Manual Pages  — SEND

#include <sys/socket.h>

The send() and sendmmsg() functions, and sendto() and sendmsg() 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 connected state. The functions sendto(), sendmsg() and sendmmsg() may be used at any time if the socket is connectionless-mode. If the socket is connection-mode, the protocol must support implied connect (currently tcp(4) is the only protocol with support) or the socket must be in a connected state before use.

The address of the target is given by to with tolen specifying its size, or the equivalent msg_name and msg_namelen in struct, msghdr. If the socket is in a connected state, the target address passed to sendto(), sendmsg() or sendmmsg() is ignored. 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 sendmmsg() 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 send(). 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 send() 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_DONTWAIT    0x00080 /* do not block */
#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.amp; 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. The MSG_DONTWAIT flag request the call to return when it would block otherwise. 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.

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

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.