The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


send - send a message on a socket


#include <sys/socket.h>

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


Specifies the socket file descriptor.
Points to the buffer containing the message to send.
Specifies the length of the message in bytes.
Specifies the type of message transmission. Values of this argument are formed by logically OR'ing zero or more of the following flags:
Terminates a record (if supported by the protocol)
Sends out-of-band data on sockets that support out-of-band communications. The significance and semantics of out-of-band data are protocol-specific.

The send() function initiates transmission of a message from the specified socket to its peer. The send() function sends a message only when the socket is connected (including when the peer of a connectionless socket has been set via connect()).

The length of the message to be sent is specified by the length argument. If the message is too long to pass through the underlying protocol, send() fails and no data is transmitted.

Successful completion of a call to send() does not guarantee delivery of the message. A return value of -1 indicates only locally-detected errors.

If space is not available at the sending socket to hold the message to be transmitted and the socket file descriptor does not have O_NONBLOCK set, send() blocks until space is available. If space is not available at the sending socket to hold the message to be transmitted and the socket file descriptor does have O_NONBLOCK set, send() will fail. The select() and poll() functions can be used to determine when it is possible to send more data.

The socket in use may require the process to have appropriate privileges to use the send() function.


Upon successful completion, send() returns the number of bytes sent. Otherwise, -1 is returned and errno is set to indicate the error.


The send() function is identical to sendto() with a null pointer dest_len argument, and to write() if no flags are used.


The send() function will fail if:

The socket's file descriptor is marked O_NONBLOCK and the requested operation would block.
The socket argument is not a valid file descriptor.
A connection was forcibly closed by a peer.
The socket is not connection-mode and no peer address is set.
The buffer parameter can not be accessed.
A signal interrupted send() before any data was transmitted.
The message is too large be sent all at once, as the socket requires.
The socket is not connected or otherwise has not had the peer prespecified.
The socket argument does not refer to a socket.
The socket argument is associated with a socket that does not support one or more of the values set in flags.
The socket is shut down for writing, or the socket is connection-mode and is no longer connected. In the latter case, and if the socket is of type SOCK_STREAM, the SIGPIPE signal is generated to the calling process.

The send() function may fail if:

The calling process does not have the appropriate privileges.
An I/O error occurred while reading from or writing to the file system.
The local interface used to reach the destination is down.
No route to the network is present.
Insufficient resources were available in the system to perform the operation.
There were insufficient STREAMS resources available for the operation to complete.


connect(), getsockopt(), poll(), recv(), recvfrom(), recvmsg(), select(), sendmsg(), sendto(), setsockopt(), shutdown(), socket(), <sys/socket.h>.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
[ Main Index | XSH | XCU | XBD | XCURSES | XNS ]