Question

I’m wondering what happens to a struct member that is pointing to a non-dynamically allocated variable. So:

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

typedef struct {
    int value;
    int *pointer;
} MyStruct;

int year = 1989;

int main (int argc, const char * argv[]) {

    MyStruct *myStruct = (MyStruct *) malloc(sizeof(MyStruct));

    myStruct->value = 100;
    myStruct->pointer = &year;

    year++;

    printf("%d \n", *myStruct->pointer);

    // what happens to the myStruct->pointer member when we free myStruct ?
    free(myStruct);

    return EXIT_SUCCESS;
}

I assume it’s destroyed an no longer points to year correct? If that is the case, the same would be true if *pointer where pointing to a function right?

like this:

typedef struct {
    int value;
    void (*someFunc)();
} MyStruct;

Then later:

void sayHi(){
  printf("hi");
}

...

myStruct->someFunc = sayHi;

No special cleanup needed except free() if our struct was created with malloc? Thanks for any insights anyone has.

Answer 1

If you didn’t malloc (or calloc/strdup/realloc) it you don’t need to free it. Nothing special is needed – the member variable just points at something, it doesn’t logically “own” the pointed at memory.

Your year member variable will still exist and is perfectly valid after you free(myStruct) – myStruct->pointer will be invalid to use though