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RECVMSG(3P) POSIX Programmer's Manual RECVMSG(3P)
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.
recvmsg — receive a message from a socket
#include <sys/socket.h>
ssize_t recvmsg(int socket, struct msghdr *message, int flags);
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.
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.
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.
None.
The select() and poll() functions can be used to determine when data
is available to be received.
None.
None.
poll(3p), pselect(3p), recv(3p), recvfrom(3p), send(3p), sendmsg(3p),
sendto(3p), shutdown(3p), socket(3p)
The Base Definitions volume of POSIX.1‐2008, sys_socket.h(0p)
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 .
IEEE/The Open Group 2013 RECVMSG(3P)
Pages that refer to this page: sys_socket.h(0p), pselect(3p), recv(3p), recvfrom(3p), send(3p), sendmsg(3p), sendto(3p), shutdown(3p), sockatmark(3p), socket(3p)