How free handles null pointer [duplicate]

This question already has answers here: Closed 11 years ago.

Possible Duplicates:

checking for NULL before calling free

What happens when you try to free() already freed memory in 开发者_开发技巧c?

Hope this isn't a totally dumb question. Anyway...

What happens when a null pointer is passed to free.

Thank you!

P.S. What about a standard compliant allocator? Thanks again!

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It immediately returns without doing anything.

Per 7.20.3.2, paragraph 3:

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. Otherwise, if the argument does not match a pointer earlier returned by the calloc, malloc, or realloc function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.

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The man page says:

free() frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Otherwise, or if free(ptr) has already been called before, undefined behaviour occurs. If ptr is NULL, no operation is performed.

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free(ptr) should do nothing if ptr is NULL.

In the real-world programming, this feature helps make your code even simpler like this:

class YourClass
{
public:

YourClass()
{
    m_ptr = (int*)malloc(sizeof(int));

    //Validate m_ptr and perform other initialization here
}

~YourClass()
{
    // You don't have to validate m_ptr like this
    // if (m_ptr)
    // {
    //     delete m_ptr;
    // }

    // Instead, just call free(m_ptr)
    // Notice: generally you should avoid managing the pointer by yourself,
    // i.e., RAII like smart pointers is a better option.
    free(m_ptr);
}

private:

int *m_ptr;
}