Using setpgid in a mini-shell breaks interactive commands - c

Question

I'm trying to write a mini-shell in C using this template. But whenever I try to use interactive commands such as less and vi, the shell gets stuck on waitpid (with WUNTRACED enabled these commands return immediately because they are stopped by a job control signal as indicated by ps) . Other commands that don't require input such as ls are fine. The root cause is setpgid, which seems to puts the forked child process (such as less and vi) into a different process group which no longer shares a terminal. The child process is therefore stopped by a job control signal. Deleting setpgid will make the mini-shell work again, but it can't be removed since the mini-shell needs to control its foreground processes as a group (e.g. if the foreground process P forks additional processes P1 and P2, the shell, upon receiving a SIGTSTP from the user, should stop P, P1, and P2. This can be conveniently done if P, P1, P2 are in the same process group whose pgid is the same as P's pid. We can just send SIGTSTP to the entire process group).
I have tried to use tcsetpgrp to fix my shell. Although it'll make commands such as vi functional again, the mini-shell will automatically exit upon completion of the forked child, presumably because the parent shell mistakenly views the completion of the forked child as also the completion of the mini-shell.
Is there a fix which will still allow me to keep setpgid?
// full code is in the provided link
if (!builtin_command(argv)) {
if ((pid = Fork()) == 0) { /* Child runs user job */
if (execve(argv[0], argv, environ) < 0) {
execvp(argv[0], argv);
printf("%s: Command not found.\n", argv[0]);
exit(0);
}
}
// call wrapper function for error handling
// set process group id of child to the pid of child
Setpgid(pid, 0);
if (!bg) {
// foreground process, should wait for completion
// tcsetpgrp does make vi and less work,
// but their completion also terminates the mini-shell
// tcsetpgrp(STDERR_FILENO, pid);
int status;
if (waitpid(pid, &status, 0) < 0) {
unix_error("waitfg: waitpid error");
}
} else {
// background process
printf("%d %s", pid, cmdline);
}
}

Answer 1

The solution is to relinquish control of the tty to the other process group using tcsetpgrp, and when the child is done, take back the control of the tty using tcsetpgrp again. Note that tcsetpgrp sends SIGTTOU to its caller if the calling process belongs to a background process group, so SIGTTOU and SIGTTIN must be blocked.
// error handling is omitted for brevity
// these two signal functions can be anywhere before tcsetpgrp
// alternatively, they can be masked during tcsetpgrp
signal(SIGTTOU, SIG_IGN);
signal(SIGTTIN, SIG_IGN);
if (!builtin_command(argv)) {
if ((pid = Fork()) == 0) {
if (execve(argv[0], argv, environ) < 0) {
execvp(argv[0], argv);
printf("%s: Command not found.\n", argv[0]);
exit(0);
}
}
if (!bg) {
setpgid(pid, 0);
tcsetpgrp(STDERR_FILENO, pid);
int status;
if (waitpid(pid, &status, 0) < 0) {
unix_error("waitfg: waitpid error");
}
tcsetpgrp(STDERR_FILENO, getpgrp());
} else {
printf("%d %s", pid, cmdline);
}
}
This is a rather crude implementation. Consult Craig's comments for this question for where to find bash's implementation.