Re: CLK_TCK undefined

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 18/10/23 06:55, Florian Weimer via Glibc-bugs wrote:
> * Andrea Monaco via Glibc-bugs:
> 
>> The constant CLK_TCK seems not defined in my glibc headers, although
>> it's required by C89 (according to the draft I'm using).  Is that
>> intended?
> 
> We have a conditional definition in <time.h>:
> 
> #if (!defined __STRICT_ANSI__ || defined __USE_POSIX) \
>    && !defined __USE_XOPEN2K
> /* Even though CLOCKS_PER_SEC has such a strange value CLK_TCK
>    presents the real value for clock ticks per second for the system.  */
> extern long int __sysconf (int);
> # define CLK_TCK ((__clock_t) __sysconf (2))    /* 2 is _SC_CLK_TCK */
> #endif
> 
> Maybe your choice of feature selection macros suppresses the definition?

It was removed by POSIX 2001 [1]:

  Issue 6

  The symbolic name CLK_TCK is removed. Application usage is added describing 
  how its equivalent functionality can be obtained using sysconf().

You need to specify an old POSIX version to actually use it:

$ cat t.c
#include <time.h>
#include <stdio.h>
#include <stdint.h>

int main (int argc, char *argv[])
{
  printf ("%jd\n", (intmax_t) CLK_TCK);
  return 0;
}

$ gcc -Wall t.c -o t -D_POSIX_C_SOURCE=200112L
t.c: In function ‘main’:
t.c:7:31: error: ‘CLK_TCK’ undeclared (first use in this function)
    7 |   printf ("%jd\n", (intmax_t) CLK_TCK);
      |                               ^~~~~~~
t.c:7:31: note: each undeclared identifier is reported only once for each function it appears in
$ gcc -Wall t.c -o t -D_POSIX_C_SOURCE=199506
$ ./t
100


[1] https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html