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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.
NAME
recvmsg — 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 message. 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 terminated, if MSG_PEEK was specified, or if an error is pending
for the socket.
The recvmsg() function shall receive messages from unconnected or connected 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 member may be a null pointer if the source address is not required.
Otherwise, if the socket is unconnected, the msg_name member points to a sockaddr structure in which the
source address is to be stored, and the msg_namelen member on input specifies the length of the supplied
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 sockaddr structure, the stored address shall be
truncated. If the socket is connected, the msg_name and msg_namelen members shall be ignored. The msg_iov
and msg_iovlen fields are used to specify where the received data shall be stored. The msg_iov member
points to an array of iovec structures; the msg_iovlen member 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.
Otherwise, -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 message 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 ALSO
poll(), pselect(), recv(), recvfrom(), send(), sendmsg(), sendto(), shutdown(), socket()
The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
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 .
IEEE/The Open Group 2017 RECVMSG(3POSIX)