From 87a944fb56ba25e845598d8c7a8761affc100964 Mon Sep 17 00:00:00 2001
From: Trent Huber <trentmhuber@gmail.com>
Date: Mon, 5 May 2025 16:38:10 -0400
Subject: [PATCH] Integrate with TTY device

---
 README.md |   5 +++
 main.c    | 120 ++++++++++++++++++++++++++++++++++++++++++++++++++----
 2 files changed, 118 insertions(+), 7 deletions(-)

diff --git a/README.md b/README.md
index df172fb..fa0c22b 100644
--- a/README.md
+++ b/README.md
@@ -1 +1,6 @@
 # Ash
+
+## Resources
+
+[TTY Demystified](http://www.linusakesson.net/programming/tty/)
+[Process Groups and Terminal Signaling](https://cs162.org/static/readings/ic221_s16_lec17.html)
diff --git a/main.c b/main.c
index eab4047..e707e55 100644
--- a/main.c
+++ b/main.c
@@ -1,3 +1,4 @@
+#include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -7,6 +8,13 @@
 
 #define BUFLEN 1024
 
+void nlprompt(int sig) {
+	(void)sig;
+
+	printf("\n%% ");
+	fflush(stdout);
+}
+
 char **tokenize(char *p) {
 	size_t i;
 	static char *result[BUFLEN / 2];
@@ -22,12 +30,97 @@ char **tokenize(char *p) {
 	return result;
 }
 
+#define BGMAX 128
+struct {
+	pid_t pgids[BGMAX];
+	size_t bp, sp;
+} bgps;
+
+void pushbgps(pid_t pgid) {
+	if ((bgps.sp + 1) % BGMAX == bgps.bp) killpg(bgps.pgids[bgps.bp++], SIGKILL);
+	bgps.pgids[++bgps.sp] = pgid;
+// printf("PUSHED pgid %d to stack: bp = %zu, sp = %zu\n", pgid, bgps.bp, bgps.sp);
+}
+
+pid_t popbgps(void) {
+	return bgps.sp != bgps.bp ? bgps.pgids[bgps.sp--] : -1;
+}
+
+void await(pid_t cpgid) {
+	int status;
+	pid_t pgid;
+
+	if (waitpid(cpgid, &status, WUNTRACED) == -1) {
+		dprintf(STDERR_FILENO, "ash: Unable to await previous command: %s\n",
+				strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+	if (!WIFSTOPPED(status)) { // Reap
+		while (waitpid(-cpgid, NULL, 0) != -1);
+		if (errno != ECHILD) {
+			dprintf(STDERR_FILENO, "ash: Unable to await previous command: %s\n",
+					strerror(errno));
+			exit(EXIT_FAILURE);
+		}
+		errno = 0;
+	}
+	if ((pgid = getpgid(0)) == -1) {
+		dprintf(STDERR_FILENO, "ash: Unable to get group process id: %s\n",
+				strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+	if (signal(SIGTTOU, SIG_IGN) == SIG_ERR) {
+		dprintf(STDERR_FILENO, "ash: Unable to ignore SIGTTOU signal: %s\n",
+				strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+	if (tcsetpgrp(STDIN_FILENO, pgid) == -1) {
+		dprintf(STDERR_FILENO, "ash: Unable to set tty foreground "
+				"process group: %s\n", strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+	if (signal(SIGTTOU, SIG_DFL) == SIG_ERR) {
+		dprintf(STDERR_FILENO, "ash: Unable to set SIGTTOU signal: %s\n",
+				strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+
+	if (WIFSIGNALED(status)) printf("\n");
+	if (WIFSTOPPED(status)) pushbgps(cpgid);
+}
+
+int builtin(char **tokens) {
+	pid_t fgpgid;
+
+	if (strcmp(tokens[0], "cd") == 0) {
+		if (chdir(tokens[1]) == -1) {
+			dprintf(STDERR_FILENO, "ash: Unable to cd: %s\n", strerror(errno));
+			exit(EXIT_FAILURE);
+		}
+	} else if (strcmp(tokens[0], "fg") == 0) {
+		if ((fgpgid = popbgps()) == -1) return 1;
+		if (tcsetpgrp(STDIN_FILENO, fgpgid) == -1) {
+			dprintf(STDERR_FILENO, "ash: Unable to set tty foreground "
+			        "process group: %s\n", strerror(errno));
+			exit(EXIT_FAILURE);
+		}
+		if (killpg(fgpgid, SIGCONT) == -1) {
+			dprintf(STDERR_FILENO, "ash: Unable to wake up process group "
+			        "%d to wake up: %s\n", fgpgid, strerror(errno));
+			exit(EXIT_FAILURE);
+		}
+		await(fgpgid);
+	} else return 0;
+
+	return 1;
+}
+
 int main(void) {
 	char buffer[BUFLEN];
 	char **tokens;
-	pid_t cpid;
-	int status;
+	pid_t cpgid;
 
+	signal(SIGINT, nlprompt);
 	for (;;) {
 		printf("%% ");
 		if (fgets(buffer, BUFLEN, stdin) == NULL) {
@@ -42,18 +135,31 @@ int main(void) {
 		}
 		tokens = tokenize(buffer);
 
-		if ((cpid = fork()) == 0)
+		if (builtin(tokens)) continue;
+
+		if ((cpgid = fork()) == 0) {
 			if (execvp(tokens[0], tokens) == -1) {
 				dprintf(STDERR_FILENO, "ash: Unable to run command `%s': %s\n",
 				        tokens[0], strerror(errno));
 				exit(EXIT_FAILURE);
 			}
-		if (waitpid(cpid, &status, 0) == -1) {
-			dprintf(STDERR_FILENO, "ash: Unable to await command `%s': %s\n",
-			        tokens[0], strerror(errno));
+		}
+		if (setpgid(cpgid, cpgid) == -1) {
+			dprintf(STDERR_FILENO, "ash: Unable to change child group process id: %s\n",
+					strerror(errno));
+			exit(EXIT_FAILURE);
+		}
+		if (tcsetpgrp(STDIN_FILENO, cpgid) == -1) {
+			dprintf(STDERR_FILENO, "ash: Unable to set tty foreground "
+					"process group: %s\n", strerror(errno));
+			exit(EXIT_FAILURE);
+		}
+		if (killpg(cpgid, SIGCONT) == -1) {
+			dprintf(STDERR_FILENO, "ash: Unable to signal child to wake up: %s\n",
+			        strerror(errno));
 			exit(EXIT_FAILURE);
 		}
-		// TODO: Handle WIFEXITED, WIFSIGNALED, WIFSTOPPED differently
+		await(cpgid);
 	}
 	return 0;
 }
-- 
2.51.0