RECV(2)

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

LIBRARY
libc

SYNOPSIS
#include <sys/socket.h>
ssize_t
recv(int s, void *buf, size_t len, int flags);
ssize_t
recvfrom(int s, void * restrict buf, size_t len, int flags, struct sockaddr * restrict from, socklen_t * restrict fromlen);
ssize_t
recvmsg(int s, struct msghdr *msg, int flags);
int
recvmmsg(int s, struct mmsghdr *mmsg, unsigned int vlen, unsigned int flags, struct timespec *timeout);

DESCRIPTION
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 connection-oriented.

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 there.

The recvmmsg() call can be used to receive multiple messages in the same call using an array of mmsghdr elements with the following form, as defined in <sys/socket.h>:

    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 arrives.

The flags argument to a recv call is formed by or'ing one or more of the values:
MSG_CMSG_CLOEXEC | set the close on exec property for passed file descriptors
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 {
    	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 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() 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 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
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() call returns the number of messages received, or -1 if an error occurred.

ERRORS
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 received.

[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.

[ENOBUFS]
    A message was not delivered because it would have overflowed the buffer.

[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}.

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

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