sendmsg Subroutine

AIX Version 4.3 Communications Technical Reference, Volume 2

sendmsg Subroutine

Purpose

Sends a message from a socket using a message structure.

Library

Standard C Library (libc.a)

Syntax

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

int sendmsg (Socket, Message, Flags)
int Socket;
const struct msghdr Message [ ];
int Flags;

Description

The sendmsg subroutine sends messages through connected or unconnected sockets using the msghdr message structure. The /usr/include/sys/socket.h file contains the msghdr structure and defines the structure members. In BSD 4.4, the size and members of the msghdr message structure have been modified. Applications wanting to start the old structure need to compile with COMPAT_43 defined. The default behaviour is that of BSD 4.4.

To broadcast on a socket, the application program must first issue a setsockopt subroutine using the SO_BROADCAST option to gain broadcast permissions.

The sendmsg subroutine supports only 15 message elements.

Parameters

Socket

Specifies the socket descriptor.

Message

Points to the msghdr message structure containing the message to be sent.

Flags

Allows the sender to control the message transmission. The sys/socket.h file contains the Flags parameter. The Flags parameter used to send a call is formed by logically ORing one or both of the following values:

MSG_OOB

Processes out-of-band data on sockets that support SOCK_STREAM.

Note: The following value is not for general use. It is an administrative tool used for debugging or for routing programs.

MSG_DONTROUTE	Sends without using routing tables.
MSG_MPEG2	Indicates that this block is a MPEG2 block. It only applies to SOCK_CONN_DGRAM types of sockets only.

Return Values

Upon successful completion, the sendmsg subroutine returns the number of characters sent.

If the sendmsg subroutine is unsuccessful, the system handler performs the following functions:

Returns a value of -1 to the calling program.
Moves an error code, indicating the specific error, into the errno global variable.

Error Codes

The sendmsg subroutine is unsuccessful if any of the following errors occurs:

EBADF	The Socket parameter is not valid.
ENOTSOCK	The Socket parameter refers to a file, not a socket.
EWOULDBLOCK	The socket is marked nonblocking, and no connections are present to be accepted.

Implementation Specifics

The sendmsg subroutine is part of Base Operating System (BOS) Runtime.

All applications containing the sendmsg subroutine must be compiled with _BSD set to a specific value. Acceptable values are 43 and 44. In addition, all socket applications must include the BSD libbsd.a library.

Related Information

The on command.

The connect subroutine, getsockopt subroutine, recv subroutine, recvfrom subroutine, recvmsg subroutine, select subroutine, send subroutine, sendto subroutine, setsockopt subroutine. shutdown subroutine, socket subroutine.

Sockets Overview and Understanding Socket Data Transfer in AIX Version 4.3 Communications Programming Concepts.