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SEND(2) System Calls Manual SEND(2)

send, sendto, sendmsg, sendmmsg — send message(s) 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);

ssize_t
sendmmsg(int s, struct mmsghdr * restrict msgvec, size_t vlen, int flags);

The () and sendmmsg() functions, and () and () system calls are used to transmit one or more messages (with the sendmmsg() call) to another socket. The send() function may be used only when the socket is in a state, while sendto(), sendmsg() and sendmmsg() may be used at any time.

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.

The () function sends multiple messages at a call. They are given by the msgvec vector along with vlen specifying the vector size. The number of octets sent per each message is placed in the msg_len field of each processed element of the vector after transmission.

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) system call may be used to determine when it is possible to send more data.

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

#define	MSG_OOB		0x00001 /* process out-of-band data */
#define	MSG_DONTROUTE	0x00004 /* bypass routing, use direct interface */
#define MSG_EOR		0x00008 /* data completes record */
#define	MSG_EOF		0x00100 /* data completes transaction */
#define	MSG_NOSIGNAL	0x20000 /* 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_EOF requests that the sender side of a socket be shut down, and that an appropriate indication be sent at the end of the specified data; this flag is only implemented for SOCK_STREAM sockets in the PF_INET protocol family. MSG_DONTROUTE is usually used only by diagnostic or routing programs. MSG_NOSIGNAL is used to prevent SIGPIPE generation when writing a socket that may be closed.

See recv(2) for a description of the msghdr structure and the mmsghdr structure.

The send(), sendto() and sendmsg() calls return the number of octets sent. The sendmmsg() call returns the number of messages sent. If an error occurred a value of -1 is returned.

The send() and sendmmsg() functions and sendto() and sendmsg() system calls fail if:

[]
An invalid descriptor was specified.
[]
The destination address is a broadcast address, and SO_BROADCAST has not been set on the socket.
[]
The argument s is not a socket.
[]
An invalid user space address was specified for an argument.
[]
The socket requires that message be sent atomically, and the size of the message to be sent made this impossible.
[]
The socket is marked non-blocking and the requested operation would block.
[]
The system was unable to allocate an internal buffer. The operation may succeed when buffers become available.
[]
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 remote host was unreachable.
[]
A destination address was specified and the socket is already connected.
[]
The socket received an ICMP destination unreachable message from the last message sent. This typically means that the receiver is not listening on the remote port.
[]
The remote host was down.
[]
The remote network was down.
[]
The process using a SOCK_RAW socket was jailed and the source address specified in the IP header did not match the IP address bound to the prison.
[]
The socket is unable to send anymore data (SBS_CANTSENDMORE has been set on the socket). This typically means that the socket is not connected.

fcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2), CMSG_DATA(3)

The send() function appeared in 4.2BSD. The sendmmsg() function appeared in FreeBSD 11.0.

Because sendmsg() does not necessarily block until the data has been transferred, it is possible to transfer an open file descriptor across an AF_UNIX domain socket (see recv(2)), then close() it before it has actually been sent, the result being that the receiver gets a closed file descriptor. It is left to the application to implement an acknowledgment mechanism to prevent this from happening.

August 19, 2018 FreeBSD-12.0