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RECV(2) System Calls Manual RECV(2)

recv, recvfrom, recvmsg — receive a message from a socket

Standard C Library (libc, -lc)

#include <sys/socket.h>

ssize_t
recv(int socket, void *buffer, size_t length, int flags);

ssize_t
recvfrom(int socket, void *restrict buffer, size_t length, int flags, struct sockaddr *restrict address, socklen_t *restrict address_len);

ssize_t
recvmsg(int socket, struct msghdr *message, int flags);

The () and recvmsg() 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 address is not a null pointer and the socket is not connection-oriented, the source address of the message is filled in. The address_len argument is a value-result argument, initialized to the size of the buffer associated with address, and modified on return to indicate the actual size of the address stored there.

The () function is normally used only on a socket (see connect(2) or connectx(2)) and is identical to recvfrom() with a null pointer passed as its address argument. 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) system call may be used to determine when more data arrive.

If no messages are available to be received and the peer has performed an orderly shutdown, the value 0 is returned.

The flags argument to a () function is formed by 'ing one or more of the values:

process out-of-band data
peek at incoming message
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 () 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). msg_iovlen shall be set to the dimension of this array. In each iovec structure, the iov_base field specifies a storage area and the iov_len field gives its size in bytes. Each storage area indicated by msg_iov is filled with received data in turn until all of the received data is stored or all of the areas have been filled.

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 {
	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 () with no data buffer provided immediately after an () 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 msg_flags field is set on return according to the message received. MSG_EOR indicates end-of-record; the data returned completed a record. 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.

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

For TCP sockets, the return value 0 means the peer has closed its half side of the connection.

The calls fail if:

[]
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 argument socket is an invalid descriptor.
[]
The connection is closed by the peer during a receive attempt on a socket.
[]
The receive buffer pointer(s) point outside the process's address space.
[]
The receive was interrupted by delivery of a signal before any data were available.
[]
MSG_OOB is set, but no out-of-band data is available.
[]
An attempt to allocate a memory buffer fails.
[]
The socket is associated with a connection-oriented protocol and has not been connected (see connect(2,) connectx(2,) and accept(2)).
[]
The argument socket does not refer to a socket.
[]
The type and/or protocol of socket do not support the option(s) specified in flags.
[]
The connection timed out.

The recvfrom() call may also fail if:

[]
The total of the iov_len values overflows a ssize_t.

The recvmsg() call may also fail if:

[]
The msg_iovlen member of the msghdr structure pointed to by message is less than or equal to 0, or is greater than IOV_MAX.
[]
Insufficient memory is available.

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

The recv() function appeared in 4.2BSD.

March 18, 2015 macOS