How can you dereference and use a matrix you passed to a POSIX thread in C?

Question

For each cell, I want to create a thread that will multiply its current row number by its current column number. The problem is that I don't know how to properly dereference the matrix and use it. Valgrind says that there is an uninitialized value of size 8 used in fillcell. Since it's size 8 I suspect it is the pointer. How can I properly convert it?

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

#include "myutils.h"
int i = 10;
int j = 10;

int var = 0;
int w,x;


void * fillcell(void * param){

    int** value = (int**) param;

    value[x][w]= x*w;

    printf("%d \n",w*x);


}


int main(int argc, char * argv[])
{
    pthread_t * tid;
    tid = malloc(i*j*sizeof(pthread_t));
    
    int A[i][j];

    for ( x = 0; x < i; x++){
        for( w = 0; w < j; w++){
            
            pthread_create(&tid[var],NULL , fillcell,&A);
            pthread_join(tid[var],NULL);  
            var++:
        }
    }

    free(tid);
}

You should absolutely read some [pthread tutorial](https://computing.llnl.gov/tutorials/pthreads/) — Basile Starynkevitch, Feb 14 '18 at 17:46
Possible duplicate of [Create a pointer to two-dimensional array](https://stackoverflow.com/questions/1052818/create-a-pointer-to-two-dimensional-array) — Tsyvarev, Feb 15 '18 at 10:27

Useless · Accepted Answer · 2018-02-14T20:40:08.977

int i = 10;
int j = 10;

int var = 0;
int w,x;

Globals: just say no.

They're not generally a great idea in single-threaded programs, but using mutable globals (which start off uninitialized) in multiple threads is ... risky.

void * fillcell(void * param) {
    int** value = (int**) param;
    value[x][w]= x*w;

your program has a single variable x, and a single variable w. They're shared between all threads. Creating a thread doesn't create a snapshot of all global variables for the new thread's use, they're all just shared. There is absolutely no way to know, in this program, what values of x and w a given thread will see in this function.

You can fix this by passing an explicit {x, w, A} parameter structure to each thread - and I mean a different instance of that structure for each thread.

Finally, int A[10][10] is not an int**, and it doesn't decay to one. To prove this, just print the address of each A[x][w] and then print the address of each value[x][w]. See how your 2D array is really laid out. Printing all your addresses will also make it easy to understand the address valgrind is complaining about.

How can you dereference and use a matrix you passed to a POSIX thread in C?

1 Answers1