How to realloc a memory? I keep getting segmentation fault

Question

I keep geeting a segfault in the main function. I create a pointer to a struct I created and I pass it's reference to another function which allocates and reallocates memory. Then accessing it in the main function (where it was originally defined) causes a segfault.

Here is my test code:

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

typedef struct {
  char desc[20];
  int nr;
} Kap;

void read(Kap *k);

int main(void) {
  Kap *k = NULL;
  read(k);

  printf("__%s %d\n", k[4].desc, k[4].nr);
  return 0;
}

void read(Kap *k) {
  int size = 3;
  k = (Kap*) calloc(size, sizeof(Kap));

  k[0].nr = 1;
  k[1].nr = 2;
  k[2].nr = 3;

  strcpy(k[0].desc, "hello0");
  strcpy(k[1].desc, "hello1");
  strcpy(k[2].desc, "hello2");

  size *= 2;
  k = (Kap*) realloc(k, sizeof(Kap)*size);
  if(k == NULL) {
    printf("ERROR\n");
    exit(EXIT_FAILURE);
  }


  k[3].nr = 4;
  k[4].nr = 5;
  k[5].nr = 6;

  strcpy(k[3].desc, "hello3");
  strcpy(k[4].desc, "hello4");
  strcpy(k[5].desc, "hello5");
}

What am I doing wrong?

Answer 1

Here's a simplified version of the problem you are having:

#include <stdio.h>

void func(int x)
{
    x = 10;
}

int main()
{
    int x = 5;
    printf("x = %d\n", x);
    func(x);
    printf("x = %d\n", x);
}

The same reason x does not change is the reason that k does not change in your program. A simple way to fix it is this:

Kap *read()
{
    Kap *k = calloc(...);
    ...
    k = realloc(k, ...);
    ...
    return k;
}

int main()
{
    Kap *k = read();
    ...
}