Using semaphore for multithreading in C

Question

The Idea here is to create a file to be written to. I'm trying to create ten threads and have them print to the file 10 times each. Using a semaphore to stop multiple threads from writing to the file at once. Everything compiles and I don't get an error exit, however I can't understand why running the program numerous times: 1)It doesn't print 100 lines to the file, infact it's far less 2) The number of lines printed to the file vary each time.

    #include <stdio.h>
    #include <sys/types.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <pthread.h>
    #include <semaphore.h>

    #define READ    "r"
    #define NEW     "w"
    #define ADD     "a"
    #define NL      "\n"
    #define TAB     "\t"

    #define FNAME   "PROCTAB.txt"
    #define MAX_STRING_LEN  80
    #define NUMBER_OF_THREADS       10
    FILE *fp;
    sem_t mutex;
    int counter;

    FILE *makeTextFile(char *fname, char mode){
       FILE *localFP;
       localFP = fopen(fname, &mode);
       return (localFP);
}

    void *print_message(void *tid){
        int i;
        for (i = 0; i < 10; i++){
            sem_wait(&mutex);
            fp = fopen(FNAME, ADD);
            fprintf(fp, "Thread %d is running.\n", tid);
            fclose(fp);
        sem_post(&mutex);
     }
    }

    int threads(){
     const char *fName = "PROCTAB.txt";
         int status;
         pthread_t threads[NUMBER_OF_THREADS];
      fp = makeTextFile(FNAME, 'w');
         fprintf(fp, "Process ID: %ld\n", (long)getpid());
         fclose(fp);
     int i;
         for (i =0; i < NUMBER_OF_THREADS; i++){
            status = pthread_create(&threads[i], NULL, &print_message, (void *)i);
            if (status != 0){
                printf("pthread_create returned error code %d\n", status);
                exit(-1);
                 }
            }
      return 0;
   }

My main function is contained in a separate file.

Test fp in fp = fopen(FNAME, ADD); to insure you did open the file successfully all the times you think you did. Also make fp local to print_message(). — chux
– chux, Commented Jun 5, 2013 at 22:50

Mzf · Accepted Answer · 2013-06-05 22:46:13Z

2

You need to wait for all thread to finish before you exit the program.

if you add trace you will see which thread is finish.

void *print_message(void *tid){
    int i;
    for (i = 0; i < 10; i++){
        sem_wait(&mutex);
        fp = fopen(FNAME, ADD);
        fprintf(fp, "Thread %d is running.\n", tid);
        fclose(fp);
    sem_post(&mutex);
    printf ( "Thread %d has finished.\n", tid);
 }
}

This is how you wait for all threads to finish

   /* Wait for Threads to Finish */
    for (i=0; i<NUMTHREADS; i++) {
        pthread_join(thread[i], NULL);
    }

answered Jun 5, 2013 at 22:46

Mzf

5,2902 gold badges27 silver badges38 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user1822789 Over a year ago

Ah hah, I think that was a big issue was that it wasn't waiting for each thread to finish. Thanks

Mzf Over a year ago

So, it answer your question ?

Collectives™ on Stack Overflow

Using semaphore for multithreading in C

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related