On 04/10/2018 14:27, Liu Hao wrote:
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; };
*/
```
Thanks for highlighting this C/C++ difference with respect to pointer
operands. In C++ it looks better.
#ifdef __cplusplus
#define _Static_assert static_assert
#endif
typedef struct { int i; } sem_t;
struct static_assert_sem_failed { int unused : ((void *)0 == (sem_t *)0)
? 1 : -1; };
_Static_assert((void *)0 == (sem_t *)0, "sem_failed");
void f(void)
{
struct f_static_assert_sem_failed { int unused : ((void *)0 == (sem_t
*)0) ? 1 : -1; };
_Static_assert((void *)0 == (sem_t *)0, "sem_failed");
}
gcc -S -x c++ -o - sa.c > /dev/null -Wall -Wextra -pedantic
No warnings or errors.
gcc -S -x c -o - sa.c > /dev/null -Wall -Wextra -pedantic
sa.c:7:39: warning: bit-field ‘unused’ width not an integer constant
expression [-Wpedantic]
struct static_assert_sem_failed { int unused : ((void *)0 == (sem_t
*)0) ? 1 : -1; };
^~~~~~
sa.c:9:26: warning: expression in static assertion is not an integer
constant expression [-Wpedantic]
_Static_assert((void *)0 == (sem_t *)0, "sem_failed");
~~~~~~~~~~^~~~~~~~~~~~~
sa.c: In function ‘f’:
sa.c:13:43: warning: bit-field ‘unused’ width not an integer constant
expression [-Wpedantic]
struct f_static_assert_sem_failed { int unused : ((void *)0 ==
(sem_t *)0) ? 1 : -1; };
^~~~~~
sa.c:15:28: warning: expression in static assertion is not an integer
constant expression [-Wpedantic]
_Static_assert((void *)0 == (sem_t *)0, "sem_failed");
Only a pedantic warning unlike to the variant with the array.
--
Sebastian Huber, embedded brains GmbH
Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone : +49 89 189 47 41-16
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