Why is pidof not working?

Question

In the output of ps aux, I can see the process just fine:

# ps aux | grep diff
root      7787 28.7  0.0   9368  4516 pts/3    D+   13:56  20:33 diff -qr mnt/mnt/md/ mnt/mnt2/
root     13130  0.0  0.0   6144   876 pts/4    S+   15:07   0:00 grep diff

But pidof claims not to be able to find anything:

# pidof diff
# echo $?
1

Looking at the man page, there is no info on what to do when lost a process, pidof has. /proc/7787/exe is a symlink to /usr/bin/diff and /usr/bin/diff itself is a regular file and an ELF. According to the man page, this ought to match.

Answer 1

After some investigation with strace, it seems that pidof also checks the status of the processes. My diff process was in D state most of the time, meaning it is waiting for I/O a lot. With this knowledge, I ran pidof a bunch of times (this is within ~3 seconds):

# pidof diff
7787
# pidof diff
# pidof diff
7787
# pidof diff
# pidof diff
# pidof diff
# pidof diff
# pidof diff
# pidof diff
# pidof diff
# pidof diff
# pidof diff
# pidof diff
7787
# pidof diff
7787

It indeed returns it 'sometimes', seeming to confirm the suspicion that pidof only return processes that are not in D state.

Checking the source code of pidof in src/killall5.c (obtained using apt source sysvinit-utils), the answer lies on line 599:

if ( (strchr(process_status, 'D') != NULL) ||
     (strchr(process_status, 'Z') != NULL) ){
   /* Ignore zombie processes or processes in
      disk sleep, as attempts
      to access the stats of these will
      sometimes fail. */

Answer 2

In sysvinit 2.96 and later, pidof -z will include processes that are in disk I/O ('D') or zombie ('Z') state.