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SEND(2) System Calls Manual SEND(2)

send, sendto, sendmsg, sendmmsg — send a message from a socket

library “libc”

#include <sys/socket.h>

ssize_t
send(int s, const void *msg, size_t len, int flags);

ssize_t
sendto(int s, const void *msg, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);

ssize_t
sendmsg(int s, const struct msghdr *msg, int flags);

int
sendmmsg(int s, struct mmsghdr *mmsg, unsigned int vlen, unsigned int flags);

(), (), (), and sendmmsg() are used to transmit a message to another socket. send() may be used only when the socket is in a state, while sendto(), sendmsg() and sendmmsg() may be used at any time.

The () call can be used to send 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 transmitted */
};

The msg_len member contains the number of bytes sent 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.

The address of the target is given by to, with tolen specifying its size. The length of the message is given by len. If the message is too long to pass atomically through the underlying protocol, the error EMSGSIZE is returned, and the message is not transmitted.

No indication of failure to deliver is implicit in a (). Locally detected errors are indicated by a return value of -1.

If no messages space is available at the socket to hold the message to be transmitted, then () normally blocks, unless the socket has been placed in non-blocking I/O mode. The select(2) or poll(2) call may be used to determine when it is possible to send more data. Unfortunately this does not work when the interface queue which is used to send the message is full, and the call returns ENOBUFS.

The flags parameter may include one or more of the following:

#define	MSG_OOB		0x0001 /* process out-of-band data */
#define	MSG_PEEK	0x0002 /* peek at incoming message */
#define	MSG_DONTROUTE	0x0004 /* bypass routing, use direct interface */
#define	MSG_EOR		0x0008 /* data completes record */
#define	MSG_NOSIGNAL	0x0400 /* do not generate SIGPIPE on EOF */

The flag MSG_OOB is used to send “out-of-band” data on sockets that support this notion (e.g. SOCK_STREAM); the underlying protocol must also support “out-of-band” data. MSG_EOR is used to indicate a record mark for protocols which support the concept. MSG_DONTROUTE is usually used only by diagnostic or routing programs.

See recv(2) for a description of the msghdr structure. MSG_NOSIGNAL is used to prevent SIGPIPE generation when writing a socket that may be closed.

The send(), sendto(), and sendmsg() calls return the number of characters sent, or -1 if an error occurred. The sendmmsg() call returns the number of messages sent, or -1 if an error occured.

send(), sendto(), sendmsg(), and sendmmsg() fail if:

[]
The SO_BROADCAST option is not set on the socket, and a broadcast address was given as the destination.
[]
Addresses in the specified address family cannot be used with this socket.
[]
The socket is marked non-blocking and the requested operation would block.
[]
An invalid descriptor was specified.
[]
In a non-connected socket a destination address has not been specified.
[]
An invalid user space address was specified for a parameter.
[]
The destination is a host on the local subnet and does not respond to arp(4).
[]
The destination for the message is unreachable.
[]
The total length of the I/O is more than can be expressed by the ssize_t return value.
[]
The socket requires that message be sent atomically, and the size of the message to be sent made this impossible.
[]
The system was unable to allocate an internal buffer. The operation may succeed when buffers become available.

An alternative reason: the output queue for a network interface was full. This generally indicates that the interface has stopped sending, but may be caused by transient congestion.

[]
The argument s is not a socket.
[]
In a connected socket the connection has been broken.

sendto() will also fail if:

[]
A destination address was specified and the socket is already connected.

sendmsg() and sendmmsg() will also fail if:

[]
The msg_iovlen member of the msg structure is less than or equal to 0 or is greater than {IOV_MAX}.

fcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2)

The send() function call appeared in 4.2BSD. The sendmmsg() function call appeared in Linux 3.0 and NetBSD 7.0.

June 22, 2012 NetBSD-9.2