On 05/05/2017 08:21 AM, Prathamesh Kulkarni wrote:
Hi,
I was wondering what constraints apply to function that is annotated
with malloc attribute ?
For instance is the function allowed to modify global data ?
I'm not familiar with how GCC handles the attribute beyond what
I've gathered from experimenting with it and reading the manual
but given that the attribute applies to the malloc family of
functions which modify global data I'd say the answer must be
yes[*].
As far as I understand, from the doc a function could be annotated
with malloc attribute if it returns a pointer to a newly allocated
memory block (uninitialized or zeroed) and the pointer does not alias
any other valid pointer ?
Right. An example helps illustrate it:
void* __attribute__ ((alloc_size (1), malloc)) f (int);
void* __attribute__ ((alloc_size (1))) g (int);
void* foo (int n)
{
int *p = (int*)f (n);
int *q = (int*)f (n);
*p = 123;
*q = 456;
if (*p != 123) // must be false
__builtin_abort (); // eliminated
return p;
}
void* bar (int n)
{
int *p = (int*)g (n);
int *q = (int*)g (n);
*p = 123;
*q = 456;
if (*p != 123) // may be true
__builtin_abort (); // not eliminated
return p;
}
But a quick search for calls to lookup_attribute with "malloc"
as an argument didn't reveal any instances in GCC so I can't
say where this happens.
Martin
[*] This example shows that GCC assumes the function can modify
global data.
void* __attribute__ ((alloc_size (1), malloc)) f (int);
void* foo (int *p, int n)
{
*p = 123;
int *q = (int*)f (n); // f could modify *p
*q = 456;
if (*p != 123) // may be true
__builtin_abort (); // not eliminated
return p;
}
