[Bug c/54202] New: Overeager warning about freeing non-heap objects
thiago at kde dot org
gcc-bugzilla@gcc.gnu.org
Wed Aug 8 13:36:00 GMT 2012
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54202
Bug #: 54202
Summary: Overeager warning about freeing non-heap objects
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: thiago@kde.org
GCC 4.7 has a warning about freeing non-heap objects that is way too eager.
Compiling the following code in C or C++:
======
#include <stdlib.h>
typedef struct Data
{
int refcount;
} Data;
extern const Data shared_null;
Data *allocate()
{
return (Data *)(&shared_null);
}
void dispose(Data *d)
{
if (d->refcount == 0)
free(d);
}
void f()
{
Data *d = allocate();
dispose(d);
}
====
Produces the following warning:
test.c: In function 'f'
test.c:17:13: warning: attempt to free a non-heap object 'shared_null'
[-Wfree-nonheap-object]
The warning is overeager because it says "attempt to free" without indicating
that it's only a possibility. GCC cannot prove that the call to free() will
happen with that particular pointer, as the value of shared_null.refcount is
not known.
The warning should either:
a) be modified to indicate it's only a possibility and the compiler can't
prove it;
b) be issued only when the compiler is sure that the free will happen on
non-heap objects.
Or both, by having two warnings: one for when it's sure and one for when it
isn't.
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