RECV(2)                       System Calls Manual                      RECV(2)

     recv, recvfrom, recvmsg, recvmmsg -- receive a message 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 * restrict 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 *mmsg, unsigned int vlen,
         unsigned int flags, struct timespec *timeout);

     recvfrom(), recvmsg() and recvmmsg() are used to receive messages from a
     socket, and may be used to receive data on a socket whether or not it is

     If from is non-nil, and the socket is not connection-oriented, the source
     address of the message is filled in.  fromlen is a value-result
     parameter, initialized to the size of the buffer associated with from,
     and modified on return to indicate the actual size of the address stored

     The recvmmsg() call be used to receive multiple messages in the same call
     using an array of mmsghdr elements with the following form, as defined in

     struct mmsghdr {
             struct msghdr   msg_hdr;        /* the message to be sent */
             unsigned int    msg_len;        /* number of bytes received */

     The msg_len member contains the number of bytes received for each msg_hdr
     member.  The array has vlen elements, which is limited to 1024.  If there
     is an error, a number fewer than vlen may be returned, and the error may
     be retrieved using getsockopt(2) with SO_ERROR.  If the flag
     MSG_WAITFORONE is set in flags then the recvmmsg() call will wait for one
     message, and set MSG_DONTWAIT for the rest.  If the timeout parameter is
     not NULL, then recvmmsg() will return if that time is exceeded.

     The recv() call is normally used only on a connected socket (see
     connect(2)) and is identical to recvfrom() with a nil from parameter.  As
     it is redundant, it may not be supported in future releases.

     recv(), recvfrom() and recvmsg() routines return the length of the
     message on successful completion.  recvmmsg() returns the number of
     messages received.  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.  If no data is available and the remote peer was shut down, 0
     is returned.  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) or poll(2) call may be used to determine when more data

     The flags argument to a recv call is formed by or'ing one or more of the

           MSG_CMSG_CLOEXEC    set the close on exec property for passed file
           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

     The recvmsg() call uses a msghdr structure to minimize the number of
     directly supplied parameters.  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.  If the socket is connected, msg_name and
     msg_namelen are ignored.  msg_iov and msg_iovlen describe scatter gather
     locations, as discussed in read(2).  msg_control, 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() call.

     Open file descriptors are now passed as ancillary data for AF_LOCAL
     domain sockets, with cmsg_level set to SOL_SOCKET and cmsg_type set to

     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 recv(), recvfrom() and recvmsg() calls return the number of bytes
     received, or -1 if an error occurred.  For connected sockets whose remote
     peer was shut down, 0 is returned when no more data is available.  The
     recvmmsg() calls return the number of messages received, or -1 if an
     error occurred.

     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

     [EBADF]            The argument s is an invalid descriptor.

     [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]           The total length of the I/O is more than can be
                        expressed by the ssize_t return value.

     [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.

     recvmsg() will also fail if:

     [EMSGSIZE]         The msg_iovlen member of the msg structure is less
                        than or equal to 0 or is greater than {IOV_MAX}.

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

     The recv() function call appeared in 4.2BSD.  The recvmmsg() function
     call appeared in Linux 2.6.32 and NetBSD 7.0.

NetBSD 7.99                      June 22, 2012                     NetBSD 7.99