recv(2)                       4 BSD                       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 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 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 non-
     blocking (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 receive call is formed by or'ing one
     or more of the values,



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recv(2)                       4 BSD                       recv(2)



          MSGPEEK  /* peek at incoming message */
          MSGOOB   /* process out-of-band data */

     The recvmsg call uses a msghdr structure to minimize the
     number of directly supplied parameters, and to receive file
     descriptors from another process (Unix domain only).  This
     structure has the following form, as defined in
     <sys/socket.h>:

     struct msghdr {
          caddrt     msgname;       /* optional address */
          int         msgnamelen;    /* size of address */
          struct iov *msgiov;        /* scatter/gather array */
          int         msgiovlen;     /* # elements in msgiov */
          caddrt     msgaccrights;  /* access rights sent/received */
          int         msgaccrightslen;
     };

     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 loca-
     tions, as described in read(2).  Access rights to be sent
     along with the message are specified in msg_accrights, which
     has length msg_accrightslen.

     RECEIVING OUT-OF-BAND DATA IN INTERNET DOMAIN

     When out-of-band data arrives at a socket, the system could
     signal a group of processes, or a specific process, or no
     process at all.

          - If the socket's process group id is positive, the
          system signals all  process in that process group.

          - If the socket's process group id is negative, the
          system only signals the process whose process id is the
          absolute value of the socket's process group id.

          - If the socket's process group id is 0 (the default
          value), the system does not signal any process.

     To assign a socket process group id, use the ioctl(2) system
     call with the command SIOCSPGRP.

     RECEIVING FILE DESCRIPTORS IN CX/UX DOMAIN

     In Unix domain, process A can open a set of files, then send
     these file descriptors to process B. Process B then can read
     or write those files which were already opened (and perhaps
     accessible only) by A.




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recv(2)                       4 BSD                       recv(2)



     The active process stores the file descriptors in the field
     msg_accrights of the structure msghdr, and use the system
     call sendmsg(2) to send the descriptors. The passive process
     then can receive the descriptors via the system call
     recvmsg(2); the descriptors are returned in the field
     msg_accrights of msghdr.

NOTE
     The ability to send and receive file descriptors is not
     available in CX/SX when configured to B1 level security.

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 avail-
                         able for the receive.

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

NOTE
     These functions are defined in the 88open Binary and Object
     Compatibility Standards' Networking Supplements (BCSNS and
     OCSNS) for use in BCSNS/OCSNS compliant applications on
     Series.  OCSNS-defined functions may be accessed by passing
     OCS options to cc(1) and ld(1).

SEE ALSO
     read(2), send(2), socket(2). See also bind(2) for address
     formats in different communication domains.












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