# strace -s100 -f /usr/sbin/ntpd -p /var/lib/ntp/var/run/ntp/ntpd.pid -u ntp -i /var/lib/ntp
[...]
open("/var/lib/ntp", O_RDONLY)          = 8
fstat(8, {st_mode=S_IFDIR|0755, st_size=144, ...}) = 0
mmap(NULL, 65536, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2000000000100000
read(8, 0x2000000000100000, 16384)      = -1 EISDIR (Is a directory)
close(8)                                = 0
munmap(0x2000000000100000, 65536)       = 0
write(2, "usage: /usr/sbin/ntpd [ -abdgmnx ] [ -c config_file ] [ -e e_delay ]\n", 69) = 69
write(2, "\t\t[ -f freq_file ] [ -k key_file ] [ -l log_file ]\n", 51) = 51
write(2, "\t\t[ -p pid_file ] [ -r broad_delay ] [ -s statdir ]\n", 52) = 52
write(2, "\t\t[ -t trust_key ] [ -v sys_var ] [ -V default_sysvar ]\n", 56) = 56
write(2, "\t\t[ -P fixed_process_priority ]\n", 32) = 32
exit_group(2)                           = ?