Its quite simple:
free de-allocates the memory the pointer is pointing to.
BUT: it does NOT change the value of p. So p still holds the address.
(free cant change the variable it got hand over since c calls goes by copy, not by reference ....)
The undefined behavior results from using the pointer still holding the address while a subsequent malloc can re-use the memory.
Therefore its a good practice always set the pointer to NULL directly after freeing it.
cheers
OJ
-----Original-Nachricht-----
Betreff: Re: Undefined behavior or not?
Datum: 2017-03-10T23:25:42+0100
Von: "Martin Sebor" <msebor@xxxxxxxxx>
An: "Segher Boessenkool" <segher@xxxxxxxxxxxxxxxxxxx>, "Liu Hao" <lh_mouse@xxxxxxx>
On 03/10/2017 09:12 AM, Segher Boessenkool wrote:
> On Fri, Mar 10, 2017 at 11:57:28PM +0800, Liu Hao wrote:
>>> So does this mean, that the above C code uses undefined behavior?
>> Yes.
>>
>> 6.2.4 Storage durations of objects
>> 1 ... Allocated storage is described in 7.22.3.
>> 2 ... The value of a pointer becomes indeterminate when the object it
>> points to (or just past) reaches the end of its lifetime.
>>
>> 7.22.3 Memory management functions
>> 1 ... The lifetime of an allocated object extends from the allocation
>> until the deallocation. ...
>
> But a null pointer does not point to an (allocated) object. So the
> code has undefined behaviour only if ever the pointer is not a null
> pointer?
That's right. Same as in:
char *p = 0;
if (rand () & 1) {
char c;
p = &c;
}
if (p)
puts ("p i non-null");
else
puts (p is null");
Martin
