Sample code:
#include <stdio.h>
#define __STDC_WANT_IEC_60559_TYPES_EXT__
#include <float.h>
#ifdef FLT16_MAX
_Float16 f16;
int main(void)
{
printf("%f\n", f16);
return 0;
}
#endif
Invocation:
# gcc trunk on linux on x86_64
$ gcc t0.c -std=c11 -Wall
Expected diagnostics:
<nothing>
Actual diagnostics:
t0.c:9:14: warning: format '%f' expects argument of type 'double', but argument 2 has type '_Float16' [-Wformat=]
9 | printf("%f\n", f16);
| ~^ ~~~
| | |
| | _Float16
| double
Does it mean that under __STDC_WANT_IEC_60559_TYPES_EXT__ AND if FLT16_MAX defined the gcc is unaware that printf may be used with _Float16? Should it be aware?
Also: printf("%f\n", f); when f is a float leads to no warning above despite the fact that format '%f' expects argument of type 'double', but argument 2 has type 'float'. Confused.
From the clang manual:
Because default argument promotion only applies to the standard floating-point types,
_Float16values are not promoted todoublewhen passed as variadic or untyped arguments. As a consequence, some caution must be taken when using certain library facilities with_Float16; for example, there is noprintfformat specifier for_Float16, and (unlikefloat) it will not be implicitly promoted todoublewhen passed toprintf, so the programmer must explicitly cast it to double before using it with an%for similar specifier.