Shared Output: Use semaphore

We use semaphore to provide mutual exclusion to the standard error. If the process is using, the another process will wait until the semaphore is unlocked.

After we use semaphore

$ gcc process-1.c -pthread -o process-1
$ gcc process-2.c -pthread -o process-2
$ ./process-1 & ./process-2 &

/*process-1.c*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <semaphore.h>
#include <unistd.h>
int main(int argc, char * argv[]) {
       sem_t * mutex;
       char * c = "This is CSCI3150--An operating system course.\n";
    // specify no buffering for stderr
    setbuf(stderr, NULL);
       mutex = sem_open("mutex_for_stderr", O_CREAT, 0666, 1);
       sem_wait(mutex);
       while (* c != '\0') {
            fputc(* c, stderr);
            c++;
            sleep(1);
         }
    sem_post(mutex);
    sem_close(mutex);
    sem_unlink("mutex_for_stderr");
    return 0;
}

/*process-2.c*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <semaphore.h>
#include <unistd.h>
int main(int argc, char * argv[]) {
    sem_t * mutex;
    char * c = "This is CSCI3150--An operating system course.\n";
    // specify no buffering for stderr
    setbuf(stderr, NULL);  
    mutex = sem_open("mutex_for_stderr", O_CREAT, 0666, 1);
    sem_wait(mutex);
    while (* c != '\0') {
        fputc(* c, stderr);
        c++;
        sleep(rand()%2+1);
    }
    sem_post(mutex);
    sem_close(mutex);
    sem_unlink("mutex_for_stderr");
    return 0;
}