On 25.07.2023 15:26, Jason Andryuk wrote: > On Tue, Jul 25, 2023 at 2:27 AM Jan Beulich <[email protected]> wrote: >> >> On 24.07.2023 21:49, Jason Andryuk wrote: >>> On Mon, Jul 24, 2023 at 12:15 PM Jan Beulich <[email protected]> wrote: >>>> On 24.07.2023 14:58, Jason Andryuk wrote: >>>>> --- /dev/null >>>>> +++ b/xen/arch/x86/acpi/cpufreq/hwp.c >>>>> +#define hwp_err(cpu, fmt, ...) \ >>>>> + printk(XENLOG_ERR "HWP: CPU%u error: " fmt, cpu, ##__VA_ARGS__) >>>>> +#define hwp_info(fmt, ...) printk(XENLOG_INFO "HWP: " fmt, >>>>> ##__VA_ARGS__) >>>> >>>> I'm not convinced ", ##__VA_ARGS__" is a good construct to use. I notice >>>> we have a few instances (mainly in code inherited from elsewhere), but I >>>> think it ought to either be plain C99 style (without the ##) or purely >>>> the gcc extension form (not using __VA_ARGS__). >>> >>> Using plain __VA_ARGS__ doesn't work for the cases without arguments: >>> arch/x86/acpi/cpufreq/hwp.c:78:53: error: expected expression before ‘)’ >>> token >>> 78 | printk(XENLOG_DEBUG "HWP: " fmt, __VA_ARGS__); \ >>> | ^ >>> arch/x86/acpi/cpufreq/hwp.c:201:9: note: in expansion of macro ‘hwp_verbose’ >>> 201 | hwp_verbose("disabled: No energy/performance >>> preference available"); >>> | ^~~~~~~~~~~ >> >> Of course. >> >>> I can use "__VA_OPT__(,) __VA_ARGS__" though. >> >> __VA_OPT__ is yet newer than C99, so this is an option only if all >> compilers we continue to support actually know of this. > > Right, sorry. > >> We have no >> uses of it in the codebase so far, which suggests you might best go >> with the longstanding gcc extension here. > > FTAOD, "##__VA_ARGS__" is the longstanding extension?
No. But you've apparently found it ... > It's the only > one I've been able to get to compile. I'm reading > https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html and it > mentions a few different extensions. > > This part seemed promising: > """ > This has been fixed in C++20, and GNU CPP also has a pair of > extensions which deal with this problem. > > First, in GNU CPP, and in C++ beginning in C++20, you are allowed to > leave the variable argument out entirely: > > eprintf ("success!\n") > → fprintf(stderr, "success!\n", ); > """ > > However, it doesn't seem to actually work for me. I still get an > error like the one above for plain __VA_ARGS__. That is for: > > #define hwp_err(cpu, fmt, args...) \ > printk(XENLOG_ERR "HWP: CPU%u error: " fmt, cpu, args) ..., just that you're missing the ##: #define hwp_err(cpu, fmt, args...) \ printk(XENLOG_ERR "HWP: CPU%u error: " fmt, cpu, ## args) Jan
