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Man Pages


Manual Reference Pages  -  MSGSND (2)

NAME

msgsnd - send a message to a message queue

CONTENTS

Library
Synopsis
Description
Return Values
Errors
History
Bugs

LIBRARY


.Lb libc

SYNOPSIS


.In sys/types.h
.In sys/ipc.h
.In sys/msg.h int msgsnd int msqid const void *msgp size_t msgsz int msgflg

DESCRIPTION

The msgsnd function sends a message to the message queue specified in msqid. The msgp argument points to a structure containing the message. This structure should consist of the following members:
    long mtype;    /* message type */
    char mtext[1]; /* body of message */

mtype is an integer greater than 0 that can be used for selecting messages (see msgrcv(2)), mtext is an array of msgsz bytes. The argument msgsz can range from 0 to a system-imposed maximum, MSGMAX.

If the number of bytes already on the message queue plus msgsz is bigger than the maximum number of bytes on the message queue ( msg_qbytes, see msgctl(2)), or the number of messages on all queues system-wide is already equal to the system limit, msgflg determines the action of msgsnd. If msgflg has IPC_NOWAIT mask set in it, the call will return immediately. If msgflg does not have IPC_NOWAIT set in it, the call will block until:

  • The condition which caused the call to block does no longer exist. The message will be sent.
  • The message queue is removed, in which case -1 will be returned, and errno is set to EINVAL.
  • The caller catches a signal. The call returns with errno set to EINTR.

After a successful call, the data structure associated with the message queue is updated in the following way:

  • msg_cbytes is incremented by the size of the message.
  • msg_qnum is incremented by 1.
  • msg_lspid is set to the pid of the calling process.
  • msg_stime is set to the current time.

RETURN VALUES


.Rv -std msgsnd

ERRORS

The msgsnd function will fail if:
[EINVAL]
  The msqid argument is not a valid message queue identifier.

The message queue was removed while msgsnd was waiting for a resource to become available in order to deliver the message.

The msgsz argument is greater than msg_qbytes.

The mtype argument is not greater than 0.

[EACCES]
  The calling process does not have write access to the message queue.
[EAGAIN]
  There was no space for this message either on the queue, or in the whole system, and IPC_NOWAIT was set in msgflg.
[EFAULT]
  The msgp argument points to an invalid address.
[EINTR]
  The system call was interrupted by the delivery of a signal.

HISTORY

Message queues appeared in the first release of AT&T Unix System V.

BUGS


.Nx and
.Fx do not define the EIDRM error value, which should be used in the case of a removed message queue.
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