I am writing a mini-shell(no, not for school :P; for my own enjoyment) and most of the basic functionality is now done but I am stuck when trying to handle SIGTSTP.
Supposedly, when a user presses Ctrl+Z
, SIGTSTP should be sent to the Foreground process of the shell if it exists, and Shell should continue normally.
After creating each process(if it's a Foreground process), the following code waits:
if(waitpid(pid, &processReturnStatus, WUNTRACED)>0){//wait stopped too
if(WIFEXITED(processReturnStatus) || WIFSIGNALED(processReturnStatus))
removeFromJobList(pid);
}
And I am handling the signal as follows:
void sigtstpHandler(int signum)
{
signum++;//Just to remove gcc's warning
pid_t pid = findForegroundProcessID();
if(pid > -1){
kill(-pid, SIGTSTP);//Sending to the whole group
}
}
What happens is that when I press Ctrl+Z
, the child process does get suspended indeed(using ps -all
to view the state of the processes) but my shell hangs at waitpid
it never returns even though I passed WUNTRACED
flag which as far as I understood is supposed to make waitpid
return when the process is stopped too.
So what could I have possible done wrong? or did I understand waitpid's behavior incorrectly?
Notes:
-findForegroundProcessID() returns the right pid; I double checked that.
-I am changing each process's group when right after I fork
-Handling Ctrl+C
is working just fine
-If I use another terminal to send SIGCONT after my shell hangs, the child process resumes its work and the shell reaps it eventually.
-I am catching SIGTSTP which as far as I read(and tested) can be caught.
-I tried using waitid instead of waitpid just in case, problem persisted.
EDIT:
void sigchldHandler(int signum)
{
signum++;//Just to remove the warning
pid_t pid;
while((pid = waitpid(-1, &processReturnStatus, 0)) > 0){
removeFromJobList(pid);
}
if(errno != ECHILD)
unixError("kill error");
}
My SIGCHLD handler.
SIGCHLD
is delivered for stopped children. The waitpid()
call in the signal handler - which doesn't specify WUNTRACED
- blocks forever.
You should probably not have the removeFromJobList()
processing in two different places. If I had to guess, it sounds like it touches global data structures, and doesn't belong in a signal handler.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With