Send signal from parent to specific child from

Question

I have a tree process with a parent and two child, and I want to send a signal and only the child2 receive it, making a handler for child2 and show a message when he receive it in the handler.

pid_t pid, pid1, pid2;

if((pid=fork())==0)
printf("child1");
 else if((pid2=fork())=0)
   printf("child2);
   signal(SIGUSR1, handler); //EDITED
   else
     printf("parent");
     kill(pid2, SIGUSR1); //EDITED


void handler(int signal)
{
  printf("received");

but I dont know how to send signal SIGUSR1 only for child2

The signal call allows you to specify the process id of the process that the signal should be sent to. So you can just specify `child2`'s process id. — David Choweller, Dec 12 '16 at 18:56
yes, I should be to create a hanlder like a function but, how call it from child2, can you do a example in my code? — Aitor Ramos Pajares, Dec 12 '16 at 19:02
@AitorRamosPajares, no, we are not going to write the code for you. But note that the point of a signal handler is that your code *doesn't* call it. The system will call it, when appropriate, to handle the signal. This is separate from the regular control flow of your program. — John Bollinger, Dec 12 '16 at 19:47
It goes in the second `if`: `printf("child2"); signal(SIGUSR1, siguser_handler);` — Barmar, Dec 12 '16 at 19:47
You need to use more braces. What is currently in the question won't compile (`else` with no `if`). Always end messages with a newline; otherwise, they won't necessarily appear in a timely fashion. Remember [How to avoid using `printf()` in a signal handler](http://stackoverflow.com/questions/16891019/), though it probably won't affect you in this code. Please post an MCVE ([MCVE]). — Jonathan Leffler, Dec 12 '16 at 21:57

score 0 · Answer 1 · answered Sep 04 '18 at 13:30

Assuming that the indentation of your pseudo code represents the nesting depth, it is now correct in principle, but it suffers from timing problems:

The parent could send the signal to child2 before child2 has installed the signal handler. To remedy this, you can let the parent install the handler (which is inherited by the child) before forking child2.
child2 could end before the parent gets to send the signal, so the handler wouldn't be invoked. If you want to see it invoked, you can give child2 something to do, even if it's only a sleep(…).

score 0 · Answer 2 · answered Mar 01 '19 at 04:45

I hope this is a pseudocode of your actual problem. In that case the problem is that the parent needs to wait till the child has actually installed the handler. This can be achieved by making the parent wait till the child signals it("Telling Hi i am up and ready to go").

have a flag in the parent code as bChildUp. and wait as

while (!bChildUp)
{
// do nothing till the child is up.
}

send a signal from child2 to parent process. The parent process on receiving the signal can change the bChildUp= TRUE. The parent will exit the wait and your task can go on(However you can check if the signal is really from the child2 by getting the pid of the process which sent the signal using siginfo_t and using sigaction to register the signal).

These kind of problems are encountered by

having a file which the child creates and the parent comes to know that the child is successfully created.
Or based on signalling mechanism.

I Would suggest to just create a file in the child2 after it is up and let the parent process wait till the file is created.

Send signal from parent to specific child from

2 Answers2