Question

I’m not sure whats wrong with the program below, but it doesn’t print each and every language once, but randomly some more often, some more less and some wont print

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



char *messages[] = { 
    "English: Hello World!", 
    "French: Bonjour, le monde!",
    "Spanish: Hola al mundo", 
    "Klingon: Nuq neH!",
    "German: Guten Tag, Welt!", 
    "Russian: Zdravstvytye, mir!",
    "Japan: Sekai e konnichiwa!", 
    "Latin: Orbis, te saluto!"
};
#define NUM_MESSAGES (sizeof( messages ) / sizeof( char* ))


void *PrintHello( void *messageid )
{
    pthread_t taskid;
    int *id_ptr, message_num;

    taskid = pthread_self();
    printf( "This is thread with ID %lu.\n", taskid );

    message_num = *((int *) messageid);
    printf( "%s \n", messages[message_num] );

    pthread_exit( NULL );
}

int main( int argc, char *argv[] ) 
{
    pthread_t threads[NUM_MESSAGES];
    int rc, i;


    for( i = 0; i < NUM_MESSAGES; i++ ) {
        void * argument = (void*) &i;
        rc = pthread_create( &threads[i], NULL, PrintHello, argument );
        if( rc ) {
            printf( "ERROR; return code from pthread_create() is %d\n", rc );
            exit( -1 );
        }
    }




    for( i = 0; i < NUM_MESSAGES; i++ ) {
        pthread_join( threads[i], NULL );
    }

    pthread_exit( NULL );
}

I guess the problem is somehow connected with the argument pointer. I tried to lock different parts but with no success.

Answer 1

You’re passing the address of the variable i, which the main thread keeps changing. So you’re at the mercy of scheduling: will anyone change i before your thread has a chance to run ?

Instead, you could try directly passing the string, greatly simplifying the code:

rc = pthread_create( &threads[i], NULL, PrintHello, messages[i]);

void *PrintHello(void *arg)
{
    char *msg = arg;
    printf("%s\n", msg);

    return NULL;
}

---

There’s another, poorer alternative where you pass the actual value of i as the argument (not its address).