what PID does os.killpg look for

Question

I am writing code to kill a process and all children processes.

According to this post, all children processes can be killed within the same process group by using os.killpg(pro.pid, signal.SIGTERM)

During test, I launched this process manually which spawned 5 subprocesses.

    UID      PID  PPID  C STIME TTY          TIME CMD
    ddd    25066 19475  0 Nov03 ?        00:00:00 /bin/sh -c gtdownload -c ~/.cghub.key --max-children 4 -vv -d https://cghub.ucsc.edu/cghub/data/analysis/download/ab0e89b4-5310-11e4-88da-adc9fc308db6 2
    ddd    25067 25066  0 Nov03 ?        00:00:07 /rsrch1/rists/djiao/apps/cghub/libexec/gtdownload -c /rsrch1/rists/djiao/.cghub.key --max-children 4 -vv -d https://cghub.ucsc.edu/cghub/data/analysis/d
    ddd    25073 25067  0 Nov03 ?        00:00:18 /rsrch1/rists/djiao/apps/cghub/libexec/gtdownload -c /rsrch1/rists/djiao/.cghub.key --max-children 4 -vv -d https://cghub.ucsc.edu/cghub/data/analysis/d
    ddd    25077 25067  0 Nov03 ?        00:00:18 /rsrch1/rists/djiao/apps/cghub/libexec/gtdownload -c /rsrch1/rists/djiao/.cghub.key --max-children 4 -vv -d https://cghub.ucsc.edu/cghub/data/analysis/d
    ddd    25081 25067  0 Nov03 ?        00:00:18 /rsrch1/rists/djiao/apps/cghub/libexec/gtdownload -c /rsrch1/rists/djiao/.cghub.key --max-children 4 -vv -d https://cghub.ucsc.edu/cghub/data/analysis/d
    ddd    25085 25067  0 Nov03 ?        00:00:18 /rsrch1/rists/djiao/apps/cghub/libexec/gtdownload -c /rsrch1/rists/djiao/.cghub.key --max-children 4 -vv -d https://cghub.ucsc.edu/cghub/data/analysis/d

However when I ran os.killpg(25066, signal.SIGTERM), I got the error "OSError: [Errno 3] No such process". Why can't it find the process with that ID?

Answer 1

You need to set process group using os.setpgrp() before calling os.killpg(). If you don't set any process group then you won't be able to kill it using os.killpg()

You can create process group using following ways:

1. os.setpgrp() -- If no argument is passed then it is equivalent to os.setpgrp(0,0). This will create a process group with id same as the calling process id.
2. os.setpgrp(0, 999) -- It will create a process group with id 999 and the current process will be part of that group. You can use any process id instead of 0 to make it part of that process group.

os.setpgrp() actually calls linux system call setpgrp(). See following linux man page for detail: https://linux.die.net/man/2/setpgrp

http://man7.org/linux/man-pages/man2/setpgid.2.html