Does free(ptr) where ptr is NULL corrupt memory?

user_134713 0
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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?

free pointers memory-management null c
10 Answers

7.20.3.2 The free function

Synopsis 

#include <stdlib.h> 
void free(void *ptr);

Description

The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs.

See ISO-IEC 9899.

That being said, when looking at different codebases in the wild, you'll notice people sometimes do:

if (ptr)
  free(ptr);

This is because some C runtimes (I for sure remember it was the case on PalmOS) would crash when freeing a NULL pointer.

But nowadays, I believe it's safe to assume free(NULL) is a nop as per instructed by the standard.

user_2160630

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);
user_298090

If ptr is NULL, no operation is performed.

says the documentation.

user_953820

I remember working on PalmOS where free(NULL) crashed.

user_2382700 
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.

user_1655200

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.

user_456540

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.

user_317820

free(ptr) is save in C if ptr is NULL, however, what most people don't know is that NULL need not be equal to 0. I have a nice old-school example: On the C64, on address 0, there is an IO-Port. If you wrote a program in C accessing this port, you'd need a pointer whose value is 0. The corresponding C library would have to distinguish between 0 and NULL then.

Kind regards.

user_88000910

not memory corruption, but behavior depends on implementation. By standard, it should be a legal code.

user_736560
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