Thread: Semaphors Understanding/Semafor with fork()

Hi, I need to help with semafor implementation understanding. I meant I understood the logic of it based on available sources, but during implementation I am not able to get the desire outcome.

My goal is to lead with the help of semafor writing into pipes.

This is my implementation:

Code:

#include <stdio.h>#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <semaphore.h>
#include <errno.h>
#include <string.h>
#include <sys/wait.h>


#define SEM_WRITE "SemaphorForControlWrite"


int main()
{
    if(mkfifo("mySemaphorPipe", 0777) == -1)
    {
        if(errno != EEXIST)
        {
            printf("Something is wrong in Create...\n");
            return 1;
        }
    }


    sem_unlink(SEM_WRITE);


    sem_t *sem_PipeWrite = sem_open(SEM_WRITE, IPC_CREAT, 0660, 0);


    if(fork()==0)
    {
        // Create few children
        fork();
        fork();
        fork();


        while ( 1 )
        {   
            sem_wait(sem_PipeWrite);
            int fd = open("mySemaphorPipe", O_WRONLY);
            if (fd == -1)
            {
                printf("Something is wrong in Open...\n");
                return 1;
            }


            char buf[ BUFSIZ ];
            srand(getpid());
            sprintf( buf, "Process: %d - Send Number: %d", getpid(), rand() % 10000 );
            
            if(write( fd, buf, strlen(buf))==-1)
            {
                printf("Write ERRoR of process %d\n", getpid());
                fflush(stdout);
            }
            //sleep (1);
            close(fd);
            sem_post(sem_PipeWrite);
        }
        return 0;
    }


    printf("READ Pipe ... \n");
    fflush(stdout);


    while(1)
    {
        int fd = open("mySemaphorPipe", O_RDWR);
        if (fd == -1)
        {
            printf("Something is wrong in Open...\n");
            return 1;
        }
        char recievedData[1024]="";
        if(read(fd, &recievedData, sizeof(recievedData)) == -1)
        {
            printf("Something is wrong in read...\n");
            return 1;
        }
        printf("I get from pipe: %s", recievedData);
        fflush(stdout);
        close(fd);
     }


    return 0;
}

anzwaz if I use semafors, I will gain data only from one process and I cannot figure out where I am doing mistake in my logic.
Could I ask for help and need to explain where I am doing mistake and how to avoid it.

How many processes do you want? Because you 7 running, I think.

Originally Posted by hamster_nz

How many processes do you want? Because you 7 running, I think.

I don't care about a specific count. And in this case i think i have 8+1 process.

Oh, you also need to have a non-blocking open on the pipe:

Code:

       int fd = open("mySemaphorPipe", O_RDWR|O_NONBLOCK);

From "man 7 fifo"

The FIFO must be opened on both ends (reading and writing) before data can be passed. Normally, opening the FIFO blocks until the other end is opened also.