View Source: sendmsg(2) - Waikato Linux Users Group

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

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

 __int sendmsg(int__ ''s''__, const struct msghdr *__''msg''__, int__ ''flags''__);__

!!DESCRIPTION
sendmsg(2) is used to transmit a message to another socket. sendmsg(2) may be used at any time, the socket does not have to be connected.

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 sendmsg(2). Locally detected errors are indicated by a return value of -1.

When the message does not fit into the send buffer of the socket, sendmsg(2) 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).

!!RETURN VALUE

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).

__MSG_CONFIRM__ is a Linux extension.
!!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), send(2), sendto(2)
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Last edited on Tuesday, July 15, 2003 11:07:11 pm by PerryLorier
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