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RECV(2) System Calls Manual RECV(2)

recv, recvfrom, recvmsg, recvmmsgreceive a message from a socket

#include <sys/socket.h>

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

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

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

recvmmsg(int s, struct mmsghdr *mmsg, unsigned int vlen, int flags, struct timespec *timeout);

(), (), recvmsg(), and recvmmsg() are used to receive messages from a socket, s. recv() is normally used only on a socket (see connect(2)). recvfrom(), recvmsg(), and recvmmsg() may be used to receive data on a socket whether or not it is connection-oriented.

() is identical to () with a null from parameter.

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.

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:

process out-of-band data
peek at incoming message
wait for full request or error
don't block
set the close-on-exec flag on received file descriptors

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 () and recvmmsg() (see below) have their close-on-exec flag set.

The () 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:

Returned to indicate that expedited or out-of-band data was received.
Indicates end-of-record; the data returned completed a record (generally used with sockets of type SOCK_SEQPACKET).
Indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied.
Indicates that some control data were discarded due to lack of space in the buffer for ancillary data.
Indicates that the packet was received as broadcast.
Indicates that the packet was received as multicast.

The () call uses an array of the mmsghdr structure of length vlen to group multiple msghdr structures into a single system call. vlen is capped at maximum 1024 messages that are received in a single call. The flags field allows setting MSG_WAITFORONE to wait for one msghdr, and set MSG_DONTWAIT for all subsequent messages. A provided timeout limits the time spent in the function but it does not limit the time spent in lower parts of the kernel.

The mmsghdr structure has the following form, as defined in <sys/socket.h>:

struct mmsghdr {
	struct msghdr msg_hdr;
	unsigned int msg_len;

Here msg_len indicated the number of bytes received for each msg_hdr member.

The recv(), recvfrom(), and recvmsg() calls return the number of bytes received, or -1 if an error occurred. The recvmmsg() call returns the number of messages received, or -1 if an error occurred before the first message has been received.

recv(), recvfrom(), recvmsg(), and recvmmsg() fail if:

The argument s is an invalid descriptor.
The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)).
The argument s does not refer to a socket.
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.
The receive was interrupted by delivery of a signal before any data were available.
The receive buffer pointer(s) point outside the process's address space.
A socket operation was attempted to an unreachable host.
A socket operation failed because the destination host was down.
A socket operation encountered a dead network.
The socket is associated with a connection-oriented protocol and the connection was forcefully rejected (see connect(2)).

In addition, recv() and recvfrom() may return the following error:

len was larger than SSIZE_MAX.

And recvmsg() and recvmmsg() may return one of the following errors:

The sum of the iov_len values in the msg_iov array overflowed an .
The msg_iovlen member of msg was less than 0 or larger than IOV_MAX.
The receiving program did not have sufficient free file descriptor slots. The descriptors are closed and any pending data can be returned by another call to recvmsg().

connect(2), fcntl(2), getsockopt(2), poll(2), read(2), select(2), socket(2), socketpair(2), CMSG_DATA(3), sockatmark(3)

The recv(), recvfrom(), and recvmsg() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”). The MSG_DONTWAIT, MSG_BCAST, and MSG_MCAST flags are extensions to that specification.

The recv() function call appeared in 4.1cBSD. The recvmmsg() syscall first appeared in Linux 2.6.33 and was added to OpenBSD 7.2.

Calling recvmsg() with a control message having no or an empty scatter/gather array exposes variations in implementations. To avoid these, always use an iovec with at least a one-byte buffer and set msg_iov and an msg_iovlen to use this vector.

August 17, 2023 OpenBSD-7.5