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What happens inside memory if we try to free a pointer which is pointing to NULL? Is that ever valid?
Why does it not show any warning/error messages?
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If ptr is a null pointer, no action occurs.The standard has always required free to behave this way.
Dangling pointers can lead to exploitable double-free and access-freed-memory vulnerabilities. A simple yet effective way to eliminate dangling pointers and avoid many memory-related vulnerabilities is to set pointers to NULL after they are freed or to set them to another valid object.
delete and free() in C++ In C++, the delete operator should only be used either for the pointers pointing to the memory allocated using new operator or for a NULL pointer, and free() should only be used either for the pointers pointing to the memory allocated using malloc() or for a NULL pointer.
The function free takes a pointer as parameter and deallocates the memory region pointed to by that pointer. The memory region passed to free must be previously allocated with calloc , malloc or realloc . If the pointer is NULL , no action is taken.
From C99 section 7.20.3.2 : The free function
Synopsis
1 #include <stdlib.h> void free(void *ptr); Description
2 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.
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From http://linux.die.net/man/3/malloc:
If ptr is NULL, no operation is performed.
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