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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-oriented, 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 sup-ported supported
     ported in future releases.

     All three routines return the length of the message on successful completion.  If a message is too long
     to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the mes-sage message
     sage 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 socketlevel
     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 argu-ments. arguments.
     ments.  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).
     msg_iovlen shall be set to the dimension of this array. In each iovec structure, the iov_base field
     specifies a storage area and the iov_len field gives its size in bytes. Each storage area indicated by
     msg_iov is filled with received data in turn until all of the received data is stored or all of the
     areas have been filled.

     The msg_control argument, which has length msg_controllen, points to a buffer for other protocol con-trol control
     trol related messages or other miscellaneous ancillary 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 requesting 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; end-ofrecord;
     record; the data returned completed a record (generally used with sockets of type SOCK_SEQPACKET).
     MSG_TRUNC indicates 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 con-nected connected
                        nected (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 msg_iovlen member of the msghdr structure pointed to by message is less than or
                        equal to 0, or is greater than IOV_MAX.

     [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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