A Tiny Unix Shell (tsh) – Part IV
Introduction
This is the fourth piece of the project on process control and signaling. In this assignment you will extend
your shell to process signals received from the keyboard. This assignment must be completed individually.
Keyboard Signals
Recall that a signal is a small message that notifies a process that an event of some type has occurred in the
system. Typing ctrl-c causes a SIGINT signal to be delivered to each process in the foreground job. The
default action for SIGINT is to terminate the process.
Similarly, typing ctrl-z causes a SIGTSTP signal to be delivered to each process in the foreground job. The
default action for SIGTSTP is to place a process in the stopped state, where it remains until it is awakened
by the receipt of a SIGCONT signal.
What to do
1. Implement signal handlers for the SIGINT and SIGTSTP signals. Typing ctrl-c (ctrl-z) should cause
aSIGINT (SIGTSTP) signal to be sent to the current foreground job, as well as any descendents of
that job (e.g., any child processes that it forked). If there is no foreground job, then the signal should
have no effect.
Your shell should catch these signals and send them to the entire foreground process group, using
“-pid” instead of “pid” in the argument to the kill function, i.e.
if (kill(-pid, SIGINT) < 0)
printf("kill (sigint) error");
Here is one problem: When you run your shell from the standard Unix shell, your shell is running in
the foreground process group. If your shell then creates a child process, by default that child will also
be a member of the foreground process group. Since typing ctrl-c sends a SIGINT to every process
in the foreground group, typing ctrl-c will send a SIGINT to your shell, as well as to every process
that your shell created, which obviously is not correct.
Here is the workaround: After the fork, but before the execvp, the child process should call
setpgid(0, 0), which puts the child in a new process group whose group ID is identical to the
childs PID. This ensures that there will be only one process, your shell, in the foreground process
group. When you type ctrl-c, the shell should catch the resulting SIGINT and then forward it to the
appropriate foreground job (or more precisely, the process group that contains the foreground job).
Important Note. Do not forget to add SIGINT and SIGTSTP to the mask set of blocked signals
before the child gets forked. This eliminates race conditions where the child receives one of these
signals before the parent calls addjob.
2. Thoroughly test your code for ctrl-c before moving on to step 3. It should properly terminate pro-
cesses.
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