msgsnd(2) send a message to a message queue

LIBRARY

Lb libc

SYNOPSIS

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

DESCRIPTION

The Fn msgsnd function sends a message to the message queue specified in Fa msqid . The Fa 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 Fa msgsz bytes. The argument Fa msgsz can range from 0 to a system-imposed maximum, MSGMAX

If the number of bytes already on the message queue plus Fa 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, Fa msgflg determines the action of Fn msgsnd . If Fa msgflg has IPC_NOWAIT mask set in it, the call will return immediately. If Fa 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 Er EINVAL .
  • The caller catches a signal. The call returns with errno set to Er 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 Fn msgsnd function will fail if:

Bq Er EINVAL
The Fa msqid argument is not a valid message queue identifier.

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

The Fa msgsz argument is greater than msg_qbytes

The Fa mtype argument is not greater than 0.

Bq Er EACCES
The calling process does not have write access to the message queue.
Bq Er EAGAIN
There was no space for this message either on the queue, or in the whole system, and IPC_NOWAIT was set in Fa msgflg .
Bq Er EFAULT
The Fa msgp argument points to an invalid address.
Bq Er 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 Er EIDRM error value, which should be used in the case of a removed message queue.

TRANSLATIONS

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