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RECV(2)                     BSD 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 socket, void *buffer, size_t length, int flags);

     ssize_t
     recvfrom(int socket, void *restrict buffer, size_t length, int flags,
         struct sockaddr *restrict address, socklen_t *restrict address_len);

     ssize_t
     recvmsg(int socket, struct msghdr *message, int flags);

DESCRIPTION
     The recvfrom() and recvmsg() system calls 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 address is not a null pointer and the socket is not connection-ori-ented, connection-oriented,
     ented, the source address of the message is filled in.  The address_len
     argument is a value-result argument, initialized to the size of the
     buffer associated with address, and modified on return to indicate the
     actual size of the address stored there.

     The recv() function is normally used only on a connected socket (see
     connect(2)) and is identical to recvfrom() with a null pointer passed as
     its address argument.  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. completion.
     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) system call may be used to determine when more data arrive.

     If no messages are available to be received and the peer has performed an
     orderly shutdown, the value 0 is returned.

     The flags argument to a recv() function 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() system call uses a msghdr structure to minimize the number
     of directly supplied arguments.  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 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 arguments
     describe scatter gather locations, as discussed in read(2).  The
     msg_control argument, which has length msg_controllen, points to a buffer
     for other protocol control related messages or other miscellaneous ancil-lary ancillary
     lary data.  The messages are of the form:

     struct cmsghdr {
             u_int   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 requesting
     ing a recvmsg() with no data buffer provided immediately after an
     accept() system call.

     Open file descriptors are now passed as ancillary data for AF_UNIX 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 indicates
     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 outof-band
     of-band data were received.

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

     For TCP sockets, the return value 0 means the peer has closed its half
     side of the connection.

ERRORS
     The calls fail if:

     [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.

     [EBADF]            The argument socket is an invalid descriptor.

     [ECONNRESET]       The connection is closed by the peer during a receive
                        attempt on a socket.

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

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

     [EINVAL]           MSG_OOB is set, but no out-of-band data is available.

     [ENOBUFS]          An attempt to allocate a memory buffer fails.

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

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

     [EOPNOTSUPP]       The type and/or protocol of socket do not support the
                        option(s) specified in flags.

     [ETIMEDOUT]        The connection timed out.

     The recvfrom() call may also fail if:

     [EINVAL]           The total of the iov_len values overflows a ssize_t.

     The recvmsg() call may also fail if:

     [EMSGSIZE]         The requested message size is invalid.

     [ENOMEM]           Insufficient memory is available.

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

HISTORY
     The recv() function appeared in 4.2BSD.

BSD                              May 15, 2006                              BSD
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