RECVMSG(3)                 POSIX Programmer's Manual                RECVMSG(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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(3)
