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I'm new to multithreading so I was trying to create a basic program that creates an array of size 1000 of random integers and create two threads, one that sorts the array ascendingly and the other sorts the array descendingly (without any concurrency control). So my problem is that the two threads does not read the proper size of the array and when I print out the array length it gives 2, so if I add another argument to the function indicating the size of the array(which is the regular fix) I won't be able to pass that extra argument to the thread created as it only takes 1 argument for the function . Here is what the code looks like : 

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

void *Function1(int *arr[]){

    int i,j,k;
    int n = ( sizeof(arr) / sizeof(int));
printf("Size of array = %d\n", sizeof(arr));
printf("Size of int = %d\n", sizeof(int));
printf("no. of elements in array = %d\n", n);

    for(i=0;i<n;i++){
        for(j=0;j<n-i;j++){
            if(arr[j]>arr[j+1]){
               int tmp=arr[j];
                arr[j]=arr[j+1];
                arr[j+1]=tmp;
            }
        }

    }
    for(k=0; k<n; k++){
        printf("a[%d] is %d  Thread#1 \n", k, arr[k]);
   }
}


void *Function2(int *arr[]){

    int i,j,k;
    int n = ( sizeof(arr) / sizeof(int));
printf("Size of array = %d\n", (int)sizeof(arr));
printf("Size of int = %d\n", sizeof(int));
printf("no. of elements in array = %d\n", n);
    for (i = 0; i < n; i++){
        for (j = i + 1; j < n; j++){
            if (arr[i] < arr[j])
            {
                int tmp = arr[i];
                arr[i] = arr[j];
                arr[j] = tmp;
            }
        }

    }
    for(k=0; k<n; k++){
        printf("a[%d] is %d  Thread#2\n", k, arr[k]);
   }
}

int main(){

int i,j;
int array[1000];
    srand(time(NULL));
    for(i=0; i<1000;i++){
        array[i] = (rand() % 1000) +1;  
    }

pthread_t t1,t2;
pthread_create(&t1, NULL, Function1, array);
pthread_create(&t2, NULL, Function2, array);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
return 0;
}

Here is what gets printed :

Size of array = 8
Size of array = 8
Size of int = 4
Size of int = 4
no. of elements in array = 2
no. of elements in array = 2
a[0] is 365  Thread#1 
a[0] is 365  Thread#2
a[1] is 72  Thread#1 
a[1] is 72  Thread#2
Code Warrior
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  • `sizeof(arr)` give the size of a pointer (to an `int`), you can't work out the number of elements like that unless the array is directly in the function's scope. Arrays are not passed by copy but by reference, and you just get the reference (pointer). The usual way is to pass the number of elements as another parameter to the function. – cdarke Jul 21 '17 at 20:38
  • Thanks for your reply, but then how would i pass that extra parameter to the thread since pthread_create takes only one argument for the function (as far i know). – Code Warrior Jul 21 '17 at 20:52
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    @CodeWarrior pass the address of a malloced struct containing fields for all you need for the thread work, inputs, results, error-messages:) – Martin James Jul 21 '17 at 21:03
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    Re, "...how would i pass that extra parameter...?" That is a problem that you will encounter over and over again: A library that tunnels one argument from somewhere to somewhere else, and you want to pass more than one thing through the tunnel. The answer is the same every time: Declare a struct. Give the struct named fields for all of the args that you want to pass, and then pass [a pointer to] the whole struct through the tunnel. The struct only counts as one "thing". – Solomon Slow Jul 21 '17 at 23:18
  • @CodeWarrior: just saw your reply. I really can't add anything to the other comments - pass a pointer to a struct. – cdarke Jul 23 '17 at 07:09

0 Answers0