[Bug c++/86879] G++ should warn about redundant tests for null pointers returned from functions with __attribute__((returns_nonnull))
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86879
Paul Cercueil changed:
What|Removed |Added
CC||paul at crapouillou dot net
--- Comment #3 from Paul Cercueil ---
I would also like to see a warning, and also in C code, for a different reason.
I have functions that return pointers to opaque structures. In case of an
error, instead of returning NULL and setting errno, it encodes the error code
into the pointer value.
The error code can then be retrieved with the following inline function:
static inline int is_err(const void *ptr)
{
return (uintptr_t) ptr >= (uintptr_t) -4095 ? (int)(intptr_t) ptr : 0;
}
if is_err(ptr) returns 0, then the pointer is valid - otherwise it returns the
error code.
Note that this was inspired on the Linux kernel, which has the exact same
mechanism.
What I want to prevent (and warn on), is incorrect error-checking of the
functions using this mechanism. Most often than not, callers will do this:
struct foo *ptr = maybe_return_errptr(arg);
if (!ptr) {
printf("Error!\n");
return NULL;
}
To avoid this mistake, I could tag my "maybe_return_errptr()" function with
__attribute__((returns_nonnull)). However, even with that, GCC does not warn
about the NULL-check; and it'd be great if it would.
[Bug c++/86879] G++ should warn about redundant tests for null pointers returned from functions with __attribute__((returns_nonnull))
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86879
Martin Sebor changed:
What|Removed |Added
Last reconfirmed|2018-08-07 00:00:00 |2021-4-8
--- Comment #2 from Martin Sebor ---
No progress in GCC 11.
Clang issues -Wpointer-bool-conversion and -Wundefined-bool-conversion:
cat pr86879.C && clang -S -Wall pr86879.C
void* get() __attribute__((returns_nonnull));
int f() { return get() ? 0 : 1; }
int& ref();
int g()
{
return () ? 0 : 1;
}
pr86879.C:3:18: warning: nonnull function call 'get()' will evaluate to 'true'
on first encounter [-Wpointer-bool-conversion]
int f() { return get() ? 0 : 1; }
^ ~
pr86879.C:1:28: note: declared 'returns_nonnull' here
void* get() __attribute__((returns_nonnull));
^
pr86879.C:9:11: warning: reference cannot be bound to dereferenced null pointer
in well-defined C++ code; pointer may be assumed to always convert to
true
[-Wundefined-bool-conversion]
return () ? 0 : 1;
^ ~
pr86879.C:5:6: note: 'ref' returns a reference
int& ref();
^
2 warnings generated.
[Bug c++/86879] G++ should warn about redundant tests for null pointers returned from functions with __attribute__((returns_nonnull))
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86879 Martin Sebor changed: What|Removed |Added Status|UNCONFIRMED |NEW Last reconfirmed||2018-08-07 CC||msebor at gcc dot gnu.org Ever confirmed|0 |1 --- Comment #1 from Martin Sebor --- Confirmed. It sounds like a useful enhancement. It also shouldn't be hard to implement -- similar functionality already exists for function arguments declared nonnull, and for addresses of functions and non-allocated objects.
