Is Msgsnd blocked?

Is Msgsnd blocked?

The msgsnd(2) and msgrcv(2) functions can be performed as either blocking or non-blocking operations. A blocked message operation remains suspended until one of the following three conditions occurs: The call succeeds.

What is Msgsnd?

The msgsnd() function is used to send a message to the queue associated with the message queue identifier specified by msgid. The structure member, mtype, must be a nonzero positive value that can be used by the receiving process for message selection.

What does Msgsnd return in C?

The msgsnd() function returns immediately with a return value of -1 and errno set to EAGAIN.

What does IPC_NOWAIT do?

IPC_NOWAIT. If the message queue is full, then the message is not written to the queue, and control is returned to the calling process. If not specified, then the calling process will suspend (block) until the message can be written.

How do I increase the message queue size in Linux?

According to documentation, /proc/sys/fs/mqueue/msg_max can be used in order to increase the limit of messages in the queue. The documentation also says, that the limit should not exceed HARD_MSGMAX , which is 65,536 since Linux 3.5.

What is message queue in Linux?

Message queues allow one or more processes to write messages which will be read by one or more reading processes. Linux maintains a list of message queues, the msgque vector; each element of which points to a msqid_ds data structure which fully describes the message queue.

Is Mq_receive blocking?

If the specified message queue is empty and O_NONBLOCK is not set in the message queue description associated with mqdes, mq_receive() shall block until a message is enqueued on the message queue or until mq_receive() is interrupted by a signal.

What is Msgctl in C?

The msgctl() function allows the caller to control the message queue specified by the msqid parameter. A message queue is controlled by setting the cmd parameter to one of the following values: IPC_RMID (0x00000000) Remove the message queue identifier msqid from the system and destroy any messages on the message queue.

Why is MQ needed?

MQ simply stands for Message Queue. You would use one when you need to reliably send a inter-process/cross-platform/cross-application message that isn’t time dependent. The Message Queue receives the message, places it in the proper queue, and waits for the application to retrieve the message when ready.

What is IPC_RMID?

IPC_RMID Immediately remove the message queue, awakening all waiting reader and writer processes (with an error return and errno set to EIDRM).

What is IPC_STAT?

Description. IPC_STAT. Copy information from the kernel data structure associated with msqid into the msqid_ds structure pointed to by buf. The caller must have read permission on the message queue.

What protocol does MQ use?

Types of transmission protocol for MQI channels

What is msgsz and msgflg?

The msgsz argument contains the size of the message in bytes, excluding the length of the message type (4 byte long). The msgflg argument can be set to 0 (ignored), or: If the message queue is full, then the message is not written to the queue, and control is returned to the calling process.

What is the difference between msgsnd and MSGP?

The first argument to msgsnd is our queue identifier, returned by a previous call to msgget. The second argument, msgp, is a pointer to our redeclared and loaded message buffer. The msgsz argument contains the size of the message in bytes, excluding the length of the message type (4 byte long). The msgflg argument can be set to 0 (ignored), or:

What happens if IPC_NOWAIT flag is on in msgflg?

If the IPC_NOWAIT flag is on in msgflg, the message will not be sent and the calling process will return immediately. msgsnd () will return -1 and set errno to EAGAIN. The condition responsible for the suspension no longer exists, in which case the message is sent.

How does msgsnd work in Linux?

The msgsnd () system call appends a copy of the message pointed to by msgp to the message queue whose identifier is specified by msqid . If sufficient space is available in the queue, msgsnd () succeeds immediately. (The queue capacity is defined by the msg_qbytes field in the associated data structure for the message queue.