Question

I write a code to print out strings: "Thread 0" to "Thread 4" using pthread.

Here is my code:

Case 1:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

void *print_message_function(void* parameter) {
    long *i = (long *)parameter;
    printf("Thread %ld\n", *i);
    pthread_exit(0);
}

int main(int argc, char *argv[]) {
    pthread_t threads[5];
    long i = 0;
    for (i = 0; i < 5; i++) {
        pthread_create(&threads[i], 0, print_message_function, (void *)&i);
    }
    
    pthread_exit(NULL);
}

But the result is:

Thread 2
Thread 3
Thread 3
Thread 4
Thread 5

or:

Thread 0
Thread 0
Thread 0
Thread 0
Thread 0

It changed when I run it again. So I don’t know why the values I passed are (2 to 5) or all (0) or ….. (many cases). I think my arguments I passed is from 0 to 4.

Case 2:

When I change to the new code:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

void *print_message_function(void *parameter);

int main(int argc, char *argv[]) {
    pthread_t threads[5];
    int i = 0;
    for (i = 0; i < 5; i++) {
        char *msg = (char*)malloc(sizeof(char));
        sprintf(msg, "Thread %d", i);
        pthread_create(&threads[i], 0, print_message_function, (void *)msg);
    }
}

void *print_message_function(void *parameter) {
    printf("%s\n", (char *)parameter);
}

The result is:

Thread 1
Thread 0
Thread 3
Thread 2
Thread 4
Thread 4

It means the loop run 6 times! Why?

Answer 1

Change Case 1 to this:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

void *print_message_function(void* parameter) {
    long i = (long)parameter;  // <<<
    printf("Thread %ld\n", i); // <<<
    pthread_exit(0);
}

int main(int argc, char *argv[]) {
    pthread_t threads[5];
    long i = 0;
    for (i = 0; i < 5; i++) {
        pthread_create(&threads[i], 0, print_message_function, (void *)i); // <<<
    }

    pthread_exit(NULL);
}

The reason that you were seeing inconsistent results before was because you were passing a pointer to each thread where each pointer was pointing at the same local variable, which you were then modifying.

In Case 2 you are mallocing only a single char and then trying to write a string to it. It should be fairly easy to fix.