RECV(2)

NAME
recv, recvfrom, recvmsg, recvmmsg - receive message(s) 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 *buf, size_t len, int flags, struct sockaddr * restrict from, socklen_t * restrict fromlen);
ssize_t
recvmsg(int s, struct msghdr *msg, int flags);
ssize_t
recvmmsg(int s, struct mmsghdr * restrict msgvec, size_t vlen, int flags, const struct timespec * restrict timeout);

DESCRIPTION
The recvfrom(), recvmsg(), and recvmmsg() system calls 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 not a null pointer and the socket is not connection-oriented, the source address of the message is filled in. The fromlen argument is a value-result argument, 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() 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'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 strict ANSI or 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 destination 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.

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 close-on-exec flag on received descriptors is set according to the MSG_CMSG_CLOEXEC flag passed to recvmsg().

Process credentials can also be passed as ancillary data for AF_UNIX domain sockets using a cmsg_type of SCM_CREDS. In this case, cmsg_data should be a structure of type cmsgcred, which is defined in <sys/socket.h> as follows:

    struct cmsgcred {
    	pid_t	cmcred_pid;		/* PID of sending process */
    	uid_t	cmcred_uid;		/* real UID of sending process */
    	uid_t	cmcred_euid;		/* effective UID of sending process */
    	gid_t	cmcred_gid;		/* real GID of sending process */
    	short	cmcred_ngroups;		/* number or groups */
    	gid_t	cmcred_groups[CMGROUP_MAX];	/* groups */
    };

If a sender supplies ancillary data with enough space for the above struct tagged as SCM_CREDS control message type to the sendmsg() system call, then kernel will fill in the credential information of the sending process and deliver it to the receiver. Since receiver usually has no control over a sender, this method of retrieving credential information isn't reliable. For reliable retrieval of remote side credentials it is advised to use the LOCAL_CREDS socket option on the receiving socket. 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.

RETURN VALUES
These calls except recvmmsg() return the number of bytes received. recvmmsg() returns the number of messages received. A value of -1 is returned if an error occurred.

ERRORS
The calls fail if:

[EBADF]
    The argument s is an invalid descriptor.

[ECONNRESET]
    The remote socket end is forcibly closed.

[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.

[EMSGSIZE]
    The recvmsg() system call was used to receive rights (file descriptors) that were in flight on the connection. However, the receiving program did not have enough free file descriptor slots to accept them. In this case the descriptors are closed, any pending data can be returned by another call to recvmsg().

[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.

SEE ALSO
fcntl(2), getsockopt(2), read(2), select(2), socket(2), unix(4)

HISTORY
The recv() function appeared in 4.2BSD. The recvmmsg() function appeared in FreeBSD 11.0.