recvfrom

RECV(2)                       System Calls Manual                      RECV(2)

NAME
     recv, recvfrom, recvmsg, recvmmsg — receive message(s) 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 *buf, size_t len, int flags,
         struct sockaddr * restrict from, socklen_t * restrict fromlen);

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

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

DESCRIPTION
     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-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.

     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 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.

     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.

RETURN VALUES
     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.

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), CMSG_DATA(3),
     unix(4)

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

                                August 19, 2018