Manual Pages  — RECV

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 Ap 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 -ansiC 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.