PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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
                   protocol).

       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:

       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         top

       None.

APPLICATION USAGE         top

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

RATIONALE         top

       None.

FUTURE DIRECTIONS         top

       None.

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.1‐2008, sys_socket.h(0p)

COPYRIGHT         top

       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)