sendmsg(2) - Waikato Linux Users Group

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).