SEND

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

#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);

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 is a flagword and can contain the following flags:

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

Dont't use a gateway to send out the packet, only send to hosts on directly connected networks. This is usually used only by diagnostic or routing programs. This is only defined for protocol families that route; packet sockets don't.

MSG_DONTWAIT

Enables non-blocking operation; if the operation would block, EAGAIN is returned (this can also be enabled using the O_NONBLOCK with the F_SETFL fcntl(2)).

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.

MSG_CONFIRM (Linux 2.3+ only)

Tell the link layer that forward process happened: you got a successful reply from the other side. If the link layer doesn't get this it'll regularly reprobe the neighbour (e.g. via a unicast ARP). Only valid on SOCK_DGRAM and SOCK_RAW sockets and currently only implemented for IPv4 and IPv6. See arp(7) for details.

The definition of the msghdr structure follows. See recv(2) and below for an exact description of its fields.

struct msghdr {
    void         * msg_name;     /* optional address */
    socklen_t    msg_namelen;    /* size of address */
    struct iovec * msg_iov;      /* scatter/gather array */
    size_t       msg_iovlen;     /* # elements in msg_iov */
    void         * msg_control;  /* ancillary data, see below */
    socklen_t    msg_controllen; /* ancillary data buffer len */
    int          msg_flags;      /* flags on received message */
};

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).

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

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.

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

MSG_CONFIRM is a Linux extension.

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).

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