SEND(2)

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

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

int send(int s, const void *msg, size_t len, int flags);
int sendto(int s, const void *msg, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);
int 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.

When the message does not fit into the send buffer of the socket, send normally blocks, unless the socket has been placed in non-blocking I/O mode. In non-blocking mode it would return EAGAIN in this case. 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_DONTROUTE	0x4	/* bypass routing, use direct interface */
    #define MSG_DONTWAIT	0x40	/* don't block */
    #define MSG_NOSIGNAL	0x2000	/* don't raise SIGPIPE */

MSG_OOB
    Sends 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_DONTROUTE
    Bypasses the usual routing table lookup and sends the packet directly to the interface described by the destination address. This is usually used only by diagnostic or routing programs.

MSG_DONTWAIT
    Enables non-blocking operation; if the operation would block, EAGAIN is returned.

MSG_NOSIGNAL
    Requests not to send SIGPIPE on errors on stream oriented sockets when the other end breaks the connection. The EPIPE error is still returned.

See recv(2) for a description of the msghdr structure. You may send control information using the msg_control and msg_controllen members. The maximum control buffer length the kernel can process is limited per socket by the net.core.optmem_max sysctl; see socket(7).

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

ERRORS
These are some standard errors generated by the socket layer. Additional errors may be generated and returned from the underlying protocol modules; see their respective manual pages.

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.

EAGAIN or EWOULDBLOCK
    The socket is marked non-blocking and the requested operation would block.

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. (This cannot occur in Linux, packets are just silently dropped when a device queue overflows.)

EINTR
    A signal occurred.

ENOMEM
    No memory available.

EINVAL
    Invalid argument passed.

EPIPE
    The local end has been shut down on a connection oriented socket. In this case the process will also receive a SIGPIPE unless MSG_NOSIGNAL is set.

CONFORMING TO
4.4BSD, SVr4, POSIX 1003.1g draft (these function calls appeared in 4.2BSD).

NOTE
The prototypes given above follow the Single Unix Specification, as glibc2 also does; the flags argument was `int' in BSD 4.*, but `unsigned int' in libc4 and libc5; the len argument was `int' in BSD 4.* and libc4, but `size_t' in libc5; the tolen argument was `int' in BSD 4.* and libc4 and libc5. See also accept(2).

SEE ALSO
fcntl(2), recv(2), select(2), getsockopt(2), sendfile(2), socket(2), write(2), socket(7), ip(7), tcp(7), udp(7)