Trying to understand how pthread works in C

Question

I am trying to use pthread in C to compare two strings. The idea is to see if the complete string 2 is in string 1 (For Example, if string1 = lkajdsgl and string2 = jd then I would have one match). What I don't understand is how the pthread works in a general manner. Here I am creating my pthreads, assuming NUM_THREADS=3 , then I should have 3 threads, threads[0], threads[1], and threads[2]. Each will call on the Pthrd_Substring function. The strings will be read in from a file and analyzed in the function. However, what I don't understand is how to use pthread_join . If the string is 12 characters long and I only have 3 threads how does the system know to keep analyzing the string with the 3 threads until all 12 characters have been examined? (The function looks at each letter in string 1 and compares it with the first letter in string 2 before determining if the full string 2 is present.)

int main(int argc, char *argv[])
{
    pthread_t threads[NUM_THREADS];
    int count, rc;
    long t;

    for(t=0;t<NUM_THREADS;t++){
     printf("In main: creating thread %ld\n", t);
     rc = pthread_create(&threads[t], NULL, Pthrd_Substring, (void *)t);
     if (rc){
       printf("ERROR; return code from pthread_create() is %d\n", rc);
       exit(-1);
       }
     }

    printf("The number of substrings is: %d\n", count);
    return 1;
}

I can use something like:

pthread_join(threads0, NULL);
 partial_sum += t.partial_count;
pthread_join(threads1, NULL);
 partial_sum += t.partial_count;
pthread_join(threads2, NULL);
 partial_sum += t.partial_count;

And have a global total in the function? But, does this somehow get every letter in the string examined?

---

I was hesitant to include this part because I haven't worked it all out since I don't understand how the pthread call works exactly in the main. However, this is the pseudo-code I have for the function, here n1 is the string length of string1 and n2 is the lenght of string2 :

void *Pthrd_Substring(void *thrdptr)
{
    int i,j,k;
    int count;
    int total = 0;

    for (i = thrdptr; i <= (n1-n2); i++){       
        count=0;
        for(j = i,k = 0; k < n2; j++,k++){  /*search for the next string of size of n2*/  
            if (*(s1+j)!=*(s2+k)){
                break;
            }
            else
                count++;
            if(count==n2)    
                total++;        /*find a substring in this step*/                          
        }
    }
    partial_count = total
}

Answer 1

If the string is 12 characters long and I only have 3 threads how does the system know to keep analyzing the string with the 3 threads until all 12 characters have been examined?

The system doesn't know that - just like with non-threaded programming. If you want every character to be analyzed then you need to write your program so it analyzes every character.

However, what I don't understand is how to use pthread_join.

pthread_join just waits for a thread to exit. That's all.