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SEND(2) System Calls Manual SEND(2)

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

library “libc”

#include <sys/types.h>
#include <sys/socket.h>

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

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

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

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

The socket file descriptor is given by s. msgbuf points to a buffer containing the message. msg points to a msghdr structure. 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) call may be used to determine when it is possible to send more data.

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

#define	MSG_OOB		0x1   /* process out-of-band data */
#define	MSG_PEEK	0x2   /* peek at incoming message */
#define	MSG_DONTROUTE	0x4   /* bypass routing, use direct interface */
#define MSG_EOR		0x8   /* data completes record */
#define	MSG_EOF		0x100 /* data completes transaction */
#define MSG_NOSIGNAL	0x400 /* No SIGPIPE to unconnected socket stream */

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 requests not to send the SIGPIPE signal if an attempt to send is made on a stream-oriented socket that is no longer connected.

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

Upon successful completion the number of characters which were sent is returned. Otherwise -1 is returned and the global variable errno is set to indicate the error.

Send(), sendto(), and sendmsg() 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 a parameter.
[]
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.
[]
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 socket is unable to send anymore data (SS_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)

The send() function call appeared in 4.2BSD.

Because sendmsg() doesn't 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.

October 6, 2010 DragonFly-5.6.1