How do I use mqueue in a c program on a Linux based system?

The following is a simple example of a server that receives messages from clients until it receives an "exit" message telling it to stop.

The code for the server:

#include <stdlib.h>#include <stdio.h>#include <string.h>#include <sys/stat.h>#include <sys/types.h>#include <errno.h>#include <mqueue.h>#include "common.h"int main(int argc, char **argv){    mqd_t mq;    struct mq_attr attr;    char buffer[MAX_SIZE + 1];    int must_stop = 0;    /* initialize the queue attributes */    attr.mq_flags = 0;    attr.mq_maxmsg = 10;    attr.mq_msgsize = MAX_SIZE;    attr.mq_curmsgs = 0;    /* create the message queue */    mq = mq_open(QUEUE_NAME, O_CREAT | O_RDONLY, 0644, &attr);    CHECK((mqd_t)-1 != mq);    do {        ssize_t bytes_read;        /* receive the message */        bytes_read = mq_receive(mq, buffer, MAX_SIZE, NULL);        CHECK(bytes_read >= 0);        buffer[bytes_read] = '\0';        if (! strncmp(buffer, MSG_STOP, strlen(MSG_STOP)))        {            must_stop = 1;        }        else        {            printf("Received: %s\n", buffer);        }    } while (!must_stop);    /* cleanup */    CHECK((mqd_t)-1 != mq_close(mq));    CHECK((mqd_t)-1 != mq_unlink(QUEUE_NAME));    return 0;}

The code for the client:

#include <stdlib.h>#include <stdio.h>#include <string.h>#include <sys/stat.h>#include <sys/types.h>#include <mqueue.h>#include "common.h"int main(int argc, char **argv){    mqd_t mq;    char buffer[MAX_SIZE];    /* open the mail queue */    mq = mq_open(QUEUE_NAME, O_WRONLY);    CHECK((mqd_t)-1 != mq);    printf("Send to server (enter \"exit\" to stop it):\n");    do {        printf("> ");        fflush(stdout);        memset(buffer, 0, MAX_SIZE);        fgets(buffer, MAX_SIZE, stdin);        /* send the message */        CHECK(0 <= mq_send(mq, buffer, MAX_SIZE, 0));    } while (strncmp(buffer, MSG_STOP, strlen(MSG_STOP)));    /* cleanup */    CHECK((mqd_t)-1 != mq_close(mq));    return 0;}

The common header:

#ifndef COMMON_H_#define COMMON_H_#define QUEUE_NAME  "/test_queue"#define MAX_SIZE    1024#define MSG_STOP    "exit"#define CHECK(x) \    do { \        if (!(x)) { \            fprintf(stderr, "%s:%d: ", __func__, __LINE__); \            perror(#x); \            exit(-1); \        } \    } while (0) \#endif /* #ifndef COMMON_H_ */

Compiling:

gcc -o server server.c -lrtgcc -o client client.c -lrt

#include <stdio.h>#include <fcntl.h>#include <mqueue.h>int main(int argc, char *argv[]){    mqd_t mq;               // message queue    struct mq_attr ma;      // message queue attributes    int status = 0;    int a = 5;    int b = 0;    printf("a = %d, b = %d\n", a, b);    // Specify message queue attributes.    ma.mq_flags = 0;                // blocking read/write    ma.mq_maxmsg = 16;              // maximum number of messages allowed in queue    ma.mq_msgsize = sizeof(int);    // messages are contents of an int    ma.mq_curmsgs = 0;              // number of messages currently in queue    // Create the message queue with some default settings.    mq = mq_open("/test_queue", O_RDWR | O_CREAT, 0700, &ma);    // -1 indicates an error.    if (mq == -1)    {        printf("Failed to create queue.\n");        status = 1;    }    if (status == 0)    {        status = mq_send(mq, (char *)(&a), sizeof(int), 1);    }    if (status == 0)    {        status = mq_receive(mq, (char *)(&b), sizeof(int), NULL);    }    if ((status == 0) && (mq_close(mq) == -1))    {        printf("Error closing message queue.\n");        status = 1;    }    if ((status == 0) && (mq_unlink("test_queue") == -1))    {        printf("Error deleting message queue.\n");        status = 1;    }    printf("a = %d, b = %d\n", a, b);    return status;} 

mq_send(mq, (char *)(&a), sizeof(int), 1) copies sizeof(int) bytes from buffer &a in this case, it does not carry the pointer of variable a, but carries the value of variable a from one process to another process. Implementation is right.