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RECV(2)                       System Calls Manual                      RECV(2)



NAME
       recv, recvfrom, recvmsg - receive a message from a socket

SYNOPSIS
       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/socket.h>&gt;

       int recv(s, buf, len, flags)
       int s;
       char *buf;
       int len, flags;

       int recvfrom(s, buf, len, flags, from, fromlen)
       int s;
       char *buf;
       int len, flags;
       struct sockaddr *from;
       int *fromlen;

       int recvmsg(s, msg, flags)
       int s;
       struct msghdr *msg;
       int flags;

DESCRIPTION
       s  is  a  socket  created  with  socket(2).   recv(),  recvfrom(),  and
       recvmsg() are used to receive messages from another socket.  recv() may
       be  used  only on a connected socket (see connect(2)), while recvfrom()
       and recvmsg() may be used to receive data on a socket whether it is  in
       a connected state or not.

       If  from  is  not  a NULL pointer, the source address of the message is
       filled in.  fromlen is a value-result  parameter,  initialized  to  the
       size  of  the  buffer  associated  with from, and modified on return to
       indicate the actual size of the address stored there.   The  length  of
       the  message  is returned.  If a message is too long to fit in the sup-
       plied buffer, excess bytes may be discarded depending on  the  type  of
       socket the message is received from (see socket(2)).

       If  no messages are available at the socket, the receive call waits for
       a message to arrive, unless the socket is non-blocking  (see  ioctl(2))
       in  which  case  -1 is returned with the external variable errno set to
       EWOULDBLOCK.

       The select(2) call may be used to determine when more data arrive.

       If the process calling recv(), recvfrom() or recvmsg() receives a  sig-
       nal  before any data are available, the system call is restarted unless
       the calling process explicitly set the signal to interrupt these  calls
       using  sigvec()  or sigaction() (see the discussions of SV_INTERRUPT on
       sigvec(2), and SA_INTERRUPT on sigaction(3V)).

       The flags parameter is formed by ORing one or more of the following:

       MSG_OOB        Read any  "out-of-band"  data  present  on  the  socket,
                      rather than the regular "in-band" data.

       MSG_PEEK       "Peek"  at  the data present on the socket; the data are
                      returned, but not consumed, so that a subsequent receive
                      operation will see the same data.

       The  recvmsg()  call  uses a msghdr structure to minimize the number of
       directly  supplied  parameters.    This   structure   is   defined   in
       <&lt;sys/socket.h>&gt;, and includes the following members:

              caddr_t      msg_name;         /* optional address */
              int          msg_namelen;      /* size of address */
              struct iovec *msg_iov;         /* scatter/gather array */
              int          msg_iovlen;       /* # elements in msg_iov */
              caddr_t      msg_accrights;    /* access rights sent/received */
              int          msg_accrightslen;

       Here  msg_name  and  msg_namelen specify the destination address if the
       socket is unconnected; msg_name may be given as a NULL  pointer  if  no
       names are desired or required.  The msg_iov and msg_iovlen describe the
       scatter-gather locations,  as  described  in  read(2V).   A  buffer  to
       receive  any  access rights sent along with the message is specified in
       msg_accrights, which has length msg_accrightslen.

RETURN VALUES
       These calls return the number of bytes received,  or  -1  if  an  error
       occurred.

ERRORS
       EBADF               s is an invalid descriptor.

       EFAULT              The  data were specified to be received into a non-
                           existent or protected part of the  process  address
                           space.

       EINTR               The  calling  process  received a signal before any
                           data were available to be received, and the  signal
                           was set to interrupt the system call.

       ENOTSOCK            s is a descriptor for a file, not a socket.

       EWOULDBLOCK         The socket is marked non-blocking and the requested
                           operation would block.

SEE ALSO
       connect(2), fcntl(2V), getsockopt(2),  ioctl(2),  read(2V),  select(2),
       send(2), socket(2)



                                21 January 1990                        RECV(2)