当我在执行 mq_send()/mq_receive() 之前和之后使用“cat /dev/mqueue/myqueue”检查消息队列的 QSIZE 时，mq_recieve() 之后似乎有一些剩余字节留在队列中。我的小测试程序如下：
#include <stdio.h>
#include <stdlib.h>
#include <mqueue.h>
#define MSG_LEN 8192 // As per mq_msgsize as returned by mq_getattr() on CentOS 7 x86_64
int main(void)
{
mqd_t mq;
/* Create a message queue */
if ((mq = mq_open("/myqueue", O_RDWR|O_NONBLOCK|O_CREAT|O_EXCL, S_IRUSR|S_IWUSR, NULL)) == -1) {
perror("Error calling mq_open()");
exit(EXIT_FAILURE);
}
/* Create a message */
char msg[MSG_LEN] = {" "};
int msglen = snprintf(msg, MSG_LEN, "Hello, World!"); // msglen is 13 without the terminating '\0'
if (msglen < 0) {
fprintf(stderr, "Error calling snprintf()\n");
exit(EXIT_FAILURE);
}
/*
* QSIZE here using "cat /dev/mqueue/myqueue" is: 0
* OK
*/
/* Send the message */
if (mq_send(mq, msg, msglen + 1, 1) == -1) { // add 1 to msglen for the termining '\0'
perror("Error calling mq_send()");
exit(EXIT_FAILURE);
}
/*
* QSIZE here using "cat /dev/mqueue/myqueue" is: 62
* Why not 14?
*/
/* Receive the message */
if (mq_receive(mq, msg, MSG_LEN, NULL) == -1) {
perror("Error calling mq_receive()");
exit(EXIT_FAILURE);
}
/*
* QSIZE here using "cat /dev/mqueue/myqueue" is: 48
* Why not 0?
*/
exit(EXIT_SUCCESS);
}
我不明白为什么最初将 62 字节放入队列中，而在发送和接收 14 字节消息后留下了 48 字节的剩余部分。任何帮助将非常感激。
亲切的问候
约翰·达菲