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PROLOG

       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

       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.1‐2008, <sys_socket.h>

COPYRIGHT

       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 .