Diffenret process wait for different type of message-CodePudding

Is it possible, in c, that 2 different process wait to receive a different type of message in the same queue??

Example: Process 1 wait Message type 4 and Process 2 wait Message type 3 in queue key = 0x000200

Cause i have 2 process that get the id of the queue and wait to a message but only when i remove one of the msgrcv it works.

Or maybe the problem is that i used as type = 0...?

CodePudding user response：

Or maybe the problem is that i used as type = 0...?

Yes, the sender should set the message type into (e.g.) mtype of the [given] message struct. And, the receiver should pass the desired message type as the msgtyp argument to msgrcv.

Here is some example code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/msg.h>
#include <sys/wait.h>

int qid;

typedef struct {
    long mtype;
    char mtext[1];
} mymsg_t;

// dosnd -- sender process
void
dosnd(void)
{
    mymsg_t msg;

    for (int idx = 0;  idx <= 20;    idx) {
        // set message type in message!
        msg.mtype = (idx & 1)   1;

        printf("dosnd: sending %ld\n",msg.mtype);

        msgsnd(qid,&msg,sizeof(msg.mtext),0);
    }

    exit(0);
}

// rcvone -- receive messages of a given type
void
rcvone(int type)
{
    mymsg_t msg;

    for (int idx = 0;  idx < 5;    idx) {
        msgrcv(qid,&msg,sizeof(msg.mtext),type,0);

        printf("rcvone: got %ld\n",msg.mtype);
    }
}

// rcvall -- receive all [remaining] messages of any type
void
rcvall(void)
{
    mymsg_t msg;

    while (1) {
        if (msgrcv(qid,&msg,sizeof(msg.mtext),0,IPC_NOWAIT) < 0)
            break;
        printf("rcvall: got %ld\n",msg.mtype);
    }
}

// dorcv -- receiver process
void
dorcv(void)
{

    sleep(2);
    rcvone(1);
    rcvone(2);
    rcvall();

    exit(0);
}

int
main(void)
{

    // set output line buffering (if using logfile)
    setlinebuf(stdout);

    // get queue ID (create message queue)
    qid = msgget(IPC_PRIVATE,IPC_CREAT | 0644);

    pid_t pid;

    // create sender process
    pid = fork();
    if (pid == 0)
        dosnd();

    // create receiver process
    pid = fork();
    if (pid == 0)
        dorcv();

    // wait for all processes to complete
    while (1) {
        pid = wait(NULL);
        if (pid < 0)
            break;
    }

    return 0;
}

Here is the program output:

dosnd: sending 1
dosnd: sending 2
dosnd: sending 1
dosnd: sending 2
dosnd: sending 1
dosnd: sending 2
dosnd: sending 1
dosnd: sending 2
dosnd: sending 1
dosnd: sending 2
dosnd: sending 1
dosnd: sending 2
dosnd: sending 1
dosnd: sending 2
dosnd: sending 1
dosnd: sending 2
dosnd: sending 1
dosnd: sending 2
dosnd: sending 1
dosnd: sending 2
dosnd: sending 1
rcvone: got 1
rcvone: got 1
rcvone: got 1
rcvone: got 1
rcvone: got 1
rcvone: got 2
rcvone: got 2
rcvone: got 2
rcvone: got 2
rcvone: got 2
rcvall: got 1
rcvall: got 2
rcvall: got 1
rcvall: got 2
rcvall: got 1
rcvall: got 2
rcvall: got 1
rcvall: got 2
rcvall: got 1
rcvall: got 2
rcvall: got 1