The number of clock ticks per second can be obtained using sysconf(_SC_CLK_TCK); In POSIX-1996 the symbol CLK_TCK (defined in <time.h>) is mentioned as obsolescent. It is obsolete now.
On Linux, if the disposition of SIGCHLD is set to SIG_IGN then the times of terminated children are automatically included in the tms_cstime and tms_cutime fields, although POSIX 1003.1-2001 says that this should only happen if the calling process wait()s on its children.
Note that clock(3) returns values of type clock_t that are not measured in clock ticks but in CLOCKS_PER_SEC.
SVr4, SVID, POSIX, X/OPEN, BSD 4.3
SVr1-3 returns long and the struct members are of type time_t although they store clock ticks, not seconds since the epoch. V7 used long for the struct members, because it had no type time_t yet.
On older systems the number of clock ticks per second is given by the variable HZ.
time(1), getrusage(2), wait(2), clock(3), sysconf(3)