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NAME

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

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/types.h>
     #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

     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 from is not a null pointer and the socket is not connection-oriented,
     the source address of the message is filled in.  The fromlen argument is
     a value-result argument, 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 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 from argument.  As it is redundant, it may not be supported 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 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 fcntl(2)) in
     which case the value -1 is returned and the global variable errno is 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
     arrives.

     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
           MSG_DONTWAIT    do not block

     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 MSG_DONTWAIT flag requests the call to return when it
     would block otherwise.  If no data is available, errno is set to EAGAIN.
     This flag is not available in strict ANSI or C99 compilation mode.

     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

     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
     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
     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.

     Process credentials can also be passed as ancillary data for AF_UNIX
     domain sockets using a cmsg_type of SCM_CREDS.  In this case, cmsg_data
     should be a structure of type cmsgcred, which is defined in #include
     <sys/socket.h>
     as follows:

     struct cmsgcred {
             pid_t   cmcred_pid;             /* PID of sending process */
             uid_t   cmcred_uid;             /* real UID of sending process */
             uid_t   cmcred_euid;            /* effective UID of sending process */
             gid_t   cmcred_gid;             /* real GID of sending process */
             short   cmcred_ngroups;         /* number or groups */
             gid_t   cmcred_groups[CMGROUP_MAX];     /* groups */
     };

     The kernel will fill in the credential information of the sending process
     and deliver it to the receiver.

     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
     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 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.

     [ECONNRESET]       The remote socket end is forcibly closed.

     [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.

     [EMSGSIZE]         The recvmsg() system call was used to receive rights
                        (file descriptors) that were in flight on the
                        connection.  However, the receiving program did not
                        have enough free file descriptor slots to accept them.
                        In this case the descriptors are closed, any pending
                        data can be returned by another call to recvmsg().

     [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.

SEE ALSO

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

HISTORY

     The recv() function appeared in 4.2BSD.