Question

When I run this code:

#include <stdio.h>

typedef struct _Food
{
    char          name [128];
} Food;

int
main (int argc, char **argv)
{
    Food  *food;

food = (Food*) malloc (sizeof (Food));
snprintf (food->name, 128, "%s", "Corn");

free (food);

printf ("%d\n", sizeof *food);
printf ("%s\n", food->name);
}

I still get

128
Corn

although I have freed food. Why is this? Is memory really freed?

Answer 1

When you free ‘food’, you are saying you are done with it. However, the pointer food still points to the same address, and that data is still there (it would be too much overhead to have to zero out every bit of memory that’s freed when not necessary)

Basically it’s because it’s such a small example that this works. If any other malloc calls were in between the free and the print statements, there’s a chance that you wouldn’t be seeing this, and would most likely crash in some awful way. You shouldn’t rely on this behavior.