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 and msg_namelen members specify the  source  address
       if  the  socket  is  unconnected. If the socket is connected, 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:

       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() , recv() , recvfrom() , select() , send() , sendmsg() ,  sendto()  ,  shutdown()  ,
       socket() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/socket.h>

COPYRIGHT

       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 .