strace -fFp pid

root@nhkotest-005 vnstat-1.7]# strace -p 5831
select(0, NULL, NULL, NULL, {1, 0})     = 0 (Timeout)

clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x2b2ab7176fd0) = 30574

wait4(-1, 0x7ffff394e6fc, WNOHANG, NULL) = 0
select(0, NULL, NULL, NULL, {1, 0})     = 0 (Timeout)
wait4(-1, 0x7ffff394e6fc, WNOHANG, NULL) = 0
select(0, NULL, NULL, NULL, {1, 0})     = 0 (Timeout)

 

root@nhkotest-005 vnstat-1.7]# strace -f -p 5831
Process 5831 attached - interrupt to quit
select(0, NULL, NULL, NULL, {0, 988000}) = 0 (Timeout)
wait4(-1, 0x7ffff394e6fc, WNOHANG, NULL) = 0
select(0, NULL, NULL, NULL, {1, 0})     = 0 (Timeout)
wait4(-1, 0x7ffff394e6fc, WNOHANG, NULL) = 0
select(0, NULL, NULL, NULL, {1, 0})     = 0 (Timeout)
wait4(-1, 0x7ffff394e6fc, WNOHANG, NULL) = 0
select(0, NULL, NULL, NULL, {1, 0})     = 0 (Timeout)

clone(Process 15746 attached
child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x2b2ab7176fd0) = 15746

[pid 15746] rt_sigaction(SIGHUP, {0x454ea0, [], SA_RESTORER|SA_INTERRUPT, 0x331cc30070},  <unfinished ...>
[pid  5831] wait4(-1,  <unfinished ...>
[pid 15746] <... rt_sigaction resumed> {0x4547d0, [HUP USR1], SA_RESTORER, 0x331cc30070}, 8) = 0
[pid  5831] <... wait4 resumed> 0x7ffff394e6fc, WNOHANG, NULL) = 0
[pid 15746] rt_sigaction(SIGUSR1, {0x454ea0, [], SA_RESTORER|SA_INTERRUPT, 0x331cc30070},  <unfinished ...>
[pid  5831] select(0, NULL, NULL, NULL, {1, 0} <unfinished ...>
[pid 15746] <... rt_sigaction resumed> {0x4547d0, [HUP USR1], SA_RESTORER, 0x331cc30070}, 8) = 0
[pid 15746] rt_sigaction(SIGTERM, {0x454ea0, [], SA_RESTORER|SA_INTERRUPT, 0x331cc30070}, {0x4547b0, [], SA_RESTORER, 0x331cc30070}, 8) = 0