msgsnd

Sends a message to a message queue.

Syntax

   #include <sys/types.h>

   #include <sys/ipc.h>

   #include <sys/msg.h>



   int msgsnd (int msqid, void *msgp, int msgsz,

               int msgflg);

Parameters

msqid

Passes a message queue identifier returned by a call to msgget().

msgp

Passes a pointer to a buffer whose structure is similar to the msgbuf example template located in <msg.h>, for example:

struct example_msgbuf {



   long   mtype;                  /* message type          */

   char   mtext[your_buffer_size];  /* message text          */



   };

where your_buffer_size is an integer specifying the size of the buffer. The mtype field stores a positive integer value that can be used by a receiving process for message selection (refer to msgrcv()). The mtext field stores the text of the message. The size of mtext is specified by the msgsz argument.

msgsz

The size, in bytes, of mtext. Valid values are from 0 to a system-defined limit.

msgflg

Passes a value defining what action to take if the number of bytes passed would cause the size of the specified queue to exceed the system-defined limit (msq_bytes), the total size of message data on this queue would exceed the system-defined limit, or the system-wide message buffer pool is temporarily depleted due to the amount of data queued to all message queues.

Flags are:

IPC_NOWAIT: The calling process is not suspended. Control returns immediately with errno set to EAGAIN.

If msgflg does not specify IPC_NOWAIT and one of the previous conditions would occur, the calling process suspends execution. When the condition that caused the suspension no longer exists, msgsnd() continues execution.

Return Values

0: Success.
-1: An error occurred, and errno is set to indicate the error condition.

Description

The msgsnd() function sends a message stored in the buffer pointed to by msgp to the message queue associated with msqid.

If the message queue associated with msqid is removed from the system while the calling process is suspended waiting to send a message, msgsnd() returns an error and sets errno to EIDRM .

If msgsnd() is successful, the following fields of the data structure associated with msqid are updated to the indicated values:

msg_qnum: Incremented by 1
msg_lrpid: PID of the calling process
msg_rtime: Current time

Implementation Considerations

If a process suspended during execution of msgsnd() receives a signal, control returns to the user with errno set to EINTR. Disabled signals are ignored.

Errors

If an error occurs, errno is set to one of the following values.

`EACCES`	CAUSE	The calling process does not have permission.
	ACTION	Ensure that the calling process has write permission for the message queue.
`EAGAIN`	CAUSE	`msgflg` specifies `IPC_NOWAIT` and either the number of bytes passed would cause the size of the specified queue to exceed the system-defined limit, the total size of message data on this queue would exceed the system-defined limit, or the system-wide message buffer pool is temporarily depleted due to the amount of data queued to all message queues.
	ACTION	None. Application dependent. The send operation can be retried later.
`EFAULT`	CAUSE	The system detected a NULL or bad address in attempting to use the `msgp` argument.
	ACTION	Check to see if the pointer is correctly initialized.
`EIDRM`	CAUSE	The message queue specified by `msqid` was removed while `msgsnd()` was waiting on a message.
	ACTION	None.
`EINTR`	CAUSE	`msgsnd()` was interrupted by a signal.
	ACTION	None. Application dependent.
`EINVAL`	CAUSE	`msqid` is not a valid message queue identifier, or `msgsz` is less than 0 or greater than the system-defined limit, or `mtype` is less than 0.
	ACTION	Check the parameters to make sure the `msqid` is valid and has not been removed from the system, and that the `msgsz` and `mtype` are within valid ranges.
`ESYSERR`	CAUSE	An operating system error occurred that does not map directly to any of the above errors.
	ACTION	Examine the MPE/iX process error stack for the type of system error.

msgsnd

Technical documentation

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