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The following code starts a lenghty nmap process and then the main program tries to kill it. I am running it from shell and also have GTOP running in another window just to see if all is successfull.

#include<stdio.h> #include<string.h> #include<stdlib.h> #include<unistd.h> #include<errno.h> #include <sys/wait.h> int main() { int ret,childpid=0; ret = fork(); if (ret == 0) { printf("Here A %d.. \n", ret); char *params[5] = {"nmap","-sS","-A","192.168.0.1/24",0}; //cmd params filled childpid = execv( "/usr/bin/nmap" , params); //parameters for cmd printf("\n child exiting (%d) (%d).. \n", childpid,errno); //on successful execution of cmd, this exit never appears } else { childpid=ret; printf("parent exiting\n"); if (childpid!=0) { printf("child %d about to be killed wait 15 so that htop has time to see it\n",childpid); sleep(15); kill(childpid, SIGTERM); sleep(2); kill(childpid, SIGKILL); printf("killed wait 15 HTOP should have time to update\n"); sleep(15); } } return 1; }

HTOP see nmap starting but when I kill the process it still shows in the HTOP display. When my main program exit HTOP removes nmap from list of processes. Am I doing something wrong or misinterpreting HTOP?

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2 Answers 2

Reset to default
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You spawned a child process, killed it, and didn't wait(2) for it. The process is now a zombie, hanging around for its parent. When the parent process dies, the zombie becomes an orphan, and init takes care of it. From man 2 wait on Linux:

In the case of a terminated child, performing a wait allows the system to release the resources associated with the child; if a wait is not performed, then the terminated child remains in a "zombie" state (see NOTES below).

And from the notes:

A child that terminates, but has not been waited for becomes a "zombie". The kernel maintains a minimal set of information about the zombie process (PID, termination status, resource usage information) in order to allow the parent to later perform a wait to obtain information about the child. As long as a zombie is not removed from the system via a wait, it will consume a slot in the kernel process table, and if this table fills, it will not be possible to create further processes. If a parent process terminates, then its "zombie" children (if any) are adopted by init(1), (or by the nearest "subreaper" process as defined through the use of the prctl(2) PR_SET_CHILD_SUBREAPER operation); init(1) automatically performs a wait to remove the zombies.

So, wait() for the child process, or it will hang around until the parent process dies.

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  • My example in my original post been cut down from larger program. Eventually I plan to do the following: Commented Jun 6, 2017 at 12:16
  • 1) from my main program launch a process (here for example nmap) 2) while nmap is running I do some other stuff in my main program. 3) later I check is the process (nmap) still running. If too much time has passed I just kill the process (nmap) Commented Jun 6, 2017 at 12:22
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I think I found a solution. If I change the kill -part as follows:

 ... { printf("child %d about to be killed wait 15 so that htop has time to see it\n",childpid); sleep(15); kill(childpid, SIGKILL); waitpid(childpid,&ret,WUNTRACED); } } return 1; }

Adding that waitpid seems to clear the process from HTOP. If someone knows any caveat (why should I use SIGKILL instead of SIGTERM or should I use something else than WUNTRACED?) please let mr know.

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