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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 

.TH "RECVFROM" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"

.\" recvfrom 

.SH NAME

recvfrom \- receive a message from a socket

.SH SYNOPSIS

.LP

\fB#include <sys/socket.h>

.br

.sp

ssize_t recvfrom(int\fP \fIsocket\fP\fB, void *restrict\fP \fIbuffer\fP\fB,

size_t\fP \fIlength\fP\fB,

.br

\ \ \ \ \ \  int\fP \fIflags\fP\fB, struct sockaddr *restrict\fP \fIaddress\fP\fB,

.br

\ \ \ \ \ \  socklen_t *restrict\fP \fIaddress_len\fP\fB);

.br

\fP

.SH DESCRIPTION

.LP

The \fIrecvfrom\fP() function shall receive a message from a connection-mode

or connectionless-mode socket. It is normally used

with connectionless-mode sockets because it permits the application

to retrieve the source address of received data.

.LP

The \fIrecvfrom\fP() function takes the following arguments:

.TP 7

\fIsocket\fP

Specifies the socket file descriptor.

.TP 7

\fIbuffer\fP

Points to the buffer where the message should be stored.

.TP 7

\fIlength\fP

Specifies the length in bytes of the buffer pointed to by the \fIbuffer\fP

argument.

.TP 7

\fIflags\fP

Specifies the type of message reception. Values of this argument are

formed by logically OR'ing zero or more of the following

values: 

.TP 7

MSG_PEEK

.RS

Peeks at an incoming message. The data is treated as unread and the

next \fIrecvfrom\fP() or similar function shall still

return this data.

.RE

.TP 7

MSG_OOB

.RS

Requests out-of-band data. The significance and semantics of out-of-band

data are protocol-specific.

.RE

.TP 7

MSG_WAITALL

.RS

On SOCK_STREAM sockets this requests that the function block until

the full amount of data can be returned. The function may

return the smaller amount of data if the socket is a message-based

socket, if a signal is caught, if the connection is terminated,

if MSG_PEEK was specified, or if an error is pending for the socket.

.RE

.sp

.TP 7

\fIaddress\fP

A null pointer, or points to a \fBsockaddr\fP structure in which the

sending address is to be stored. The length and format of

the address depend on the address family of the socket.

.TP 7

\fIaddress_len\fP

Specifies the length of the \fBsockaddr\fP structure pointed to by

the \fIaddress\fP argument.

.sp

.LP

The \fIrecvfrom\fP() function shall return the length of the message

written to the buffer pointed to by the \fIbuffer\fP

argument. For message-based sockets, such as   \ SOCK_RAW,  SOCK_DGRAM,

and SOCK_SEQPACKET, the entire message shall be read in a single operation.

If a message is too long to fit

in the supplied buffer, and MSG_PEEK is not set in the \fIflags\fP

argument, the excess bytes shall be discarded. For stream-based

sockets, such as SOCK_STREAM, message boundaries shall be ignored.

In this case, data shall be returned to the user as soon as it

becomes available, and no data shall be discarded.

.LP

If the MSG_WAITALL flag is not set, data shall be returned only up

to the end of the first message.

.LP

Not all protocols provide the source address for messages. If the

\fIaddress\fP argument is not a null pointer and the protocol

provides the source address of messages, the source address of the

received message shall be stored in the \fBsockaddr\fP

structure pointed to by the \fIaddress\fP argument, and the length

of this address shall be stored in the object pointed to by the

\fIaddress_len\fP argument.

.LP

If the actual length of the address is greater than the length of

the supplied \fBsockaddr\fP structure, the stored address

shall be truncated.

.LP

If the \fIaddress\fP argument is not a null pointer and the protocol

does not provide the source address of messages, the value

stored in the object pointed to by \fIaddress\fP is unspecified.

.LP

If no messages are available at the socket and O_NONBLOCK is not set

on the socket's file descriptor, \fIrecvfrom\fP() shall

block until a message arrives. If no messages are available at the

socket and O_NONBLOCK is set on the socket's file descriptor,

\fIrecvfrom\fP() shall fail and set \fIerrno\fP to [EAGAIN] or [EWOULDBLOCK].

.SH RETURN VALUE

.LP

Upon successful completion, \fIrecvfrom\fP() shall return the length

of the message in bytes. If no messages are available to

be received and the peer has performed an orderly shutdown, \fIrecvfrom\fP()

shall return 0. Otherwise, the function shall return

-1 and set \fIerrno\fP to indicate the error.

.SH ERRORS

.LP

The \fIrecvfrom\fP() function shall fail if:

.TP 7

.B EAGAIN \fRor\fP EWOULDBLOCK

.sp

The socket's file descriptor is marked O_NONBLOCK and no data is waiting

to be received; or MSG_OOB is set and no out-of-band data

is available and either the socket's file descriptor is marked O_NONBLOCK

or the socket does not support blocking to await

out-of-band data.

.TP 7

.B EBADF

The \fIsocket\fP argument is not a valid file descriptor.

.TP 7

.B ECONNRESET

A connection was forcibly closed by a peer.

.TP 7

.B EINTR

A signal interrupted \fIrecvfrom\fP() before any data was available.

.TP 7

.B EINVAL

The MSG_OOB flag is set and no out-of-band data is available.

.TP 7

.B ENOTCONN

A receive is attempted on a connection-mode socket that is not connected.

.TP 7

.B ENOTSOCK

The \fIsocket\fP argument does not refer to a socket.

.TP 7

.B EOPNOTSUPP

The specified flags are not supported for this socket type.

.TP 7

.B ETIMEDOUT

The connection timed out during connection establishment, or due to

a transmission timeout on active connection.

.sp

.LP

The \fIrecvfrom\fP() function may fail if:

.TP 7

.B EIO

An I/O error occurred while reading from or writing to the file system.

.TP 7

.B ENOBUFS

Insufficient resources were available in the system to perform the

operation.

.TP 7

.B ENOMEM

Insufficient memory was available to fulfill the request.

.sp

.LP

\fIThe following sections are informative.\fP

.SH EXAMPLES

.LP

None.

.SH APPLICATION USAGE

.LP

The \fIselect\fP() and \fIpoll\fP() functions can

be used to determine when data is available to be received.

.SH RATIONALE

.LP

None.

.SH FUTURE DIRECTIONS

.LP

None.

.SH SEE ALSO

.LP

\fIpoll\fP() , \fIread\fP() , \fIrecv\fP() , \fIrecvmsg\fP() , \fIselect\fP()

, \fIsend\fP() , \fIsendmsg\fP() , \fIsendto\fP() , \fIshutdown\fP()

,

\fIsocket\fP() , \fIwrite\fP() , the Base Definitions volume of

IEEE\ Std\ 1003.1-2001, \fI<sys/socket.h>\fP

.SH COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form

from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology

-- Portable Operating System Interface (POSIX), The Open Group Base

Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of

Electrical and Electronics Engineers, Inc and The Open Group. In the

event of any discrepancy between this version and the original IEEE and

The Open Group Standard, the original IEEE and The Open Group Standard

is the referee document. The original Standard can be obtained online at

http://www.opengroup.org/unix/online.html .