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     recv, recvfrom, recvmsg, recvmmsg — receive message(s) from a socket


     Standard C Library (libc, -lc)


     #include <sys/socket.h>

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

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

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

     recvmmsg(int s, struct mmsghdr * restrict msgvec, size_t vlen, int flags,
         const struct timespec * restrict timeout);


     The recvfrom(), recvmsg(), and recvmmsg() 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-

     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.

     The recvmmsg() function is used to receive multiple messages at a call.  Their number is
     supplied by vlen.  The messages are placed in the buffers described by msgvec vector, after
     reception.  The size of each received message is placed in the msg_len field of each element
     of the vector.  If timeout is NULL the call blocks until the data is available for each
     supplied message buffer.  Otherwise it waits for data for the specified amount of time.  If
     the timeout expired and there is no data received, a value 0 is returned.  The ppoll(2)
     system call is used to implement the timeout mechanism, before first receive is performed.

     The recv(), recvfrom() and recvmsg() return the length of the message on successful
     completion, whereas recvmmsg() returns the number of received messages.  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 except recvmmsg() 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 recvmmsg() function implements this
     behaviour for each message in the vector.

     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
           MSG_CMSG_CLOEXEC    set received fds close-on-exec
           MSG_WAITFORONE      do not block after receiving the first message (only for
                               recvmmsg() )

     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 ANSI
     X3.159-1989 (“ANSI C89”) or ISO/IEC 9899:1999 (“ISO C99”) compilation mode.  The
     MSG_WAITFORONE flag sets MSG_DONTWAIT after the first message has been received.  This flag
     is only relevant for recvmmsg().

     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 source 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

     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.

     With AF_UNIX domain sockets, ancillary data can be used to pass file descriptors and process
     credentials.  See unix(4) for details.

     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.

     The recvmmsg() system call uses the mmsghdr structure, defined as follows in the
     <sys/socket.h> header:

     struct mmsghdr {
             struct msghdr    msg_hdr;       /* message header */
             ssize_t          msg_len;       /* message length */

     On data reception the msg_len field is updated to the length of the received message.


     These calls except recvmmsg() return the number of bytes received.  recvmmsg() returns the
     number of messages received.  A value of -1 is returned if an error occurred.


     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

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


     fcntl(2), getsockopt(2), read(2), select(2), socket(2), CMSG_DATA(3), unix(4)


     The recv() function appeared in 4.2BSD.  The recvmmsg() function appeared in FreeBSD 11.0.