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cpointersmemory-managementnullfree

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

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?

Solution

  • 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.