RECVMSG(P) POSIX Programmer's Manual RECVMSG(P)NAMErecvmsg - receive a message from a socket
SYNOPSIS
#include <sys/socket.h>
ssize_t recvmsg(int socket, struct msghdr *message, int flags);
DESCRIPTION
The recvmsg() 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.
The recvmsg() function takes the following arguments:
socket Specifies the socket file descriptor.
message
Points to a msghdr structure, containing both the buffer to
store the source address and the buffers for the incoming mes‐
sage. The length and format of the address depend on the address
family of the socket. The msg_flags member is ignored on input,
but may contain meaningful values on output.
flags Specifies the type of message reception. Values of this argument
are formed by logically OR'ing zero or more of the following
values:
MSG_OOB
Requests out-of-band data. The significance and semantics of
out-of-band data are protocol-specific.
MSG_PEEK
Peeks at the incoming message.
MSG_WAITALL
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 termi‐
nated, if MSG_PEEK was specified, or if an error is pending for
the socket.
The recvmsg() function shall receive messages from unconnected or con‐
nected sockets and shall return the length of the message.
The recvmsg() 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
flags argument, the excess bytes shall be discarded, and MSG_TRUNC
shall be set in the msg_flags member of the 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.
If the MSG_WAITALL flag is not set, data shall be returned only up to
the end of the first message.
If no messages are available at the socket and O_NONBLOCK is not set on
the socket's file descriptor, recvmsg() 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 recvmsg() function shall fail
and set errno to [EAGAIN] or [EWOULDBLOCK].
In the msghdr structure, the msg_name and msg_namelen members specify
the source address if the socket is unconnected. If the socket is con‐
nected, the msg_name and msg_namelen members shall be ignored. The
msg_name member may be a null pointer if no names are desired or
required. The msg_iov and msg_iovlen fields are used to specify where
the received data shall be stored. msg_iov points to an array of iovec
structures; msg_iovlen shall be set to the dimension of this array. In
each iovec structure, the iov_base field specifies a storage area and
the iov_len field gives its size in bytes. Each storage area indicated
by 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.
Upon successful completion, the 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:
MSG_EOR
End-of-record was received (if supported by the protocol).
MSG_OOB
Out-of-band data was received.
MSG_TRUNC
Normal data was truncated.
MSG_CTRUNC
Control data was truncated.
RETURN VALUE
Upon successful completion, recvmsg() 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, recvmsg() shall return 0. Oth‐
erwise, -1 shall be returned and errno set to indicate the error.
ERRORS
The recvmsg() function shall fail if:
EAGAIN or EWOULDBLOCK
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.
EBADF The socket argument is not a valid open file descriptor.
ECONNRESET
A connection was forcibly closed by a peer.
EINTR This function was interrupted by a signal before any data was
available.
EINVAL The sum of the iov_len values overflows a ssize_t, or the
MSG_OOB flag is set and no out-of-band data is available.
EMSGSIZE
The msg_iovlen member of the msghdr structure pointed to by mes‐
sage is less than or equal to 0, or is greater than {IOV_MAX}.
ENOTCONN
A receive is attempted on a connection-mode socket that is not
connected.
ENOTSOCK
The socket argument does not refer to a socket.
EOPNOTSUPP
The specified flags are not supported for this socket type.
ETIMEDOUT
The connection timed out during connection establishment, or due
to a transmission timeout on active connection.
The recvmsg() function may fail if:
EIO An I/O error occurred while reading from or writing to the file
system.
ENOBUFS
Insufficient resources were available in the system to perform
the operation.
ENOMEM Insufficient memory was available to fulfill the request.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
The select() and poll() functions can be used to determine when data is
available to be received.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSOpoll() , recv() , recvfrom() , select() , send() , sendmsg() , sendto()
, shutdown() , socket() , the Base Definitions volume of
IEEE Std 1003.1-2001, <sys/socket.h>
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 .
IEEE/The Open Group 2003 RECVMSG(P)
