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

Arguments in C are always passed by value. So, when you call alloc(pointer), you just pass in whatever garbage value pointer contains. Inside the function, the assignment p = (int*)... only modifies the local variable/argument p. Instead, you need to pass the address of pointer into alloc, like so:

int alloc(int **p) {
    *p = malloc(sizeof(int)); // side note - notice the lack of a cast
    ...
    **p = 4; // <---- notice the double indirection here
    printf("%d\n", **p); // <---- same here
    return 1;
}

In main, you would call alloc like this:

if (!(alloc(&pointer))) {
    ....

Then, your code will work.