Technical Reference: Kernel and Subsystems, Volume 1

kmsgsnd Kernel Service

Purpose

Sends a message using a previously defined message queue.

Syntax

#include <sys/types.h>
#include <sys/errno.h>
#include <sys/ipc.h>
#include <sys/msg.h>

int kmsgsnd (msqid, msgp, msgsz, msgflg) int msqid; struct msgbuf * msgp; int msgsz, msgflg;

Parameters

msqid	Specifies the message queue ID that indicates which message queue the message is to be sent on.
msgp	Points to an msgbuf structure containing the message. The msgbuf structure is defined in the /usr/include/sys/msg.h file.
msgsz	Specifies the size of the message to be sent in bytes. The msgsz parameter can range from 0 to a system-imposed maximum.
msgflg	Specifies the action to be taken if the message cannot be sent for one of several reasons.

Description

The kmsgsnd kernel service sends a message to the queue specified by the msqid parameter. The kmsgsnd kernel service provides the same functions for user-mode processes in kernel mode as the msgsnd subroutine performs for kernel processes or user-mode processes in user mode. The kmsgsnd service can be called by a user-mode process in kernel mode or by a kernel process. A kernel process can also call the msgsnd subroutine to provide the same function.

There are two reasons why the kmsgsnd kernel service cannot send the message:

The number of bytes already on the queue is equal to the msg_qbytes member.
The total number of messages on all queues systemwide is equal to a system-imposed limit.

There are several actions to take when the kmsgsnd kernel service cannot send the message:

If the msgflg parameter is set to IPC_NOWAIT, then the message is not sent, and the kmsgsnd service fails and returns an EAGAIN value.
If the msgflg parameter is 0, then the calling process suspends execution until one of the following occurs:
- The condition responsible for the suspension no longer exists, in which case the message is sent.
- The message queue ID specified by the msqid parameter is removed from the system. When this occurs, the kmsgsnd service fails and an EIDRM value is returned.
- The calling process receives a signal that is to be caught. In this case, the message is not sent and the calling process resumes execution as described in the sigaction kernel service.

Execution Environment

The kmsgsnd kernel service can be called from the process environment only.

The calling process must have write permission to perform the kmsgsnd operation.

Return Values

0	Indicates a successful operation.
EINVAL	Indicates that the msqid parameter is not a valid message queue ID.
EACCES	Indicates that operation permission is denied to the calling process.
EAGAIN	Indicates that the message cannot be sent for one of the reasons stated previously, and the msgflg parameter is set to IPC_NOWAIT.
EINVAL	Indicates that the msgsz parameter is less than 0 or greater than the system-imposed limit.
EINTR	Indicates that the kmsgsnd service received a signal.
EIDRM	Indicates that the message queue ID specified by the msqid parameter has been removed from the system.
ENOMEM	Indicates that the system does not have enough memory to send the message.

Related Information

The msgsnd subroutine.

Message Queue Kernel Services and Understanding System Call Execution in AIX 5L Version 5.2 Kernel Extensions and Device Support Programming Concepts.