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.
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
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 termi-
nated, if MSG_PEEK was specified, or if an error is pending for
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
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:
End-of-record was received (if supported by the protocol).
Out-of-band data was received.
Normal data was truncated.
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. Oth-
erwise, -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.
A connection was forcibly closed by a peer.
EINTR This function was interrupted by a signal before any data was
The socket argument does not refer to a socket.
The specified flags are not supported for this socket type.
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
Insufficient resources were available in the system to perform
ENOMEM Insufficient memory was available to fulfill the request.
The following sections are informative.
The select() and poll() functions can be used to determine when data is
available to be received.
poll() , recv() , recvfrom() , select() , send() , sendmsg() , sendto()
, shutdown() , socket() , the Base Definitions volume of
IEEE Std 1003.1-2001, <sys/socket.h>
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 .
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