Conference turris::digital_unix

9945.0. "What is kill(0,SIGUSR1) means in Digital UNIX ?" by HGOV08::SOCHUNWAH () Mon May 26 1997 23:56

Hello,

	This questions is raised on behalf of a remote-site enduser in China,
they are performing some C language coding and encountered problem when using 
the kill(0,SIGUSR1) function.

	According to the 'kill' man information and customer's understanding, 
kill 0 will terminate all it's owned background processes, but kill(0,SIGUSR1)
should only send a signal to SIGUSR1 of the relevant process accrodingly.

	The enduser made use of the following example to illustrate his
explaination about the kill(0,SIGUSR1) -- if they use kill(314,SIGUSR1), 
i.e. pid other than 0, there will be no problem :

> #include <signal.h>
> #include <ctype.h>
>
> main()
> {
>	printf("this is first clouse for test:);
>	kill(0,SIGUSR1);
>	printf("this is end of test");
> }

	Whenever the program run, it didn't give any printf line, and it'll
return to # prompt right away.  If we use kill(123,SIGUSR1), I can have the
printf line display on screen.

> # ./a.out
> User defined signal 1
> #

	The customer tried to setup some "menu setting" for their endusers, but
whenever they use the kill function as described above, the enduser's process
and it's background proceesses will be all terminated and fall back to login 
prompt again.

	Can anybody give an exlpaination about the phenonmean of the 
kill(0,XXX) function ?  I beleive it's normal becasue I tried it myself
on a ULTRIX system and have the same result (customer's system is running
OSF/1 V3.2B). Customer said the same command works just fine under SCO UNIX,
but I think kill 0 may means differnt thing in SCO UNIX.  

	


Thanks in Advance !
SO 

    kill(0, SIGUSR1) does send the SIGUSR1 signal to all processes whose
    process group ID is equal to the process group ID of the sender. I.e.
    it is also sent to the sending process. As this does not do any signal
    handling it is killed. As printf is line buffered but the first printf
    in the program has no trailing \n the line is not yet printed when the
    signal arrives and gets lost.
    
    SIGUSR1 is only useful for process that know what SIGUSR1 is meant for.
    By itself SIGUSR1 just has no meaning except that it's a signal. Sender
    and addressee must agree on a common understanding.
    
    What you probably would like to use is SIGTERM which is handled by most
    background - at least the well coded - processes. But be careful that
    the sending process knows how to deal with it too.
    
    	Hartmut