EPROTONOSUPPORT
1911
The specified protocol is not supported.
1914
The specified protocol does not support the creation of socket pairs.
1919
@section Using Sockets with Connections
1924
The most common communication styles involve making a connection to a
1925
particular other socket, and then exchanging data with that socket
1926
over and over. Making a connection is asymmetric; one side (the
1927
@dfn{client}) acts to request a connection, while the other side (the
1928
@dfn{server}) makes a socket and waits for the connection request.
1933
@ref{Connecting}, describes what the client program must do to
1934
initiate a connection with a server.
1937
@ref{Listening} and @ref{Accepting Connections} describe what the
1938
server program must do to wait for and act upon connection requests
1942
@ref{Transferring Data}, describes how data are transferred through the
1948
* Connecting:: What the client program must do.
1949
* Listening:: How a server program waits for requests.
1950
* Accepting Connections:: What the server does when it gets a request.
1951
* Who is Connected:: Getting the address of the
1952
other side of a connection.
1953
* Transferring Data:: How to send and receive data.
1954
* Byte Stream Example:: An example program: a client for communicating
1955
over a byte stream socket in the Internet namespace.
1956
* Server Example:: A corresponding server program.
1957
* Out-of-Band Data:: This is an advanced feature.
1961
@subsection Making a Connection
1962
@cindex connecting a socket
1963
@cindex socket, connecting
1964
@cindex socket, initiating a connection
1965
@cindex socket, client actions
1967
In making a connection, the client makes a connection while the server
1968
waits for and accepts the connection. Here we discuss what the client
1969
program must do with the @code{connect} function, which is declared in
1970
@file{sys/socket.h}.
1972
@comment sys/socket.h
1974
@deftypefun int connect (int @var{socket}, struct sockaddr *@var{addr}, socklen_t @var{length})
1975
The @code{connect} function initiates a connection from the socket
1976
with file descriptor @var{socket} to the socket whose address is
1977
specified by the @var{addr} and @var{length} arguments. (This socket
1978
is typically on another machine, and it must be already set up as a
1979
server.) @xref{Socket Addresses}, for information about how these
1980
arguments are interpreted.
1982
Normally, @code{connect} waits until the server responds to the request
1983
before it returns. You can set nonblocking mode on the socket
1984
@var{socket} to make @code{connect} return immediately without waiting
1985
for the response. @xref{File Status Flags}, for information about
1987
@c !!! how do you tell when it has finished connecting? I suspect the
1988
@c way you do it is select for writing.
1990
The normal return value from @code{connect} is @code{0}. If an error
1991
occurs, @code{connect} returns @code{-1}. The following @code{errno}
1992
error conditions are defined for this function:
1996
The socket @var{socket} is not a valid file descriptor.
1999
File descriptor @var{socket} is not a socket.
2002
The specified address is not available on the remote machine.
2005
The namespace of the @var{addr} is not supported by this socket.
2008
The socket @var{socket} is already connected.
2011
The attempt to establish the connection timed out.
2014
The server has actively refused to establish the connection.
2017
The network of the given @var{addr} isn't reachable from this host.
2020
The socket address of the given @var{addr} is already in use.
2023
The socket @var{socket} is non-blocking and the connection could not be
2024
established immediately. You can determine when the connection is
2025
completely established with @code{select}; @pxref{Waiting for I/O}.
2026
Another @code{connect} call on the same socket, before the connection is
2027
completely established, will fail with @code{EALREADY}.
2030
The socket @var{socket} is non-blocking and already has a pending
2031
connection in progress (see @code{EINPROGRESS} above).
2034
This function is defined as a cancellation point in multi-threaded
2035
programs, so one has to be prepared for this and make sure that
2036
allocated resources (like memory, files descriptors, semaphores or
2037
whatever) are freed even if the thread is canceled.
2038
@c @xref{pthread_cleanup_push}, for a method how to do this.
2042
@subsection Listening for Connections
2043
@cindex listening (sockets)
2044
@cindex sockets, server actions
2045
@cindex sockets, listening
2047
Now let us consider what the server process must do to accept
2048
connections on a socket. First it must use the @code{listen} function
2049
to enable connection requests on the socket, and then accept each
2050
incoming connection with a call to @code{accept} (@pxref{Accepting
2051
Connections}). Once connection requests are enabled on a server socket,
2052
the @code{select} function reports when the socket has a connection
2053
ready to be accepted (@pxref{Waiting for I/O}).
2055
The @code{listen} function is not allowed for sockets using
2056
connectionless communication styles.
2058
You can write a network server that does not even start running until a
2059
connection to it is requested. @xref{Inetd Servers}.
2061
In the Internet namespace, there are no special protection mechanisms
2062
for controlling access to a port; any process on any machine
2063
can make a connection to your server. If you want to restrict access to
2064
your server, make it examine the addresses associated with connection
2065
requests or implement some other handshaking or identification
2068
In the local namespace, the ordinary file protection bits control who has
2069
access to connect to the socket.
2071
@comment sys/socket.h
2073
@deftypefun int listen (int @var{socket}, int @var{n})
2074
The @code{listen} function enables the socket @var{socket} to accept
2075
connections, thus making it a server socket.
2077
The argument @var{n} specifies the length of the queue for pending
2078
connections. When the queue fills, new clients attempting to connect
2079
fail with @code{ECONNREFUSED} until the server calls @code{accept} to
2080
accept a connection from the queue.
2082
The @code{listen} function returns @code{0} on success and @code{-1}
2083
on failure. The following @code{errno} error conditions are defined
2088
The argument @var{socket} is not a valid file descriptor.
2091
The argument @var{socket} is not a socket.
2094
The socket @var{socket} does not support this operation.
2098
@node Accepting Connections
2099
@subsection Accepting Connections
2100
@cindex sockets, accepting connections
2101
@cindex accepting connections
2103
When a server receives a connection request, it can complete the
2104
connection by accepting the request. Use the function @code{accept}
2107
A socket that has been established as a server can accept connection
2108
requests from multiple clients. The server's original socket
2109
@emph{does not become part of the connection}; instead, @code{accept}
2110
makes a new socket which participates in the connection.
2111
@code{accept} returns the descriptor for this socket. The server's
2112
original socket remains available for listening for further connection
2115
The number of pending connection requests on a server socket is finite.
2116
If connection requests arrive from clients faster than the server can
2117
act upon them, the queue can fill up and additional requests are refused
2118
with an @code{ECONNREFUSED} error. You can specify the maximum length of
2119
this queue as an argument to the @code{listen} function, although the
2120
system may also impose its own internal limit on the length of this
2123
@comment sys/socket.h
2125
@deftypefun int accept (int @var{socket}, struct sockaddr *@var{addr}, socklen_t *@var{length_ptr})
2126
This function is used to accept a connection request on the server
2127
socket @var{socket}.
2129
The @code{accept} function waits if there are no connections pending,
2130
unless the socket @var{socket} has nonblocking mode set. (You can use
2131
@code{select} to wait for a pending connection, with a nonblocking
2132
socket.) @xref{File Status Flags}, for information about nonblocking
2135
The @var{addr} and @var{length-ptr} arguments are used to return
2136
information about the name of the client socket that initiated the
2137
connection. @xref{Socket Addresses}, for information about the format
2140
Accepting a connection does not make @var{socket} part of the
2141
connection. Instead, it creates a new socket which becomes
2142
connected. The normal return value of @code{accept} is the file
2143
descriptor for the new socket.
2145
After @code{accept}, the original socket @var{socket} remains open and
2146
unconnected, and continues listening until you close it. You can
2147
accept further connections with @var{socket} by calling @code{accept}
2150
If an error occurs, @code{accept} returns @code{-1}. The following
2151
@code{errno} error conditions are defined for this function:
2155
The @var{socket} argument is not a valid file descriptor.
2158
The descriptor @var{socket} argument is not a socket.
2161
The descriptor @var{socket} does not support this operation.
2164
@var{socket} has nonblocking mode set, and there are no pending
2165
connections immediately available.
2168
This function is defined as a cancellation point in multi-threaded
2169
programs, so one has to be prepared for this and make sure that
2170
allocated resources (like memory, files descriptors, semaphores or
2171
whatever) are freed even if the thread is canceled.
2172
@c @xref{pthread_cleanup_push}, for a method how to do this.
2175
The @code{accept} function is not allowed for sockets using
2176
connectionless communication styles.
2178
@node Who is Connected
2179
@subsection Who is Connected to Me?
2181
@comment sys/socket.h
2183
@deftypefun int getpeername (int @var{socket}, struct sockaddr *@var{addr}, socklen_t *@var{length-ptr})
2184
The @code{getpeername} function returns the address of the socket that
2185
@var{socket} is connected to; it stores the address in the memory space
2186
specified by @var{addr} and @var{length-ptr}. It stores the length of
2187
the address in @code{*@var{length-ptr}}.
2189
@xref{Socket Addresses}, for information about the format of the
2190
address. In some operating systems, @code{getpeername} works only for
2191
sockets in the Internet domain.
2193
The return value is @code{0} on success and @code{-1} on error. The
2194
following @code{errno} error conditions are defined for this function:
2198
The argument @var{socket} is not a valid file descriptor.
2201
The descriptor @var{socket} is not a socket.
2204
The socket @var{socket} is not connected.
2207
There are not enough internal buffers available.
2212
@node Transferring Data
2213
@subsection Transferring Data
2214
@cindex reading from a socket
2215
@cindex writing to a socket
2217
Once a socket has been connected to a peer, you can use the ordinary
2218
@code{read} and @code{write} operations (@pxref{I/O Primitives}) to
2219
transfer data. A socket is a two-way communications channel, so read
2220
and write operations can be performed at either end.
2222
There are also some I/O modes that are specific to socket operations.
2223
In order to specify these modes, you must use the @code{recv} and
2224
@code{send} functions instead of the more generic @code{read} and
2225
@code{write} functions. The @code{recv} and @code{send} functions take
2226
an additional argument which you can use to specify various flags to
2227
control special I/O modes. For example, you can specify the
2228
@code{MSG_OOB} flag to read or write out-of-band data, the
2229
@code{MSG_PEEK} flag to peek at input, or the @code{MSG_DONTROUTE} flag
2230
to control inclusion of routing information on output.
2233
* Sending Data:: Sending data with @code{send}.
2234
* Receiving Data:: Reading data with @code{recv}.
2235
* Socket Data Options:: Using @code{send} and @code{recv}.
2239
@subsubsection Sending Data
2241
@pindex sys/socket.h
2242
The @code{send} function is declared in the header file
2243
@file{sys/socket.h}. If your @var{flags} argument is zero, you can just
2244
as well use @code{write} instead of @code{send}; see @ref{I/O
2245
Primitives}. If the socket was connected but the connection has broken,
2246
you get a @code{SIGPIPE} signal for any use of @code{send} or
2247
@code{write} (@pxref{Miscellaneous Signals}).
2249
@comment sys/socket.h
2251
@deftypefun int send (int @var{socket}, void *@var{buffer}, size_t @var{size}, int @var{flags})
2252
The @code{send} function is like @code{write}, but with the additional
2253
flags @var{flags}. The possible values of @var{flags} are described
2254
in @ref{Socket Data Options}.
2256
This function returns the number of bytes transmitted, or @code{-1} on
2257
failure. If the socket is nonblocking, then @code{send} (like
2258
@code{write}) can return after sending just part of the data.
2259
@xref{File Status Flags}, for information about nonblocking mode.
2261
Note, however, that a successful return value merely indicates that
2262
the message has been sent without error, not necessarily that it has
2263
been received without error.
2265
The following @code{errno} error conditions are defined for this function:
2269
The @var{socket} argument is not a valid file descriptor.
2272
The operation was interrupted by a signal before any data was sent.
2273
@xref{Interrupted Primitives}.
2276
The descriptor @var{socket} is not a socket.
2279
The socket type requires that the message be sent atomically, but the
2280
message is too large for this to be possible.
2283
Nonblocking mode has been set on the socket, and the write operation
2284
would block. (Normally @code{send} blocks until the operation can be
2288
There is not enough internal buffer space available.
2291
You never connected this socket.
2294
This socket was connected but the connection is now broken. In this
2295
case, @code{send} generates a @code{SIGPIPE} signal first; if that
2296
signal is ignored or blocked, or if its handler returns, then
2297
@code{send} fails with @code{EPIPE}.
2300
This function is defined as a cancellation point in multi-threaded
2301
programs, so one has to be prepared for this and make sure that
2302
allocated resources (like memory, files descriptors, semaphores or
2303
whatever) are freed even if the thread is canceled.
2304
@c @xref{pthread_cleanup_push}, for a method how to do this.
2307
@node Receiving Data
2308
@subsubsection Receiving Data
2310
@pindex sys/socket.h
2311
The @code{recv} function is declared in the header file
2312
@file{sys/socket.h}. If your @var{flags} argument is zero, you can
2313
just as well use @code{read} instead of @code{recv}; see @ref{I/O
2316
@comment sys/socket.h
2318
@deftypefun int recv (int @var{socket}, void *@var{buffer}, size_t @var{size}, int @var{flags})
2319
The @code{recv} function is like @code{read}, but with the additional
2320
flags @var{flags}. The possible values of @var{flags} are described
2321
in @ref{Socket Data Options}.
2323
If nonblocking mode is set for @var{socket}, and no data are available to
2324
be read, @code{recv} fails immediately rather than waiting. @xref{File
2325
Status Flags}, for information about nonblocking mode.
2327
This function returns the number of bytes received, or @code{-1} on failure.
2328
The following @code{errno} error conditions are defined for this function:
2332
The @var{socket} argument is not a valid file descriptor.
2335
The descriptor @var{socket} is not a socket.
2338
Nonblocking mode has been set on the socket, and the read operation
2339
would block. (Normally, @code{recv} blocks until there is input
2340
available to be read.)
2343
The operation was interrupted by a signal before any data was read.
2344
@xref{Interrupted Primitives}.
2347
You never connected this socket.
2350
This function is defined as a cancellation point in multi-threaded
2351
programs, so one has to be prepared for this and make sure that
2352
allocated resources (like memory, files descriptors, semaphores or
2353
whatever) are freed even if the thread is canceled.
2354
@c @xref{pthread_cleanup_push}, for a method how to do this.
2357
@node Socket Data Options
2358
@subsubsection Socket Data Options
2360
@pindex sys/socket.h
2361
The @var{flags} argument to @code{send} and @code{recv} is a bit
2362
mask. You can bitwise-OR the values of the following macros together
2363
to obtain a value for this argument. All are defined in the header
2364
file @file{sys/socket.h}.
2366
@comment sys/socket.h
2368
@deftypevr Macro int MSG_OOB
2369
Send or receive out-of-band data. @xref{Out-of-Band Data}.
2372
@comment sys/socket.h
2374
@deftypevr Macro int MSG_PEEK
2375
Look at the data but don't remove it from the input queue. This is
2376
only meaningful with input functions such as @code{recv}, not with
2380
@comment sys/socket.h
2382
@deftypevr Macro int MSG_DONTROUTE
2383
Don't include routing information in the message. This is only
2384
meaningful with output operations, and is usually only of interest for
2385
diagnostic or routing programs. We don't try to explain it here.
2388
@node Byte Stream Example
2389
@subsection Byte Stream Socket Example
2391
Here is an example client program that makes a connection for a byte
2392
stream socket in the Internet namespace. It doesn't do anything
2393
particularly interesting once it has connected to the server; it just
2394
sends a text string to the server and exits.
2396
This program uses @code{init_sockaddr} to set up the socket address; see
2400
@include inetcli.c.texi
2403
@node Server Example
2404
@subsection Byte Stream Connection Server Example
2406
The server end is much more complicated. Since we want to allow
2407
multiple clients to be connected to the server at the same time, it
2408
would be incorrect to wait for input from a single client by simply
2409
calling @code{read} or @code{recv}. Instead, the right thing to do is
2410
to use @code{select} (@pxref{Waiting for I/O}) to wait for input on
2411
all of the open sockets. This also allows the server to deal with
2412
additional connection requests.
2414
This particular server doesn't do anything interesting once it has
2415
gotten a message from a client. It does close the socket for that
2416
client when it detects an end-of-file condition (resulting from the
2417
client shutting down its end of the connection).
2419
This program uses @code{make_socket} to set up the socket address; see
2423
@include inetsrv.c.texi
2426
@node Out-of-Band Data
2427
@subsection Out-of-Band Data
2429
@cindex out-of-band data
2430
@cindex high-priority data
2431
Streams with connections permit @dfn{out-of-band} data that is
2432
delivered with higher priority than ordinary data. Typically the
2433
reason for sending out-of-band data is to send notice of an
2434
exceptional condition. To send out-of-band data use
2435
@code{send}, specifying the flag @code{MSG_OOB} (@pxref{Sending
2438
Out-of-band data are received with higher priority because the
2439
receiving process need not read it in sequence; to read the next
2440
available out-of-band data, use @code{recv} with the @code{MSG_OOB}
2441
flag (@pxref{Receiving Data}). Ordinary read operations do not read
2442
out-of-band data; they read only ordinary data.
2444
@cindex urgent socket condition
2445
When a socket finds that out-of-band data are on their way, it sends a
2446
@code{SIGURG} signal to the owner process or process group of the
2447
socket. You can specify the owner using the @code{F_SETOWN} command
2448
to the @code{fcntl} function; see @ref{Interrupt Input}. You must
2449
also establish a handler for this signal, as described in @ref{Signal
2450
Handling}, in order to take appropriate action such as reading the
2453
Alternatively, you can test for pending out-of-band data, or wait
2454
until there is out-of-band data, using the @code{select} function; it
2455
can wait for an exceptional condition on the socket. @xref{Waiting
2456
for I/O}, for more information about @code{select}.
2458
Notification of out-of-band data (whether with @code{SIGURG} or with
2459
@code{select}) indicates that out-of-band data are on the way; the data
2460
may not actually arrive until later. If you try to read the
2461
out-of-band data before it arrives, @code{recv} fails with an
2462
@code{EWOULDBLOCK} error.
2464
Sending out-of-band data automatically places a ``mark'' in the stream
2465
of ordinary data, showing where in the sequence the out-of-band data
2466
``would have been''. This is useful when the meaning of out-of-band
2467
data is ``cancel everything sent so far''. Here is how you can test,
2468
in the receiving process, whether any ordinary data was sent before
2472
success = ioctl (socket, SIOCATMARK, &atmark);
2475
The @code{integer} variable @var{atmark} is set to a nonzero value if
2476
the socket's read pointer has reached the ``mark''.
2478
@c Posix 1.g specifies sockatmark for this ioctl. sockatmark is not
2481
Here's a function to discard any ordinary data preceding the
2486
discard_until_mark (int socket)
2490
/* @r{This is not an arbitrary limit; any size will do.} */
2492
int atmark, success;
2494
/* @r{If we have reached the mark, return.} */
2495
success = ioctl (socket, SIOCATMARK, &atmark);
2501
/* @r{Otherwise, read a bunch of ordinary data and discard it.}
2502
@r{This is guaranteed not to read past the mark}
2503
@r{if it starts before the mark.} */
2504
success = read (socket, buffer, sizeof buffer);
2511
If you don't want to discard the ordinary data preceding the mark, you
2512
may need to read some of it anyway, to make room in internal system
2513
buffers for the out-of-band data. If you try to read out-of-band data
2514
and get an @code{EWOULDBLOCK} error, try reading some ordinary data
2515
(saving it so that you can use it when you want it) and see if that
2516
makes room. Here is an example:
2523
struct buffer *next;
2526
/* @r{Read the out-of-band data from SOCKET and return it}
2527
@r{as a `struct buffer', which records the address of the data}
2530
@r{It may be necessary to read some ordinary data}
2531
@r{in order to make room for the out-of-band data.}
2532
@r{If so, the ordinary data are saved as a chain of buffers}
2533
@r{found in the `next' field of the value.} */
2536
read_oob (int socket)
2538
struct buffer *tail = 0;
2539
struct buffer *list = 0;
2543
/* @r{This is an arbitrary limit.}
2544
@r{Does anyone know how to do this without a limit?} */
2546
char *buf = (char *) xmalloc (BUF_SZ);
2550
/* @r{Try again to read the out-of-band data.} */
2551
success = recv (socket, buf, BUF_SZ, MSG_OOB);
2554
/* @r{We got it, so return it.} */
2556
= (struct buffer *) xmalloc (sizeof (struct buffer));
2558
link->size = success;
2563
/* @r{If we fail, see if we are at the mark.} */
2564
success = ioctl (socket, SIOCATMARK, &atmark);
2569
/* @r{At the mark; skipping past more ordinary data cannot help.}
2570
@r{So just wait a while.} */
2575
/* @r{Otherwise, read a bunch of ordinary data and save it.}
2576
@r{This is guaranteed not to read past the mark}
2577
@r{if it starts before the mark.} */
2578
success = read (socket, buf, BUF_SZ);
2582
/* @r{Save this data in the buffer list.} */
2585
= (struct buffer *) xmalloc (sizeof (struct buffer));
2587
link->size = success;
2589
/* @r{Add the new link to the end of the list.} */
2601
@section Datagram Socket Operations
2603
@cindex datagram socket
2604
This section describes how to use communication styles that don't use
2605
connections (styles @code{SOCK_DGRAM} and @code{SOCK_RDM}). Using
2606
these styles, you group data into packets and each packet is an
2607
independent communication. You specify the destination for each
2608
packet individually.
2610
Datagram packets are like letters: you send each one independently
2611
with its own destination address, and they may arrive in the wrong
2612
order or not at all.
2614
The @code{listen} and @code{accept} functions are not allowed for
2615
sockets using connectionless communication styles.
2618
* Sending Datagrams:: Sending packets on a datagram socket.
2619
* Receiving Datagrams:: Receiving packets on a datagram socket.
2620
* Datagram Example:: An example program: packets sent over a
2621
datagram socket in the local namespace.
2622
* Example Receiver:: Another program, that receives those packets.
2625
@node Sending Datagrams
2626
@subsection Sending Datagrams
2627
@cindex sending a datagram
2628
@cindex transmitting datagrams
2629
@cindex datagrams, transmitting
2631
@pindex sys/socket.h
2632
The normal way of sending data on a datagram socket is by using the
2633
@code{sendto} function, declared in @file{sys/socket.h}.
2635
You can call @code{connect} on a datagram socket, but this only
2636
specifies a default destination for further data transmission on the
2637
socket. When a socket has a default destination you can use
2638
@code{send} (@pxref{Sending Data}) or even @code{write} (@pxref{I/O
2639
Primitives}) to send a packet there. You can cancel the default
2640
destination by calling @code{connect} using an address format of
2641
@code{AF_UNSPEC} in the @var{addr} argument. @xref{Connecting}, for
2642
more information about the @code{connect} function.
2644
@comment sys/socket.h
2646
@deftypefun int sendto (int @var{socket}, void *@var{buffer}. size_t @var{size}, int @var{flags}, struct sockaddr *@var{addr}, socklen_t @var{length})
2647
The @code{sendto} function transmits the data in the @var{buffer}
2648
through the socket @var{socket} to the destination address specified
2649
by the @var{addr} and @var{length} arguments. The @var{size} argument
2650
specifies the number of bytes to be transmitted.
2652
The @var{flags} are interpreted the same way as for @code{send}; see
2653
@ref{Socket Data Options}.
2655
The return value and error conditions are also the same as for
2656
@code{send}, but you cannot rely on the system to detect errors and
2657
report them; the most common error is that the packet is lost or there
2658
is no-one at the specified address to receive it, and the operating
2659
system on your machine usually does not know this.
2661
It is also possible for one call to @code{sendto} to report an error
2662
owing to a problem related to a previous call.
2664
This function is defined as a cancellation point in multi-threaded
2665
programs, so one has to be prepared for this and make sure that
2666
allocated resources (like memory, files descriptors, semaphores or
2667
whatever) are freed even if the thread is canceled.
2668
@c @xref{pthread_cleanup_push}, for a method how to do this.
2671
@node Receiving Datagrams
2672
@subsection Receiving Datagrams
2673
@cindex receiving datagrams
2675
The @code{recvfrom} function reads a packet from a datagram socket and
2676
also tells you where it was sent from. This function is declared in
2677
@file{sys/socket.h}.
2679
@comment sys/socket.h
2681
@deftypefun int recvfrom (int @var{socket}, void *@var{buffer}, size_t @var{size}, int @var{flags}, struct sockaddr *@var{addr}, socklen_t *@var{length-ptr})
2682
The @code{recvfrom} function reads one packet from the socket
2683
@var{socket} into the buffer @var{buffer}. The @var{size} argument
2684
specifies the maximum number of bytes to be read.
2686
If the packet is longer than @var{size} bytes, then you get the first
2687
@var{size} bytes of the packet and the rest of the packet is lost.
2688
There's no way to read the rest of the packet. Thus, when you use a
2689
packet protocol, you must always know how long a packet to expect.
2691
The @var{addr} and @var{length-ptr} arguments are used to return the
2692
address where the packet came from. @xref{Socket Addresses}. For a
2693
socket in the local domain the address information won't be meaningful,
2694
since you can't read the address of such a socket (@pxref{Local
2695
Namespace}). You can specify a null pointer as the @var{addr} argument
2696
if you are not interested in this information.
2698
The @var{flags} are interpreted the same way as for @code{recv}
2699
(@pxref{Socket Data Options}). The return value and error conditions
2700
are also the same as for @code{recv}.
2702
This function is defined as a cancellation point in multi-threaded
2703
programs, so one has to be prepared for this and make sure that
2704
allocated resources (like memory, files descriptors, semaphores or
2705
whatever) are freed even if the thread is canceled.
2706
@c @xref{pthread_cleanup_push}, for a method how to do this.
2709
You can use plain @code{recv} (@pxref{Receiving Data}) instead of
2710
@code{recvfrom} if you don't need to find out who sent the packet
2711
(either because you know where it should come from or because you
2712
treat all possible senders alike). Even @code{read} can be used if
2713
you don't want to specify @var{flags} (@pxref{I/O Primitives}).
2716
@c sendmsg and recvmsg are like readv and writev in that they
2717
@c use a series of buffers. It's not clear this is worth
2718
@c supporting or that we support them.
2719
@c !!! they can do more; it is hairy
2721
@comment sys/socket.h
2723
@deftp {Data Type} {struct msghdr}
2726
@comment sys/socket.h
2728
@deftypefun int sendmsg (int @var{socket}, const struct msghdr *@var{message}, int @var{flags})
2730
This function is defined as a cancellation point in multi-threaded
2731
programs, so one has to be prepared for this and make sure that
2732
allocated resources (like memory, files descriptors, semaphores or
2733
whatever) are freed even if the thread is cancel.
2734
@c @xref{pthread_cleanup_push}, for a method how to do this.
2737
@comment sys/socket.h
2739
@deftypefun int recvmsg (int @var{socket}, struct msghdr *@var{message}, int @var{flags})
2741
This function is defined as a cancellation point in multi-threaded
2742
programs, so one has to be prepared for this and make sure that
2743
allocated resources (like memory, files descriptors, semaphores or
2744
whatever) are freed even if the thread is canceled.
2745
@c @xref{pthread_cleanup_push}, for a method how to do this.
2749
@node Datagram Example
2750
@subsection Datagram Socket Example
2752
Here is a set of example programs that send messages over a datagram
2753
stream in the local namespace. Both the client and server programs use
2754
the @code{make_named_socket} function that was presented in @ref{Local
2755
Socket Example}, to create and name their sockets.
2757
First, here is the server program. It sits in a loop waiting for
2758
messages to arrive, bouncing each message back to the sender.
2759
Obviously this isn't a particularly useful program, but it does show
2760
the general ideas involved.
2763
@include filesrv.c.texi
2766
@node Example Receiver
2767
@subsection Example of Reading Datagrams
2769
Here is the client program corresponding to the server above.
2771
It sends a datagram to the server and then waits for a reply. Notice
2772
that the socket for the client (as well as for the server) in this
2773
example has to be given a name. This is so that the server can direct
2774
a message back to the client. Since the socket has no associated
2775
connection state, the only way the server can do this is by
2776
referencing the name of the client.
2779
@include filecli.c.texi
2782
Keep in mind that datagram socket communications are unreliable. In
2783
this example, the client program waits indefinitely if the message
2784
never reaches the server or if the server's response never comes
2785
back. It's up to the user running the program to kill and restart
2786
it if desired. A more automatic solution could be to use
2787
@code{select} (@pxref{Waiting for I/O}) to establish a timeout period
2788
for the reply, and in case of timeout either re-send the message or
2789
shut down the socket and exit.
2792
@section The @code{inetd} Daemon
2794
We've explained above how to write a server program that does its own
2795
listening. Such a server must already be running in order for anyone
2798
Another way to provide a service on an Internet port is to let the daemon
2799
program @code{inetd} do the listening. @code{inetd} is a program that
2800
runs all the time and waits (using @code{select}) for messages on a
2801
specified set of ports. When it receives a message, it accepts the
2802
connection (if the socket style calls for connections) and then forks a
2803
child process to run the corresponding server program. You specify the
2804
ports and their programs in the file @file{/etc/inetd.conf}.
2808
* Configuring Inetd::
2812
@subsection @code{inetd} Servers
2814
Writing a server program to be run by @code{inetd} is very simple. Each time
2815
someone requests a connection to the appropriate port, a new server
2816
process starts. The connection already exists at this time; the
2817
socket is available as the standard input descriptor and as the
2818
standard output descriptor (descriptors 0 and 1) in the server
2819
process. Thus the server program can begin reading and writing data
2820
right away. Often the program needs only the ordinary I/O facilities;
2821
in fact, a general-purpose filter program that knows nothing about
2822
sockets can work as a byte stream server run by @code{inetd}.
2824
You can also use @code{inetd} for servers that use connectionless
2825
communication styles. For these servers, @code{inetd} does not try to accept
2826
a connection since no connection is possible. It just starts the
2827
server program, which can read the incoming datagram packet from
2828
descriptor 0. The server program can handle one request and then
2829
exit, or you can choose to write it to keep reading more requests
2830
until no more arrive, and then exit. You must specify which of these
2831
two techniques the server uses when you configure @code{inetd}.
2833
@node Configuring Inetd
2834
@subsection Configuring @code{inetd}
2836
The file @file{/etc/inetd.conf} tells @code{inetd} which ports to listen to
2837
and what server programs to run for them. Normally each entry in the
2838
file is one line, but you can split it onto multiple lines provided
2839
all but the first line of the entry start with whitespace. Lines that
2840
start with @samp{#} are comments.
2842
Here are two standard entries in @file{/etc/inetd.conf}:
2845
ftp stream tcp nowait root /libexec/ftpd ftpd
2846
talk dgram udp wait root /libexec/talkd talkd
2849
An entry has this format:
2852
@var{service} @var{style} @var{protocol} @var{wait} @var{username} @var{program} @var{arguments}
2855
The @var{service} field says which service this program provides. It
2856
should be the name of a service defined in @file{/etc/services}.
2857
@code{inetd} uses @var{service} to decide which port to listen on for
2860
The fields @var{style} and @var{protocol} specify the communication
2861
style and the protocol to use for the listening socket. The style
2862
should be the name of a communication style, converted to lower case
2863
and with @samp{SOCK_} deleted---for example, @samp{stream} or
2864
@samp{dgram}. @var{protocol} should be one of the protocols listed in
2865
@file{/etc/protocols}. The typical protocol names are @samp{tcp} for
2866
byte stream connections and @samp{udp} for unreliable datagrams.
2868
The @var{wait} field should be either @samp{wait} or @samp{nowait}.
2869
Use @samp{wait} if @var{style} is a connectionless style and the
2870
server, once started, handles multiple requests as they come in.
2871
Use @samp{nowait} if @code{inetd} should start a new process for each message
2872
or request that comes in. If @var{style} uses connections, then
2873
@var{wait} @strong{must} be @samp{nowait}.
2875
@var{user} is the user name that the server should run as. @code{inetd} runs
2876
as root, so it can set the user ID of its children arbitrarily. It's
2877
best to avoid using @samp{root} for @var{user} if you can; but some
2878
servers, such as Telnet and FTP, read a username and password
2879
themselves. These servers need to be root initially so they can log
2880
in as commanded by the data coming over the network.
2882
@var{program} together with @var{arguments} specifies the command to
2883
run to start the server. @var{program} should be an absolute file
2884
name specifying the executable file to run. @var{arguments} consists
2885
of any number of whitespace-separated words, which become the
2886
command-line arguments of @var{program}. The first word in
2887
@var{arguments} is argument zero, which should by convention be the
2888
program name itself (sans directories).
2890
If you edit @file{/etc/inetd.conf}, you can tell @code{inetd} to reread the
2891
file and obey its new contents by sending the @code{inetd} process the
2892
@code{SIGHUP} signal. You'll have to use @code{ps} to determine the
2893
process ID of the @code{inetd} process as it is not fixed.
2895
@c !!! could document /etc/inetd.sec
2897
@node Socket Options
2898
@section Socket Options
2899
@cindex socket options
2901
This section describes how to read or set various options that modify
2902
the behavior of sockets and their underlying communications protocols.
2904
@cindex level, for socket options
2905
@cindex socket option level
2906
When you are manipulating a socket option, you must specify which
2907
@dfn{level} the option pertains to. This describes whether the option
2908
applies to the socket interface, or to a lower-level communications
2912
* Socket Option Functions:: The basic functions for setting and getting
2914
* Socket-Level Options:: Details of the options at the socket level.
2917
@node Socket Option Functions
2918
@subsection Socket Option Functions
2920
@pindex sys/socket.h
2921
Here are the functions for examining and modifying socket options.
2922
They are declared in @file{sys/socket.h}.
2924
@comment sys/socket.h
2926
@deftypefun int getsockopt (int @var{socket}, int @var{level}, int @var{optname}, void *@var{optval}, socklen_t *@var{optlen-ptr})
2927
The @code{getsockopt} function gets information about the value of
2928
option @var{optname} at level @var{level} for socket @var{socket}.
2930
The option value is stored in a buffer that @var{optval} points to.
2931
Before the call, you should supply in @code{*@var{optlen-ptr}} the
2932
size of this buffer; on return, it contains the number of bytes of
2933
information actually stored in the buffer.
2935
Most options interpret the @var{optval} buffer as a single @code{int}
2938
The actual return value of @code{getsockopt} is @code{0} on success
2939
and @code{-1} on failure. The following @code{errno} error conditions
2944
The @var{socket} argument is not a valid file descriptor.
2947
The descriptor @var{socket} is not a socket.
2950
The @var{optname} doesn't make sense for the given @var{level}.
2954
@comment sys/socket.h
2956
@deftypefun int setsockopt (int @var{socket}, int @var{level}, int @var{optname}, void *@var{optval}, socklen_t @var{optlen})
2957
This function is used to set the socket option @var{optname} at level
2958
@var{level} for socket @var{socket}. The value of the option is passed
2959
in the buffer @var{optval} of size @var{optlen}.
2964
The return value and error codes for @code{setsockopt} are the same as
2965
for @code{getsockopt}.
2968
The return value and error codes for @code{setsockopt} are the same as
2969
for @code{getsockopt}.
2974
@node Socket-Level Options
2975
@subsection Socket-Level Options
2977
@comment sys/socket.h
2979
@deftypevr Constant int SOL_SOCKET
2980
Use this constant as the @var{level} argument to @code{getsockopt} or
2981
@code{setsockopt} to manipulate the socket-level options described in
2985
@pindex sys/socket.h
2987
Here is a table of socket-level option names; all are defined in the
2988
header file @file{sys/socket.h}.
2991
@comment sys/socket.h
2994
@c Extra blank line here makes the table look better.
2996
This option toggles recording of debugging information in the underlying
2997
protocol modules. The value has type @code{int}; a nonzero value means
2999
@c !!! should say how this is used
3000
@c OK, anyone who knows, please explain.
3002
@comment sys/socket.h
3005
This option controls whether @code{bind} (@pxref{Setting Address})
3006
should permit reuse of local addresses for this socket. If you enable
3007
this option, you can actually have two sockets with the same Internet
3008
port number; but the system won't allow you to use the two
3009
identically-named sockets in a way that would confuse the Internet. The
3010
reason for this option is that some higher-level Internet protocols,
3011
including FTP, require you to keep reusing the same port number.
3013
The value has type @code{int}; a nonzero value means ``yes''.
3015
@comment sys/socket.h
3018
This option controls whether the underlying protocol should
3019
periodically transmit messages on a connected socket. If the peer
3020
fails to respond to these messages, the connection is considered
3021
broken. The value has type @code{int}; a nonzero value means
3024
@comment sys/socket.h
3027
This option controls whether outgoing messages bypass the normal
3028
message routing facilities. If set, messages are sent directly to the
3029
network interface instead. The value has type @code{int}; a nonzero
3030
value means ``yes''.
3032
@comment sys/socket.h
3035
This option specifies what should happen when the socket of a type
3036
that promises reliable delivery still has untransmitted messages when
3037
it is closed; see @ref{Closing a Socket}. The value has type
3038
@code{struct linger}.
3040
@comment sys/socket.h
3042
@deftp {Data Type} {struct linger}
3043
This structure type has the following members:
3047
This field is interpreted as a boolean. If nonzero, @code{close}
3048
blocks until the data are transmitted or the timeout period has expired.
3051
This specifies the timeout period, in seconds.
3055
@comment sys/socket.h
3058
This option controls whether datagrams may be broadcast from the socket.
3059
The value has type @code{int}; a nonzero value means ``yes''.
3061
@comment sys/socket.h
3064
If this option is set, out-of-band data received on the socket is
3065
placed in the normal input queue. This permits it to be read using
3066
@code{read} or @code{recv} without specifying the @code{MSG_OOB}
3067
flag. @xref{Out-of-Band Data}. The value has type @code{int}; a
3068
nonzero value means ``yes''.
3070
@comment sys/socket.h
3073
This option gets or sets the size of the output buffer. The value is a
3074
@code{size_t}, which is the size in bytes.
3076
@comment sys/socket.h
3079
This option gets or sets the size of the input buffer. The value is a
3080
@code{size_t}, which is the size in bytes.
3082
@comment sys/socket.h
3085
@comment sys/socket.h
3088
This option can be used with @code{getsockopt} only. It is used to
3089
get the socket's communication style. @code{SO_TYPE} is the
3090
historical name, and @code{SO_STYLE} is the preferred name in GNU.
3091
The value has type @code{int} and its value designates a communication
3092
style; see @ref{Communication Styles}.
3094
@comment sys/socket.h
3097
@c Extra blank line here makes the table look better.
3099
This option can be used with @code{getsockopt} only. It is used to reset
3100
the error status of the socket. The value is an @code{int}, which represents
3101
the previous error status.
3102
@c !!! what is "socket error status"? this is never defined.
3105
@node Networks Database
3106
@section Networks Database
3107
@cindex networks database
3108
@cindex converting network number to network name
3109
@cindex converting network name to network number
3111
@pindex /etc/networks
3113
Many systems come with a database that records a list of networks known
3114
to the system developer. This is usually kept either in the file
3115
@file{/etc/networks} or in an equivalent from a name server. This data
3116
base is useful for routing programs such as @code{route}, but it is not
3117
useful for programs that simply communicate over the network. We
3118
provide functions to access this database, which are declared in
3123
@deftp {Data Type} {struct netent}
3124
This data type is used to represent information about entries in the
3125
networks database. It has the following members:
3129
This is the ``official'' name of the network.
3131
@item char **n_aliases
3132
These are alternative names for the network, represented as a vector
3133
of strings. A null pointer terminates the array.
3135
@item int n_addrtype
3136
This is the type of the network number; this is always equal to
3137
@code{AF_INET} for Internet networks.
3139
@item unsigned long int n_net
3140
This is the network number. Network numbers are returned in host
3141
byte order; see @ref{Byte Order}.
3145
Use the @code{getnetbyname} or @code{getnetbyaddr} functions to search
3146
the networks database for information about a specific network. The
3147
information is returned in a statically-allocated structure; you must
3148
copy the information if you need to save it.
3152
@deftypefun {struct netent *} getnetbyname (const char *@var{name})
3153
The @code{getnetbyname} function returns information about the network
3154
named @var{name}. It returns a null pointer if there is no such
3160
@deftypefun {struct netent *} getnetbyaddr (unsigned long int @var{net}, int @var{type})
3161
The @code{getnetbyaddr} function returns information about the network
3162
of type @var{type} with number @var{net}. You should specify a value of
3163
@code{AF_INET} for the @var{type} argument for Internet networks.
3165
@code{getnetbyaddr} returns a null pointer if there is no such
3169
You can also scan the networks database using @code{setnetent},
3170
@code{getnetent} and @code{endnetent}. Be careful when using these
3171
functions because they are not reentrant.
3175
@deftypefun void setnetent (int @var{stayopen})
3176
This function opens and rewinds the networks database.
3178
If the @var{stayopen} argument is nonzero, this sets a flag so that
3179
subsequent calls to @code{getnetbyname} or @code{getnetbyaddr} will
3180
not close the database (as they usually would). This makes for more
3181
efficiency if you call those functions several times, by avoiding
3182
reopening the database for each call.
3187
@deftypefun {struct netent *} getnetent (void)
3188
This function returns the next entry in the networks database. It
3189
returns a null pointer if there are no more entries.
3194
@deftypefun void endnetent (void)
3195
This function closes the networks database.