segmentation fault on wrap of realloc function

The value of p in main hasn't been changed after calling the function reallocmem, so a segmentation fault happened because the program tries to dereference the NULL pointer.

To pass a pointer as an argument and change it, we need to pass a pointer of the pointer:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int reallocmem(int **p, size_t newsize) {
  void *q;
  q = realloc(*p, newsize);
  if (q != NULL) {
    *p = q;
    return 1;
  }
  return 0;
}
int main(void) {
  int *p = 0x0;
  if (!reallocmem(&p, 10 * sizeof(int))) {
    fprintf(stderr, "Could not reallocate memory\n");
    free(p);
    return 0;
  }
  int i;
  int size = 10;
  for (i = 0; i < size; i++) {
    p[i] = i * 2;
  }

  for (i = 0; i < size; i++) {
    printf("%d\n", p[i]);
  }
  free(p);
}

Online demo

The C program passes every argument by value, if you add printf statement in your code, you will find that the p in reallocmem have a different address with the p in main, so in order to change the p in main, we need to pass the address of p(pointer to pointer) but not the value of p. Or the local copy of p in reallocmem has been changed, but it won't affect the caller function main.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int reallocmem(int *p, size_t newsize) {
  printf("%p\n", &p);
  void *q;
  q = realloc(p, newsize);
  if (q != NULL) {
    p = q;
    return 1;
  }
  return 0;
}
int main(void) {
  int *p = 0x0;
  printf("%p\n", &p);
  if (!reallocmem(p, 10 * sizeof(int))) {
    fprintf(stderr, "Could not reallocate memory\n");
  }
  int i;
  int size = 10;
  for (i = 0; i < size; i++) {
    p[i] = i * 2;
  }

  for (i = 0; i < size; i++) {
    printf("%d\n", p[i]);
  }
  free(p);
}

On top of previous answers, to avoid memory leaks in the error branches, action must be taken when realloc() returns NULL as according to the manual:

If realloc() fails, the original block is left untouched; it is not freed or moved.

So, when realloc(p, ...) returns NULL, the error branch should free what was pointed by 'p' (if it was not NULL before the call) when it is not planed to use 'p' any more...