SEND(2)

NAME
send, sendto, sendmsg - send a message from a socket

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#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);

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

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) 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 */

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, and is used to implement Transaction TCP (see ttcp(4)). MSG_DONTROUTE is usually used only by diagnostic or routing programs.

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

RETURN VALUES
The call returns the number of characters sent, or -1 if an error occurred.

ERRORS
Send(), sendto(), and sendmsg() fail if:

[EBADF]
    An invalid descriptor was specified.

[EACCES]
    The destination address is a broadcast address, and SO_BROADCAST has not been set on the socket.

[ENOTSOCK]
    The argument s is not a socket.

[EFAULT]
    An invalid user space address was specified for a parameter.

[EMSGSIZE]
    The socket requires that message be sent atomically, and the size of the message to be sent made this impossible.

[EAGAIN]
    The socket is marked non-blocking and the requested operation would block.

[ENOBUFS]
    The system was unable to allocate an internal buffer. The operation may succeed when buffers become available.

[ENOBUFS]
    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.

[EHOSTUNREACH]
    The remote host was unreachable.

BUGS
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 acknowlegment mechanism to prevent this from happening.

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

HISTORY
The send() function call appeared in 4.2BSD.