recvmsg(3p) — Linux manual page


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

PROLOG         top

       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         top

       recvmsg — receive a message from a socket

SYNOPSIS         top

       #include <sys/socket.h>

       ssize_t recvmsg(int socket, struct msghdr *message, int flags);

DESCRIPTION         top

       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

       MSG_OOB     Out-of-band data was received.

       MSG_TRUNC   Normal data was truncated.

       MSG_CTRUNC  Control data was truncated.

RETURN VALUE         top

       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         top

       The recvmsg() function shall fail if:

              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 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.

              The msg_iovlen member of the msghdr structure pointed to
              by message is less than or equal to 0, or is greater than

              A receive is attempted on a connection-mode socket that is
              not connected.

              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:

       EIO    An I/O error occurred while reading from or writing to the
              file system.

              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         top



       The select() and poll() functions can be used to determine when
       data is available to be received.

RATIONALE         top




SEE ALSO         top

       poll(3p), pselect(3p), recv(3p), recvfrom(3p), send(3p),
       sendmsg(3p), sendto(3p), shutdown(3p), socket(3p)

       The Base Definitions volume of POSIX.1‐2017, sys_socket.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                       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)