note

1.mq_open函数的参数pathname应以/开始，且最多一个/

2.mq_receive的参数msg_len应大于等于attr.msgsize

3.消息队列写方写时不要求读方就绪，读方读时不要求写方就绪(和管道不同)

code

#include <fcntl.h>
#include <sys/stat.h>
#include <mqueue.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <time.h>
#include <sys/wait.h>

// mq_open\mq_close\mq_unlink\mq_getattr\mq_setattr\mq_send\mq_receive\mq_notify

const char* msq_queue_instance = "/posix_msg_queue";
mqd_t msg_queue_id = 0;
long msglen = 0;

void parent_work(void) {
    int i = 0;
    time_t t = 0;
    int ret = -1;
    char* sendBuf = (char*)malloc(msglen);
    for (i = 0; i < 10; i++) {
        memset(sendBuf, 0, msglen);
        time(&t);
        sprintf(sendBuf, "i am parent process,%s", ctime(&t));
        ret = mq_send(msg_queue_id, sendBuf, strlen(sendBuf), 0);
        if (ret == -1) {
            fprintf(stderr, "parent process mq_send error,%s\n", strerror(errno));
        }
		else if (ret == 0) {
			fprintf(stdout, "parent process mq_send success\n");
		}
        sleep(1);
    }
    
    wait(NULL);
	free(sendBuf);
    (void)mq_unlink(msq_queue_instance);
	exit(EXIT_SUCCESS);
}

void child_work(void) {
    int i = 0;
    int ret1 = -1;
    ssize_t ret2 = -1;
    struct mq_attr attr;
    char* rcvBuf = (char*)malloc(msglen);
    for (i = 0; i < 10; ++i) {
        //fprintf(stdout, "child process in loop\n");
        ret1 = mq_getattr(msg_queue_id, &attr);
        if (ret1 == 0) {
			fprintf(stdout, "flag:%ld,maxmsg:%ld,msgsize:%ld,curmsgs:%ld\n", attr.mq_flags, attr.mq_maxmsg, attr.mq_msgsize, attr.mq_curmsgs);
            memset(rcvBuf, 0, msglen);
            ret2 = mq_receive(msg_queue_id, rcvBuf, msglen, 0);
            if (ret2 >= 0) {
                fprintf(stdout, "child process recieve msg:%s\n", rcvBuf);
            }
			else if (ret2 == -1) {
				fprintf(stderr, "child process mq_receive error,%s\n", strerror(errno));
			}
        }
		else if (ret1 == -1) {
			fprintf(stderr, "child process mq_getattr error,%s\n", strerror(errno));
		}
        sleep(1);
    }
	free(rcvBuf);
	exit(EXIT_SUCCESS);
}

int main(int argc, char** argv) {
    int ret = -1;
    struct mq_attr attr;
    pid_t pid = 0;

    msg_queue_id = mq_open(msq_queue_instance, O_CREAT|O_RDWR, 666, NULL);  // 默认属性
    if (msg_queue_id == -1) {
        fprintf(stderr, "parent process mq_open error,%s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }

    // 设置消息队列属性
    attr.mq_flags = 0;
	attr.mq_maxmsg = 100;
	attr.mq_msgsize = 1024;
	attr.mq_curmsgs = 0;
	ret = mq_setattr(msg_queue_id, &attr, NULL);
	if (ret == -1) {
        fprintf(stderr, "parent process mq_setattr error,%s\n", strerror(errno));
        (void)mq_unlink(msq_queue_instance);
        exit(EXIT_FAILURE);
	}
	
	
	ret = mq_getattr(msg_queue_id, &attr);
	if (ret == -1) {
        fprintf(stderr, "parent process mq_getattr error,%s\n", strerror(errno));
        (void)mq_unlink(msq_queue_instance);
        exit(EXIT_FAILURE);
	}
	msglen = attr.mq_msgsize;

    pid = fork();
    if (pid < 0) {
        fprintf(stderr, "parent process fork error,%s\n", strerror(errno));
        (void)mq_unlink(msq_queue_instance);
        exit(EXIT_FAILURE);
    }

    if (pid > 0) {
		fprintf(stdout, "parent process:%d\n", getpid());
        parent_work();
    }
    else if (pid == 0) {
		fprintf(stdout, "child process:%d\n", getpid());
        child_work();
    }

    return 0;
}

test