Theoretically I can say that
free(ptr);
free(ptr);
is a memory corruption since we are freeing the memory which has already been freed.
But what if
free(ptr);
ptr=NULL;
free(ptr);
As the OS will behave in an undefined manner I cannot get an actual theoretical analysis for this about what's happening. Whatever I am doing, is this memory corruption or not?
Is freeing a NULL pointer valid?
not memory corruption, but behavior depends on implementation. By standard, it should be a legal code.
If ptr is NULL, no operation is performed.
says the documentation.
free(NULL) is perfectly legal in C as well as delete (void *)0 and delete[] (void *)0 are legal in C++.
BTW, freeing memory twice usually causes some kind of runtime error, so it does not corrupt anything.
free(ptr);
ptr=NULL;
free(ptr);/*This is perfectly safe */
You can safely delete a NULL pointer. No operation will be performed in that case.In other words free() does nothing on a NULL pointer.
7.20.3.2 The
freefunctionSynopsis
#include <stdlib.h>
void free(void *ptr);
Description
The
freefunction causes the space pointed to byptrto be deallocated, that is, made available for further allocation. Ifptris a null pointer, no action occurs.
See ISO-IEC 9899.
All standards compliant versions of the C library treat free(NULL) as a no-op.
That said, at one time there were some versions of free that would crash on free(NULL) which is why you may see some defensive programming techniques recommend:
if (ptr != NULL)
free(ptr);
Recomended usage:
free(ptr);
ptr = NULL;
See:
man free
The free() function deallocates the memory allocation pointed to by ptr.
If ptr is a NULL pointer, no operation is performed.
When you set the pointer to NULL after free() you can call free() on it again and no operation will be performed.