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
