4.3BSD - /usr/man/cat2/recvmsg.0

RECV(2)                                                                RECV(2)


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

SYNOPSIS
       #include <sys/types.h>
       #include <sys/socket.h>

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

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

       cc = recvmsg(s, msg, flags)
       int cc, s;
       struct msghdr msg[];
       int flags;

DESCRIPTION
       Recv, recvfrom, and recvmsg are used to receive messages from a socket.

       The recv call is normally used only on a  connected  socket  (see  con���
       nect(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 non-zero, 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 in cc.  If a message is  too  long  to  fit  in  the  supplied
       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 nonblocking (see ioctl(2)) in
       which case a cc of -1 is returned with the external variable errno  set
       to EWOULDBLOCK.

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

       The  flags  argument  to a recv call is formed by or’ing one or more of
       the values,

              #define  MSG_OOB         0x1    /* process out-of-band data */
              #define  MSG_PEEK        0x2    /* peek at incoming message */

       The recvmsg call uses a msghdr structure  to  minimize  the  number  of
       directly  supplied  parameters.  This structure has the following form,
       as defined in <sys/socket.h>:

              struct msghdr {
                   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(2).  A buffer to receive
       any  access  rights  sent  along  with  the  message  is  specified  in
       msg_accrights,  which  has  length msg_accrightslen.  Access rights are
       currently limited to file descriptors, which each occupy the size of an
       int.

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

ERRORS
       The calls fail if:

       [EBADF]             The argument s is an invalid descriptor.

       [ENOTSOCK]          The argument s is not a socket.

       [EWOULDBLOCK]       The socket is marked non-blocking and  the  receive
                           operation would block.

       [EINTR]             The receive was interrupted by delivery of a signal
                           before any data was available for the receive.

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

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


4.2 Berkeley Distribution        May 23, 1986                          RECV(2)