在 2018-10-04 17:11, Sebastian Huber 写道:
On 04/10/2018 10:56, Jonathan Wakely wrote:
On Thu, 4 Oct 2018 at 09:51, Sebastian Huber wrote:
Hello,
I use a typedef for static assertions for pre C11 compilers. For one
static assertion
typedef struct { int i; } sem_t;
typedef int static_assert_sem_failed [((void *)0 == (sem_t *)0) ? 1 :
-1];
_Static_assert((void *)0 == (sem_t *)0, "sem_failed");
I get a warning like this:
gcc -S -o - sa.c > /dev/null
sa.c:3:1: warning: variably modified ‘static_assert_sem_failed’ at file
scope
typedef int static_assert_sem_failed [((void *)0 == (sem_t *)0) ?
1 : -1];
^~~~~~~
Is there some way to get around this warning?
You could move the homemade assertion so it's not at file scope:
__attribute__((used)) static void assertions() {
__attribute__((used)) typedef int static_assert_sem_failed [((void
*)0 == (sem_t *)0) ? 1 : -1];
}
Thanks, this works.
This works for me on GCC 4.9, 7 and 8:
```
#define FORCE_CONSTANT_TRUE(expr) (__builtin_constant_p(expr) ? (expr) : 0)
typedef int static_assert_sem_failed [FORCE_CONSTANT_TRUE((void *)0 ==
(sem_t *)0) ? 1 : -1];
```
The same expression in the
_Static_assert() doen't lead to a warning.
It does with -pedantic, and that makes the reason for the warning
clearer:
foo.c:5:26: warning: expression in static assertion is not an integer
constant expression [-Wpedantic]
_Static_assert((void *)0 == (sem_t *)0, "sem_failed");
This looks bad, so basically a pointer constant doesn't exist in
standard C?
As a difference from C++, C does not allow pointer operands in an
/integer constant expression/, despite the type of results of pointer
comparison, which is `int`.
Because of VLAs, it is discouraged to use array extents for such
purpose. You may prefer bitfield widths, which would result in a better
message:
```
struct static_assert_sem_failed
{ int unused : ((void *)0 == (sem_t *)0) ? 1 : -1; };
/*
lh_mouse@lhmouse-ideapad ~/Desktop $ gcc test.5.c -Wall -Wextra
-Wpedantic -Werror
test.5.c:4:9: error: bit-field ‘unused’ width not an integer constant
expression [-Werror=pedantic]
{ int unused : ((void *)0 == (sem_t *)0) ? 1 : -1; };
*/
```
--
Best regards,
LH_Mouse
