RECV(2)                   NetBSD System Calls Manual                   RECV(2)

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

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/socket.h>

     ssize_t
     recv(int s, void *buf, size_t len, int flags);

     ssize_t
     recvfrom(int s, void * restrict buf, size_t len, int flags,
         struct sockaddr * restrict from, socklen_t * restrict fromlen);

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

DESCRIPTION
     recvfrom() and recvmsg() are used to receive messages from a socket, and
     may be used to receive data on a socket whether or not it is connection-
     oriented.

     If from is non-nil, and the socket is not connection-oriented, the source
     address of the message is filled in.  fromlen is a value-result parame-
     ter, initialized to the size of the buffer associated with from, and mod-
     ified on return to indicate the actual size of the address stored there.

     The recv() call is normally used only on a connected socket (see
     connect(2)) and is identical to recvfrom() with a nil from parameter.  As
     it is redundant, it may not be supported in future releases.

     All three routines return the length of the message on successful comple-
     tion.  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 fcntl(2)) in
     which case the value -1 is returned and the external variable errno set
     to EAGAIN.  The receive calls normally return any data available, up to
     the requested amount, rather than waiting for receipt of the full amount
     requested; this behavior is affected by the socket-level options
     SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).

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

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

           MSG_OOB        process out-of-band data
           MSG_PEEK       peek at incoming message
           MSG_WAITALL    wait for full request or error
     The MSG_OOB flag requests receipt of out-of-band data that would not be
     received in the normal data stream.  Some protocols place expedited data
     at the head of the normal data queue, and thus this flag cannot be used
     with such protocols.  The MSG_PEEK flag causes the receive operation to
     return data from the beginning of the receive queue without removing that
     data from the queue.  Thus, a subsequent receive call will return the
     same data.  The MSG_WAITALL flag requests that the operation block until
     the full request is satisfied.  However, the call may still return less
     data than requested if a signal is caught, an error or disconnect occurs,
     or the next data to be received is of a different type than that
     returned.

     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 {
             void            *msg_name;      /* optional address */
             socklen_t       msg_namelen;    /* size of address */
             struct iovec    *msg_iov;       /* scatter/gather array */
             int             msg_iovlen;     /* # elements in msg_iov */
             void            *msg_control;   /* ancillary data, see below */
             socklen_t       msg_controllen; /* ancillary data buffer len */
             int             msg_flags;      /* flags on received message */
     };

     Here msg_name and msg_namelen specify the source address if the socket is
     unconnected; msg_name may be given as a null pointer if no names are
     desired or required.  If the socket is connected, msg_name and
     msg_namelen are ignored.  msg_iov and msg_iovlen describe scatter gather
     locations, as discussed in read(2).  msg_control, which has length
     msg_controllen, points to a buffer for other protocol control related
     messages or other miscellaneous ancillary data.  The messages are of the
     form:

     struct cmsghdr {
             socklen_t       cmsg_len;       /* data byte count, including hdr */
             int             cmsg_level;     /* originating protocol */
             int             cmsg_type;      /* protocol-specific type */
     /* followed by
             u_char          cmsg_data[]; */
     };
     As an example, one could use this to learn of changes in the data-stream
     in XNS/SPP, or in ISO, to obtain user-connection-request data by request-
     ing a recvmsg with no data buffer provided immediately after an accept()
     call.

     Open file descriptors are now passed as ancillary data for AF_LOCAL
     domain sockets, with cmsg_level set to SOL_SOCKET and cmsg_type set to
     SCM_RIGHTS.

     The msg_flags field is set on return according to the message received.
     MSG_EOR indicates end-of-record; the data returned completed a record
     (generally used with sockets of type SOCK_SEQPACKET).  MSG_TRUNC indi-
     cates that the trailing portion of a datagram was discarded because the
     datagram was larger than the buffer supplied.  MSG_CTRUNC indicates that
     some control data were discarded due to lack of space in the buffer for
     ancillary data.  MSG_OOB is returned to indicate that expedited or out-
     of-band data were received.

RETURN VALUES
     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.

     [ENOTCONN]         The socket is associated with a connection-oriented
                        protocol and has not been connected (see connect(2)
                        and accept(2)).

     [ENOTSOCK]         The argument s does not refer to a socket.

     [EAGAIN]           The socket is marked non-blocking, and the receive
                        operation would block, or a receive timeout had been
                        set, and the timeout expired before data were
                        received.

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

     [EFAULT]           The receive buffer pointer(s) point outside the
                        process's address space.

     [EINVAL]           The total length of the I/O is more than can be
                        expressed by the ssize_t return value.

     recvmsg() will also fail if:

     [EMSGSIZE]         The msg_iovlen member of the msg structure is less
                        than or equal to 0 or is greater than {IOV_MAX}.

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

HISTORY
     The recv() function call appeared in 4.2BSD.

NetBSD 5.0_STABLE               April 23, 2006               NetBSD 5.0_STABLE
