C - segmentation fault when accessing a pointer in header file

Question

I'm programming in C, and have two files, func2.h :

#define NN 20

void network_construction(int **veins, int *num_veins){

        int i, j;


        for(i=0;i<NN;i++){
            num_veins[i] = NN/2;
        }


        veins = malloc(NN * sizeof(*veins));
        for (i = 0; i < NN; i++) { veins[i] = malloc(num_veins[i] * sizeof(*(veins[i]))); }
        for (i = 0; i < NN; i++) { for (j = 0; j<num_veins[i];j++) { veins[i][j] = -1; } }


    return;
    }

and main.c :

#include <stdio.h>
#include <stdlib.h>
#include "func2.h"

void main(){

    int num_veins[NN];
    int **veins;


    network_construction(veins, num_veins);

    printf("\n%d\n", num_veins[19]);
    printf("\n%d\n", veins[2][2]);

return;
}

num_veins[] gives the right number, 10, but when trying to access to any elements of veins, it gives segmentation fault. How can I use in main.c the values of veins that are filled in func2.h ? What am I doing wrong and why is this happening ?

EDIT:

These codes are a simplified version of bigger ones. In the answers there is a way to solve the problem, but it is not exactly what I want. What I need is to do the dynamic allocation in func2.h , so I don't want any calculation involving veins[][] in main.c before the call to network_construction().

Answer 1

veins = malloc(NN * sizeof(*veins)); is your problem. You actually need an int*** to do what you want. Inside network_construction , the argument veins is actually a copy of the variable veins in your main . Therefore when you malloc it, only the copy changes, but not the variable in main.

A simpler example of the same problem would be this:

void foo(int a){
    a=5;
}
void foo2(int* a){
    *a=5;
}
int main(){
    int a=0;
    foo(a);
    printf("%d\n",a);
    foo2(&a);
    printf("%d\n",a);
}

which prints 0 5 . Your case is the same, but instead of int and int* , think of int** and int***

Answer 2

Your function call should be as follows:

network_construction(&veins, num_veins);

Then in the function network_construction catch veins as follows:

void network_construction(int ***veins, int *num_veins){

        int i, j;


        for(i=0;i<NN;i++){
            num_veins[i] = NN/2;
        }


        *veins = malloc(NN * sizeof(**veins));
        for (i = 0; i < NN; i++) { *veins[i] = malloc(num_veins[i] * sizeof(**(veins[i]))); }
        for (i = 0; i < NN; i++) { for (j = 0; j<num_veins[i];j++) { *veins[i][j] = -1; } }


    return;
    }

Answer 3

Your malloc in network_construction() function is the problem. Easy to overcome this thing is to modify your functions as follows:

Your func.c should be as follows:

 #define NN 20

    void network_construction(int **veins, int *num_veins){

            int i, j;

           for (i = 0; i < NN; i++) { for (j = 0; j<num_veins[i];j++) { veins[i][j] = -1; } }


        return;
        }

Your main function will be as follows:

#include <stdio.h>
#include <stdlib.h>
#include "func2.h"

void main(){

    int num_veins[NN];
    int **veins;
int i;


        for(i=0;i<NN;i++){
            num_veins[i] = NN/2;
        }


        veins = malloc(NN * sizeof(*veins));
        for (i = 0; i < NN; i++) { veins[i] = malloc(num_veins[i] * sizeof(*(veins[i]))); }

    network_construction(veins, num_veins);

    printf("\n%d\n", num_veins[19]);
    printf("\n%d\n", veins[2][2]);

return;
}

Answer 4

The code below achieves what I wanted:

void network_construction(int ***veins, int num_veins[]){
  int i,j;

  for(i=0;i<NN;i++){
    num_veins[i] = NN/2;
  }

  (*veins) = malloc((NN) * sizeof(**veins));

  for(i=0;i<NN;i++){
    (*veins)[i] = malloc(num_veins[i]*sizeof(*(*veins)[i]));
  }

  for (i = 0; i < NN; i++) { 
    for (j = 0; j < num_veins[i]; j++) {
      (*veins)[i][j] = -1; 
    }
  }
  return;}

To call it in main():

network_construction(&veins,num_veins);