Operating Systems 2021F: Assignment 2

This assignment is still being developed.

Questions

1. How could you fix 3000capture so it sends its output to the file specified by the CAPTURE environment variable? If there is no CAPTURE environment variable, it should use the same default as it does currently.
2. What is the purpose of the fflush() call in parent() at line ?

Code

3000capture.c

/* 3000capture.c */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <signal.h>

int result_pid = -1;
int result_status = 0;
int child_exited = 0;

void signal_handler(int the_signal)
{
        if (the_signal == SIGCHLD) {
                result_pid = wait(&result_status);
                if (result_pid == -1) {
                        return;
                }
                
                child_exited = 1;
        }
}

int parent(pid_t child_pid, char *command)
{
        int count = 0;
        const int bufsize = 256;
        char buf[bufsize];

        printf("Waiting.");
        fflush(stdout);

        while (!child_exited) {
                sleep(1);
                write(1, ".", 1);
        }

        printf("\n");
        
        if (result_pid == child_pid) {
                count += snprintf(buf + count, bufsize,
                                  "Child %d terminated", child_pid);
                if (WIFEXITED(result_status)) {
                        count += snprintf(buf + count, bufsize,
                                          " normally with status %d",
                                          WEXITSTATUS(result_status));
                } else if (WIFSIGNALED(result_status)) {
                        count += snprintf(buf + count, bufsize,
                                          " with signal %d",
                                          WTERMSIG(result_status));
                }
                
                puts(buf);
                return 0;
        } else if (result_pid == -1) {
                printf("No child to wait for.\n");
                return -1;
        } else {
                printf("wait returned unknown value %d\n", result_pid);
                return result_pid;
        }
}

int child(char *command, char *input_fn, char *output_fn, char *argv[])
{
        extern char **environ;
        int fd;

        close(1);
        fd = creat(output_fn, 0644);
        if (fd == -1) {
                fprintf(stderr, "Could not create %s for output.\n",
                        output_fn);
                exit(-3);
        }
        dup2(fd, 1);

        close(0);
        fd = open(input_fn, O_RDONLY);
        if (fd == -1) {
                fprintf(stderr, "Could not open %s for input.\n",
                        output_fn);
                exit(-4);
        }
        dup2(fd, 0);
        
        execve(command, argv, environ);
        fprintf(stderr, "execve of %s failed.\n", command);
        return(-1);
}

int main(int argc, char *argv[], char *envp[])
{
        char *command;
        char *input_fn;
        char *output_fn = "3000capture.log";
        
        pid_t pid;
        
        struct sigaction signal_handler_struct;
        
        memset(&signal_handler_struct, 0, sizeof(signal_handler_struct));
        signal_handler_struct.sa_handler = signal_handler;
        
        if (sigaction(SIGCHLD, &signal_handler_struct, NULL)) {
                fprintf(stderr, "Couldn't register SIGCHLD handler.\n");
        }

        if (argc < 3) {
                fprintf(stderr,
                        "Usage: %s <command> <input file> [command args]\n"
                        "By default saves data to %s; set CAPTURE to change.\n",
                        argv[0], output_fn);
                exit(-1);
        }
        
        command = argv[1];
        input_fn = argv[2];

        pid = fork();

        if (pid == -1) {
                fprintf(stderr, "Fork failed.\n");
                exit(-2);
        } else if (pid == 0) {
                argv[2] = command;
                return child(command, input_fn, output_fn, argv + 2);
        } else {
                return parent(pid, command);
        }
}

countdown.sh

#!/bin/bash

for i in `seq 10 -1 1`; do
    echo ">>>>  $i  <<<<"
    sleep 1
done

echo "Blastoff!"