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

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

.\" recvmsg 

.SH NAME

recvmsg \- receive a message from a socket

.SH SYNOPSIS

.LP

\fB#include <sys/socket.h>

.br

.sp

ssize_t recvmsg(int\fP \fIsocket\fP\fB, struct msghdr *\fP\fImessage\fP\fB,

int\fP \fIflags\fP\fB);

.br

\fP

.SH DESCRIPTION

.LP

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

.TP 7

\fIsocket\fP

Specifies the socket file descriptor.

.TP 7

\fImessage\fP

Points to a \fBmsghdr\fP structure, containing both the buffer to

store the source address and the buffers for the incoming

message. The length and format of the address depend on the address

family of the socket. The \fImsg_flags\fP member is ignored on

input, but may contain meaningful values on output.

.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_OOB

.RS

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

data are protocol-specific.

.RE

.TP 7

MSG_PEEK

.RS

Peeks at the incoming message.

.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

.sp

.LP

The \fIrecvmsg\fP() function shall receive messages from unconnected

or connected sockets and shall return the length of the

message.

.LP

The \fIrecvmsg\fP() function shall return the total length of the

message. For message-based sockets, such as 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 buffers,

and MSG_PEEK is not set in the \fIflags\fP argument, the excess bytes

shall be discarded, and MSG_TRUNC shall be set in the

\fImsg_flags\fP member of the \fBmsghdr\fP structure. 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

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

on the socket's file descriptor, \fIrecvmsg\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,

the \fIrecvmsg\fP() function shall fail and set \fIerrno\fP to [EAGAIN]

or [EWOULDBLOCK].

.LP

In the \fBmsghdr\fP structure, the \fImsg_name\fP and \fImsg_namelen\fP

members specify the source address if the socket is

unconnected. If the socket is connected, the \fImsg_name\fP and \fImsg_namelen\fP

members shall be ignored. The \fImsg_name\fP

member may be a null pointer if no names are desired or required.

The \fImsg_iov\fP and \fImsg_iovlen\fP fields are used to

specify where the received data shall be stored. \fImsg_iov\fP points

to an array of \fBiovec\fP structures; \fImsg_iovlen\fP

shall be set to the dimension of this array. In each \fBiovec\fP structure,

the \fIiov_base\fP field specifies a storage area and

the \fIiov_len\fP field gives its size in bytes. Each storage area

indicated by \fImsg_iov\fP is filled with received data in

turn until all of the received data is stored or all of the areas

have been filled.

.LP

Upon successful completion, the \fImsg_flags\fP member of the message

header shall be the bitwise-inclusive OR of all of the

following flags that indicate conditions detected for the received

message:

.TP 7

MSG_EOR

End-of-record was received (if supported by the protocol).

.TP 7

MSG_OOB

Out-of-band data was received.

.TP 7

MSG_TRUNC

Normal data was truncated.

.TP 7

MSG_CTRUNC

Control data was truncated.

.sp

.SH RETURN VALUE

.LP

Upon successful completion, \fIrecvmsg\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, \fIrecvmsg\fP()

shall return 0. Otherwise, -1 shall be returned and

\fIerrno\fP set to indicate the error.

.SH ERRORS

.LP

The \fIrecvmsg\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 open file descriptor.

.TP 7

.B ECONNRESET

A connection was forcibly closed by a peer.

.TP 7

.B EINTR

This function was interrupted by a signal before any data was available.

.TP 7

.B EINVAL

The sum of the \fIiov_len\fP values overflows a \fBssize_t\fP, or

the MSG_OOB flag is set and no out-of-band data is

available.

.TP 7

.B EMSGSIZE

The \fImsg_iovlen\fP member of the \fBmsghdr\fP structure pointed

to by \fImessage\fP is less than or equal to 0, or is

greater than {IOV_MAX}.

.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 \fIrecvmsg\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() , \fIrecv\fP() , \fIrecvfrom\fP() ,

\fIselect\fP() , \fIsend\fP() , \fIsendmsg\fP() ,

\fIsendto\fP() , \fIshutdown\fP() , \fIsocket\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 .