RECV(2)                       System Calls Manual                      RECV(2)


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


     #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 *from,
         socklen_t *fromlen);

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


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

     If from is non-null 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 recv() call is normally used only on a connected socket (see
     connect(2)) and is identical to recvfrom() with a null from parameter.

     On successful completion, all three routines return the number of message
     bytes read.  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.  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) system calls may be used to determine when more
     data arrive.

     The flags argument is the bitwise OR of zero or more of the following

           MSG_OOB             process out-of-band data
           MSG_PEEK            peek at incoming message
           MSG_WAITALL         wait for full request or error
           MSG_DONTWAIT        don't block
           MSG_CMSG_CLOEXEC    set the close-on-exec flag on received file

     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
     MSG_CMSG_CLOEXEC requests that any file descriptors received as ancillary
     data with recvmsg() (see below) have their close-on-exec flag set.

     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 */
             unsigned 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.  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[]; */

     See CMSG_DATA(3) for how these messages are constructed and decomposed.

     Open file descriptors are now passed as ancillary data for AF_UNIX domain
     and socketpair(2) 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.
     It will contain zero or more of the following values:

           MSG_OOB     Returned to indicate that expedited or out-of-band data
                       was received.
           MSG_EOR     Indicates end-of-record; the data returned completed a
                       record (generally used with sockets of type
           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_BCAST   Indicates that the packet was received as broadcast.
           MSG_MCAST   Indicates that the packet was received as multicast.


     These calls return the number of bytes received, or -1 if an error


     recv(), recvfrom(), and recvmsg() fail if:

     [EBADF]         The argument s is an invalid descriptor.

     [ENOTCONN]      The socket is associated with a connection-oriented
                     protocol and has not been connected (see connect(2) and

     [ENOTSOCK]      The argument s does not refer to a socket.

     [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.

     [EHOSTUNREACH]  A socket operation was attempted to an unreachable host.

     [EHOSTDOWN]     A socket operation failed because the destination host
                     was down.

     [ENETDOWN]      A socket operation encountered a dead network.

     [ECONNREFUSED]  The socket is associated with a connection-oriented
                     protocol and the connection was forcefully rejected (see

     In addition, recv() and recvfrom() may return the following error:

     [EINVAL]           len was larger than SSIZE_MAX.

     And recvmsg() may return one of the following errors:

     [EINVAL]           The sum of the iov_len values in the msg_iov array
                        overflowed an ssize_t.

     [EMSGSIZE]         The msg_iovlen member of msg was less than 0 or larger
                        than IOV_MAX.

     [EMSGSIZE]         The receiving program did not have sufficient free
                        file descriptor slots.  The descriptors are closed and
                        any pending data can be returned by another call to


     connect(2), fcntl(2), getsockopt(2), poll(2), read(2), select(2),
     socket(2), socketpair(2), CMSG_DATA(3), sockatmark(3)


     The recv(), recvfrom(), and recvmsg() functions conform to IEEE Std
     1003.1-2008 (``POSIX.1'').  The MSG_DONTWAIT, MSG_BCAST, and MSG_MCAST
     flags are extensions to that specification.


     The recv() function call appeared in 4.2BSD.

OpenBSD 5.9                    February 16, 2015                   OpenBSD 5.9

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