c - What happens to the data in memory deallocated by free()?

Question

What happens to the data that is present in a memory location that has just been freed by a free() ? Is that data also deleted and the memory will now have a garbage value ? Or that data still persists there untill a new data is stored in that memory location (in future) ?

I mean, for code below:

int *ptr;
ptr = malloc(sizeof(int));
*ptr = 1;
 // Suppose ptr = 2000
 //Free now
free(ptr);
// My question is what is the value stored in memory address 2000 now ?
// Is it still '1' or some garbage value ?

Answer 1

The result is unpredictable. There are several options that can happen. The point is that you cannot rely on any behavior of the memory released by free()

Some examples:

• the memory can be untouched (remain the same as it is with the same data).
• It can be given to another memory allocation, in which case it can be written over at any point.
• It can be zeroed.
• The page containing the memory can be returned to the OS, removing it from the memory map of your process, making your program crash if you try to access it.