gorilla@guest:~/cc65 (c_sp)$ cat foo.c
#include <stdlib.h>
#include <time.h>

int main() {
   srand(time(NULL));
   return 0;
}
gorilla@guest:~/cc65 (c_sp)$ bin/cl65 -t sim6502 foo.c
common/time.s:30: Error: Unresolved external '_clock_gettime'
ld65: Error: 1 unresolved external(s) found - cannot create output file

i guess it makes sense for it to be system dependant. that said, sim6502 has hooks for printing and file operations, seems like a hook for clock_gettime() would not be that hard.

gorilla@guest:~/cc65 (c_sp)$ grep -r clock_gettime libsrc
libsrc/apple2/gettime.s:; int __fastcall__ clock_gettime (clockid_t clk_id, struct timespec *tp);
libsrc/apple2/gettime.s:_clock_gettime:
libsrc/c64/gettime.s:; int __fastcall__ clock_gettime (clockid_t clk_id, struct timespec *tp);
libsrc/c64/gettime.s:.proc   _clock_gettime
libsrc/geos-common/system/gettime.c:int __fastcall__ clock_gettime(clockid_t, struct timespec *tp)
libsrc/common/time.s:        jsr     _clock_gettime
libsrc/common/time.s:; _clock_gettime returns 0 on success and -1 on error. Check that.
libsrc/common/time.s:        inx                     ; Did _clock_gettime return -1?
libsrc/cbm610/gettime.s:; int __fastcall__ clock_gettime (clockid_t clk_id, struct timespec *tp);
libsrc/cbm610/gettime.s:.proc   _clock_gettime
libsrc/atari/gettime.s:; int __fastcall__ clock_gettime (clockid_t clk_id, struct timespec *tp);
libsrc/atari/gettime.s:_clock_gettime:
libsrc/rp6502/gettimezone.c:int clock_gettimezone (clockid_t clock_id, struct _timezone* tz)
libsrc/rp6502/gettime.c:extern int __clock_gettimespec (struct timespec* ts, unsigned char op);
libsrc/rp6502/gettime.c:int clock_gettime (clockid_t clock_id, struct timespec* tp)
libsrc/rp6502/gettime.c:    return __clock_gettimespec (tp, RIA_OP_CLOCK_GETTIME);
libsrc/rp6502/gettimespec.c:int __clock_gettimespec (struct timespec* ts, unsigned char op)
libsrc/rp6502/gettimespec.c:/* Internal method shared by clock_getres and clock_gettime. */
libsrc/rp6502/getres.c:extern int __clock_gettimespec (struct timespec* ts, unsigned char op);
libsrc/rp6502/getres.c:    return __clock_gettimespec (res, RIA_OP_CLOCK_GETRES);
libsrc/cbm510/gettime.s:; int __fastcall__ clock_gettime (clockid_t clk_id, struct timespec *tp);
libsrc/cbm510/gettime.s:.proc   _clock_gettime
libsrc/c128/gettime.s:; int __fastcall__ clock_gettime (clockid_t clk_id, struct timespec *tp);
libsrc/c128/gettime.s:.proc   _clock_gettime
libsrc/cx16/gettime.s:; int __fastcall__ clock_gettime (clockid_t clk_id, struct timespec *tp);
libsrc/cx16/gettime.s:.proc   _clock_gettime