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
The recvmsg() function takes the following arguments:
Specifies the socket file descriptor.
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.
Specifies the type of message reception. Values of this
argument are formed by logically OR'ing zero or more of the following
Requests out-of-band data. The significance
and semantics of out-of-band data are protocol-specific.
Peeks at the incoming message.
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 and msg_namelen
members specify the source address if the socket is unconnected. If the socket
is connected, 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
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:
End-of-record was received (if supported by the
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 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.
The socket argument is not a valid open file
A connection was forcibly closed by a peer.
This function was interrupted by a signal before any data
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
The msg_iovlen member of the msghdr structure
pointed to by message is less than or equal to 0, or is greater
A receive is attempted on a connection-mode socket that is
The socket argument does not refer to a socket.
The specified flags are not supported for this socket
The connection timed out during connection establishment,
or due to a transmission timeout on active connection.
The recvmsg() function may fail if:
An I/O error occurred while reading from or writing to the
Insufficient resources were available in the system to
perform the operation.
Insufficient memory was available to fulfill the request.
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