.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "RECVMSG" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" recvmsg 
.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 \- 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 .