RECV(2)

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

SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>

int recv (int s, void *buf, size_t len, unsigned int flags);

int recvfrom (int s, void *buf, size_t len, unsigned int flags, struct sockaddr *from, int *fromlen);

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

DESCRIPTION
Recvfrom and recvmsg 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 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. As it is redundant, it may not be supported in future releases.

All three routines return the length of the message on successful completion. 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) call may be used to determine when more data arrive.

The flags argument to a recv call 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

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 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 {
	caddr_t	msg_name;	/* optional address */
	u_int	msg_namelen;	/* size of address */
	struct	iovec *msg_iov;	/* scatter/gather array */
	u_int	msg_iovlen;	/* # elements in msg_iov */
	caddr_t	msg_control;	/* ancillary data, see below */
	u_int	msg_controllen; /* ancillary data buffer len */
	int	msg_flags;	/* flags on received message */
};
Here msg_name and msg_namelen specify the destination 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 {
	u_int	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_UNIX domain 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. 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.

RETURN VALUES
These calls return the number of bytes received, or -1 if an error occurred.

ERRORS
The calls 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 accept(2)).

    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.

CONVORMANCE
The GNO prototypes of these routines differ slightly from that of 4.4BSD.

SEE ALSO
fcntl(2), read(2), select(2), getsockopt(2), socket(2)

HISTORY
The recvmsg function call appeared in 4.2BSD. The first appearance in GNO was in v2.0.5.