SEND(2)

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

LIBRARY
libc

SYNOPSIS
#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) or poll(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		0x0001 /* process out-of-band data */
    #define	MSG_PEEK	0x0002 /* peek at incoming message */
    #define	MSG_DONTROUTE	0x0004 /* bypass routing, use direct interface */
    #define	MSG_EOR		0x0008 /* data completes record */
    #define	MSG_NOSIGNAL	0x0400 /* 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_DONTROUTE is usually used only by diagnostic or routing programs.

See recv(2) for a description of the msghdr structure. MSG_NOSIGNAL is used to prevent SIGPIPE generation when writing a socket that may be closed.

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.

[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.

[EPIPE]
    In a connected socket the connection has been broken.

[EDSTADDRREQ]
    In a non-connected socket a destination address has not been specified.

[EAGAIN|EWOULDBLOCK]
    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.

[EACCES]
    The SO_BROADCAST option is not set on the socket, and a broadcast address was given as the destination.

[EHOSTUNREACH]
    The destination for the message is unreachable.

[EHOSTDOWN]
    The destination is a host on the local subnet and does not respond to arp(4).

[EINVAL]
    The total length of the I/O is more than can be expressed by the ssize_t return value.

[EAFNOSUPPORT]
    Addresses in the specified address family cannot be used with this socket.

sendto() will also fail if:

[EISCONN]
    A destination address was specified and the socket is already connected.

sendmsg() will also fail if:

[EMSGSIZE]
    The msg_iovlen member of the msg structure is less than or equal to 0 or is greater than {IOV_MAX}.

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

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