From 6a1cae3c869cc6e35b030552cc9954abdbec37c7 Mon Sep 17 00:00:00 2001
From: Amber <amber@ailuridae.io>
Date: Sun, 21 Feb 2021 18:11:53 -0500
Subject: [PATCH] Initial commit.

---
 programming-assignment.c | 186 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 186 insertions(+)
 create mode 100644 programming-assignment.c

diff --git a/programming-assignment.c b/programming-assignment.c
new file mode 100644
index 0000000..3a64f36
--- /dev/null
+++ b/programming-assignment.c
@@ -0,0 +1,186 @@
+#include <stdio.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/wait.h>
+#include <sys/time.h>
+#include <sys/resource.h>
+
+/**************************************************************************
+ * Authors: Gary Fleming, Amber McCloughan
+ * Class: CIS452 - Operating Systems Concepts
+ * Professor: Dr. Greg Wolffe
+ * Date: 1/24/2019
+ * 
+ * A simple shell that reads in commands from the user. The user can
+ * run programs from the shell with a max of 10 arguments. Usage
+ * statistics are given after a program has finished running in the
+ * shell.
+ * 
+ * Type 'quit' to exit the shell.
+***************************************************************************/
+
+// Number of characters that can be typed when shell is reading input.
+#define CHAR_LIMIT 1024
+
+// Max number of arguments that can be passed.
+#define ARG_LIMIT 32
+
+/**
+ * Clears out arguments from argument buffer for
+ * after they have been used.
+ * @param argv, the argument buffer to be cleared.
+ **/
+void clearArgs(char **argv)
+{
+    int i;
+    for (i = 0; i < ARG_LIMIT; i++)
+    {
+        argv[i] = NULL;
+    }
+}
+
+/**
+ * Main function that drives shell.
+ **/
+int main()
+{
+    // The input buffer for reading commands.
+    char *input = malloc(sizeof(char) * CHAR_LIMIT);
+
+    // The argument buffer for executing programs.
+    char **argv = malloc(sizeof(char *) * ARG_LIMIT);
+
+    // Counter for parsing arguments, child process status, child process PID.
+    int count, status, child;
+
+    // Temporary variable for time calculations.
+    long temp_micro;
+
+    // Struct for gathering usage statistics about child processes.
+    struct rusage usageStats;
+
+    printf("Welcome! Type \"quit\" to close the shell.\n");
+
+    /* 
+   * Driving loop of program.
+   * Inputting 'quit' breaks out of the loop.
+   */
+    while (1)
+    {
+        printf("$ ");
+
+        // Reading user input.
+        input = fgets(input, CHAR_LIMIT, stdin);
+
+        // Handling newline input.
+        if (!strcmp(input, "\n"))
+        {
+            continue;
+        }
+
+        // Handling loop exit.
+        if (!strcmp(input, "quit\n"))
+        {
+            printf("Goodbye!\n");
+            break;
+        }
+
+        // Removing newlines from end of input.
+        input = strtok(input, "\n");
+
+        // Reading first token.
+        char *token = strtok(input, " ");
+
+        // Setting counter from parsing tokens.
+        count = 0;
+
+        /*
+        * Loop for parsing tokens. The following source
+        * was used to figure out how to properly use
+        * the strtok() subroutine:
+        * https://www.geeksforgeeks.org/strtok-strtok_r-functions-c-examples/
+        */
+        while (token != NULL)
+        {
+            // If the counter is at the argument limit, stop.
+            if (count == ARG_LIMIT)
+            {
+                break;
+            }
+
+            // If a space is entered, replace it with a blank character.
+            if (!strcmp(token, " "))
+            {
+                argv[count] = "";
+            }
+            // Otherwise, read token into argument buffer.
+            else
+            {
+                argv[count] = token;
+            }
+
+            // Increment counter.
+            count++;
+
+            // Onto the next token!
+            token = strtok(NULL, " ");
+        }
+
+        // Forking the child process that will exec().
+        int pid = fork();
+
+        // Error handling.
+        if (pid < 0)
+        {
+            printf("Uh-oh. There was an error!\n");
+            exit(1);
+        }
+        // Child process handling.
+        else if (pid == 0)
+        {
+            // Executing user arguments.
+            execvp(argv[0], &argv[0]);
+
+            // Handling errors from bad commands.
+            printf("That didn't work.\n");
+            exit(1);
+        }
+        // Parent process handling.
+        else
+        {
+            // Waiting on child. Grabs usage stats when waiting finishes.
+            child = wait3(&status, 0, &usageStats);
+
+            // Clearing argument buffer for next run.
+            clearArgs(argv);
+
+            // Resetting counter.
+            count = 0;
+
+            // Don't print usage statistics if child errors.
+            if (status != 0)
+            {
+                continue;
+            }
+
+            // For user time calculation.
+            temp_micro = (usageStats.ru_utime.tv_sec * 1000000) + (usageStats.ru_utime.tv_usec);
+
+            // Displaying information to user.
+            printf("\nChild process with PID %d finished.\n", child);
+            printf("Child process took %ld microseconds user time.\n", temp_micro);
+            printf("It also had %ld involuntary context switches.\n", usageStats.ru_nivcsw);
+        }
+
+        printf("\n");
+    }
+
+    // Freeing allocated memory.
+    free(input);
+    free(argv);
+
+    // Goodbye.
+    return 0;
+}
\ No newline at end of file