Passing a pointer to a pointer in a procedure in C

Question

I'm trying to pass a pointer to a pointer to a procedure, but i get a segfault everytime, while passing it to a function works perfectly fine. I guess it may have something to do with the coercion C does automatically on arrays to pointers, such as in this question : passing a pointer to a pointer in C .

But i don't know how to fix it.

Here's the code :

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

void creer_matrice(int nbCol, int nbLigne, char **matrice)
{
     int i,j;

     matrice = calloc( nbCol, sizeof(char*));

     if( matrice == NULL ){

         printf("Allocation impossible");
         exit(EXIT_FAILURE);

     }

     for( i = 0 ; i < nbLigne ; i++ ){

          matrice[i] = calloc (nbCol, sizeof(char*));

            if( matrice[i] == NULL ){

             printf("Allocation impossible");
             exit(EXIT_FAILURE);

             }
      }


      /* Remplissage de test*/
     for(i = 0; i < nbLigne; i++){

           for(j = 0; j < nbCol; j++){
             matrice[i][j] = 'I';
           }

     }

   //return matrice;
}


int main(){
    int i,j;

    char **matrice;

    creer_matrice(8,6,matrice);

    //matrice = creer_matrice(8,6);

      for(i = 0; i < 6; i++){
         for(j = 0; j < 8; j++){
            printf("%c ",matrice[i][j]);
         }
      printf("\n");
      }

}

Can somebody please tell me where i am wrong and how to solve it ?

You might want to write your code with english variables and comments. — LtWorf, Feb 03 '13 at 13:44
Note: there is no distinction between "function" and "procedure" in C... — Oliver Charlesworth, Feb 03 '13 at 13:51

m0skit0 · Answer 1 · 2013-02-03T16:04:46.320

0

matrice = calloc( nbCol, sizeof(char*));

Here you're not allocating enough memory for your matrix. You need to allocate the memory depending on the size of your matrix and the type you store in it:

matrice = calloc(nbCol * nbLigne, sizeof(int));

And remove

matrice[i] = calloc (nbCol, sizeof(char*));

Try to do as less system calls as possible. If you can allocate all matrix in a single call, do it. You already know the exact size to be allocated at the start of your function.

And don't forget to free your allocated memory once you're finished.

free(matrice);

edited Feb 03 '13 at 16:04

answered Feb 03 '13 at 13:55

m0skit0

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Thanks for your reply; I forgot to change the allocation. – Prewitt Feb 03 '13 at 14:02
I also thought about allocating in one line, but the problem is, i need to access it as a matrix (such as matrice[i][j]). – Prewitt Feb 03 '13 at 14:04
Doesn't matter, one allocation is enough. I suggest you check a good C tutorial. – m0skit0 Feb 03 '13 at 14:18
Ok, thanks. Do you think there's an error in the argument i'm passing to the procedure ? – Prewitt Feb 03 '13 at 14:23
No, it's ok, but don't forget to free the memory when you're done (see my answer edit). – m0skit0 Feb 03 '13 at 16:04

yattering · Accepted Answer · 2013-02-03T14:01:40.153

0

Change this lines:

 for( i = 0 ; i < nbLigne ; i++ ){

      matrice[i] = calloc (nbCol, sizeof(char*));

To:

 for( i = 0 ; i < nbCol ; i++ ){

      matrice[i] = calloc (nbLinge, sizeof(char));

Or event better: allocate memory for whole matrix in one statement.

edited Feb 03 '13 at 14:01

answered Feb 03 '13 at 13:56

yattering

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Passing a pointer to a pointer in a procedure in C

2 Answers2