C free(): invalid pointer allocated in other function

Question

I\'m new in StackOverflow. I\'m learning C pointer now.

This is my code:

#include 
#include 

int alloc(int* p){
    p         


        

Answer 1

You are passing the pointer by value into your alloc function. Although that function takes a pointer to an int, that pointer itself cannot be modified by the function. If you make alloc accept **p, set *p = ..., and pass in &pointer from main, it should work.

#include 
#include 
int alloc(int** p){
  *p = (int*) malloc (sizeof(int)); 
  if(!*p){
    puts("fail\n"); 
    return 0; 
  } 
  **p = 4; 
  printf("%d\n",**p);
  return 1; 
} 
int main() { 
  int* pointer; 
  if(!alloc(&pointer)){ 
    return -1;
  } else { 
    printf("%d\n",*pointer);
  } 
  free(pointer);
  return 0;
}