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.TH RECVMSG "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"

.SH PROLOG

This manual page is part of the POSIX Programmer's Manual.

The Linux implementation of this interface may differ (consult

the corresponding Linux manual page for details of Linux behavior),

or the interface may not be implemented on Linux.

.SH NAME

recvmsg

\(em receive a message from a socket

.SH SYNOPSIS

.LP

.nf

#include <sys/socket.h>

.P

ssize_t recvmsg(int \fIsocket\fP, struct msghdr *\fImessage\fP, int \fIflags\fP);

.fi

.SH DESCRIPTION

The

\fIrecvmsg\fR()

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.

.P

The

\fIrecvmsg\fR()

function takes the following arguments:

.IP "\fIsocket\fR" 12

Specifies the socket file descriptor.

.IP "\fImessage\fR" 12

Points to a

.BR msghdr

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

.IR msg_flags

member is ignored on input, but may contain meaningful values on

output.

.IP "\fIflags\fR" 12

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

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

.RS 12 

.IP MSG_OOB 12

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

out-of-band data are protocol-specific.

.IP MSG_PEEK 12

Peeks at the incoming message.

.IP MSG_WAITALL 12

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

.P

The

\fIrecvmsg\fR()

function shall receive messages from unconnected or connected

sockets and shall return the length of the message.

.P

The

\fIrecvmsg\fR()

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

.IR flags

argument, the excess bytes shall be discarded, and MSG_TRUNC shall be

set in the

.IR msg_flags

member of the

.BR msghdr

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.

.P

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

the end of the first message.

.P

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

the socket's file descriptor,

\fIrecvmsg\fR()

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\fR()

function shall fail and set

.IR errno

to

.BR [EAGAIN] 

or

.BR [EWOULDBLOCK] .

.P

In the

.BR msghdr

structure, the

.IR msg_name

member may be a null pointer if the source address is not required.

Otherwise, if the socket is unconnected, the

.IR msg_name

member points to a

.BR sockaddr

structure in which the source address is to be stored, and the

.IR msg_namelen

member on input specifies the length of the supplied

.BR sockaddr

structure and on output specifies the length of the stored address.

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

supplied

.BR sockaddr

structure, the stored address shall be truncated. If the socket is

connected, the

.IR msg_name

and

.IR msg_namelen

members shall be ignored. The

.IR msg_iov

and

.IR msg_iovlen

fields are used to specify where the received data shall be stored.

The

.IR msg_iov

member points to an array of

.BR iovec

structures; the

.IR msg_iovlen

member shall be set to the dimension of this array. In each

.BR iovec

structure, the

.IR iov_base

field specifies a storage area and the

.IR iov_len

field gives its size in bytes. Each storage area indicated by

.IR msg_iov

is filled with received data in turn until all of the received data is

stored or all of the areas have been filled.

.P

Upon successful completion, the

.IR msg_flags

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:

.IP MSG_EOR 12

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

.IP MSG_OOB 12

Out-of-band data was received.

.IP MSG_TRUNC 12

Normal data was truncated.

.IP MSG_CTRUNC 12

Control data was truncated.

.SH "RETURN VALUE"

Upon successful completion,

\fIrecvmsg\fR()

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\fR()

shall return 0. Otherwise, \(mi1 shall be returned and

.IR errno

set to indicate the error.

.SH ERRORS

The

\fIrecvmsg\fR()

function shall fail if:

.TP

.BR EAGAIN " or " EWOULDBLOCK

.br

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

.BR EBADF

The

.IR socket

argument is not a valid open file descriptor.

.TP

.BR ECONNRESET

A connection was forcibly closed by a peer.

.TP

.BR EINTR

This function was interrupted by a signal before any data was

available.

.TP

.BR EINVAL

The sum of the

.IR iov_len

values overflows a

.BR ssize_t ,

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

.TP

.BR EMSGSIZE

The

.IR msg_iovlen

member of the

.BR msghdr

structure pointed to by

.IR message

is less than or equal to 0, or is greater than

{IOV_MAX}.

.TP

.BR ENOTCONN

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

connected.

.TP

.BR ENOTSOCK

The

.IR socket

argument does not refer to a socket.

.TP

.BR EOPNOTSUPP

The specified flags are not supported for this socket type.

.TP

.BR ETIMEDOUT

The connection timed out during connection establishment, or due to a

transmission timeout on active connection.

.P

The

\fIrecvmsg\fR()

function may fail if:

.TP

.BR EIO

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

system.

.TP

.BR ENOBUFS

Insufficient resources were available in the system to perform the

operation.

.TP

.BR ENOMEM

Insufficient memory was available to fulfill the request.

.LP

.IR "The following sections are informative."

.SH "EXAMPLES"

None.

.SH "APPLICATION USAGE"

The

\fIselect\fR()

and

\fIpoll\fR()

functions can be used to determine when data is available to be

received.

.SH "RATIONALE"

None.

.SH "FUTURE DIRECTIONS"

None.

.SH "SEE ALSO"

.IR "\fIpoll\fR\^(\|)",

.IR "\fIpselect\fR\^(\|)",

.IR "\fIrecv\fR\^(\|)",

.IR "\fIrecvfrom\fR\^(\|)",

.IR "\fIsend\fR\^(\|)",

.IR "\fIsendmsg\fR\^(\|)",

.IR "\fIsendto\fR\^(\|)",

.IR "\fIshutdown\fR\^(\|)",

.IR "\fIsocket\fR\^(\|)"

.P

The Base Definitions volume of POSIX.1\(hy2008,

.IR "\fB<sys_socket.h>\fP"

.SH COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form

from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology

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

Specifications Issue 7, Copyright (C) 2013 by the Institute of

Electrical and Electronics Engineers, Inc and The Open Group.

(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

Any typographical or formatting errors that appear

in this page are most likely

to have been introduced during the conversion of the source files to

man page format. To report such errors, see

https://www.kernel.org/doc/man-pages/reporting_bugs.html .