mq_send() / mq_receive()

Purpose

Sends and receives messages through a POSIX message queue.

Header

#include <mqueue.h>

Prototype

int mq_send(mqd_t mqdes, const char *msg_ptr, size_t msg_len, unsigned int msg_prio);
ssize_t mq_receive(mqd_t mqdes, char *msg_ptr, size_t msg_len, unsigned int *msg_prio);

Parameters

• mqdes: message queue descriptor.
• msg_ptr: message buffer.
• msg_len: message length. For receive, buffer size must be at least mq_msgsize.
• msg_prio: message priority.

Return Value

• mq_send(): returns 0 on success, -1 on failure.
• mq_receive(): returns received byte count on success, -1 on failure.

Minimal Example

char buf[128];
unsigned int prio;
ssize_t n = mq_receive(mq, buf, sizeof(buf), &prio);

Common Pitfalls

• The receive buffer must be large enough for the queue message size.
• Non-blocking queues return EAGAIN when empty or full.
• POSIX mqueue descriptors can be monitored by epoll() on Linux.

Related Topics

• IPC