Does freeing an uninitialized pointer result in undefined behavior?

问题

If you have a pointer that is not initialized and, by mistake, try to free it, is this going to result in undefined behavior?

Like:

int main(void){

    char *string;
    free(string);

    return 0;
}

回答1:

Does freeing an uninitialized pointer result in undefined behavior?

Yes.

However, freeing a null pointer is well-defined.

From the C99 standard:

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.

回答2:

Yes, because accessing any uninitialised variable provokes undefined behaviour.

This includes passing an uninitialise pointer tofree(). This itself also includes the case where the uninitialise pointer "by accident" may have a value equal to NULL.

回答3:

Yes, it is undefined behavior.

The pointer passed to free should be a pointer to a valid object allocated with malloc, calloc, realloc or a null pointer.

From C99:

(7.20.3.2p2) "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."

回答4:

Yes, it does, since you should only free() a pointer that 1. is NULL or 2. you obtained via a call to malloc(), calloc() or realloc().

来源：https://stackoverflow.com/questions/18160776/does-freeing-an-uninitialized-pointer-result-in-undefined-behavior