man.bsd.lv manual page server

DESCRIPTION

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 there.

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 values:

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 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[]; */
};

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: