Operating Systems 2017F: Tutorial 2

In this tutorial you will be experimenting with and extending 3000shell.c (listed below).

Getting Started

You should download and run 3000shell.c on a Ubuntu Linux 16.04 system or similar. Compile it using the command

gcc -O -Wall 3000shell.c -o 3000shell

In the labs, use either the 2401 Virtualbox VM or create your own in [openstack.scs.carleton.ca SCS Openstack] using the Ubuntu 16.04 cloud image. If you have issues with Openstack please ask the TAs for assistance.

Code

/* 3000shell.c */
/* v1 Sept. 24, 2017 */
/* based off of csimpleshell.c, Enrico Franchi © 2005
      https://web.archive.org/web/20170223203852/
      http://rik0.altervista.org/snippets/csimpleshell.html */
/* Original under "BSD" license */
/* This version is under GPLv3, copyright Anil Somayaji */
/* You really shouldn't be incorporating parts of this in any other code,
   it is meant for teaching, not production */

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

#define BUFFER_SIZE 1<<16
#define ARR_SIZE 1<<16
#define COMM_SIZE 32

const char *proc_prefix = "/proc";

void parse_args(char *buffer, char** args, 
                size_t args_size, size_t *nargs)
{
    char *buf_args[args_size]; /* You need C99 */
    char **cp;
    char *wbuf;
    size_t i, j;
    
    wbuf=buffer;
    buf_args[0]=buffer; 
    args[0] =buffer;
    
    for(cp=buf_args; (*cp=strsep(&wbuf, " \n\t")) != NULL ;){
        if ((*cp != '\0') && (++cp >= &buf_args[args_size]))
            break;
    }
    
    for (j=i=0; buf_args[i]!=NULL; i++){
        if(strlen(buf_args[i])>0)
            args[j++]=buf_args[i];
    }
    
    *nargs=j;
    args[j]=NULL;
}


/* this is kind of like getenv() */
char *find_env(char *envvar, char *notfound, char *envp[])
{
        const int MAXPATTERN = 128;
        int i, p;
        char c;
        char pattern[MAXPATTERN];
        char *value = NULL;

        p = 0;
        while ((c = envvar[p])) {
                pattern[p] = c;
                p++;
                if (p == (MAXPATTERN - 2)) {
                        break;
                }
        }
        pattern[p] = '=';
        p++;
        pattern[p] = '\0';
        
        i = 0;
        while (envp[i] != NULL) {
                if (strncmp(pattern, envp[i], p) == 0) {                        
                        value = envp[i] + p;
                }
                i++;
        }

        if (value == NULL) {
                return notfound;
        } else {
                return value;
        }
}

void find_binary(char *name, char *path, char *fn, int fn_size) {
        char *n, *p;
        int r, stat_return;

        struct stat file_status;

        if (name[0] == '.' || name[0] == '/') {
                strncpy(fn, name, fn_size);
                return;
        }
        
        p = path;
        while (*p != '\0') {       
                r = 0;
                while (*p != '\0' && *p != ':' && r < fn_size - 1) {
                        fn[r] = *p;
                        r++;
                        p++;
                }

                fn[r] = '/';
                r++;
                
                n = name;
                while (*n != '\0' && r < fn_size) {
                        fn[r] = *n;
                        n++;
                        r++;
                }
                fn[r] = '\0';

                
                stat_return = stat(fn, &file_status);

                if (stat_return == 0) {
                        return;
                }

                if (*p != '\0') {
                        p++;
                }
        }
}

void setup_comm_fn(char *pidstr, char *comm_fn)
{
        char *c;

        strcpy(comm_fn, proc_prefix);
        c = comm_fn + strlen(comm_fn);
        *c = '/';
        c++;
        strcpy(c, pidstr);
        c = c + strlen(pidstr);
        strcpy(c, "/comm");
}

void plist()
{
        DIR *proc;
        struct dirent *e;
        int result;
        char comm[COMM_SIZE];  /* seems to just need 16 */        
        char comm_fn[512];
        int fd, i, n;

        proc = opendir(proc_prefix);

        if (proc == NULL) {
                fprintf(stderr, "ERROR: Couldn't open /proc.\n");
        }
        
        for (e = readdir(proc); e != NULL; e = readdir(proc)) {
                if (isdigit(e->d_name[0])) {
                        setup_comm_fn(e->d_name, comm_fn);
                        fd = open(comm_fn, O_RDONLY);
                        if (fd > -1) {                                
                                n = read(fd, comm, COMM_SIZE);
                                close(fd);
                                for (i=0; i < n; i++) {
                                        if (comm[i] == '\n') {
                                                comm[i] = '\0';
                                                break;
                                        }
                                }
                                printf("%s: %s\n", e->d_name, comm);
                        } else {
                                printf("%s\n", e->d_name);
                        }
                }
        }
        
        result = closedir(proc);
        if (result) {
                fprintf(stderr, "ERROR: Couldn't close /proc.\n");
        }
}

void signal_handler(int the_signal)
{
        int pid, status;

        if (the_signal == SIGHUP) {
                fprintf(stderr, "Received SIGHUP.\n");
                return;
        }
        
        if (the_signal != SIGCHLD) {
                fprintf(stderr, "Child handler called for signal %d?!\n",
                        the_signal);
                return;
        }

        pid = wait(&status);

        if (pid == -1) {
                /* nothing to wait for */
                return;
        }
        
        if (WIFEXITED(status)) {
                fprintf(stderr, "\nProcess %d exited with status %d.\n",
                        pid, WEXITSTATUS(status));
        } else {
                fprintf(stderr, "\nProcess %d aborted.\n", pid);
        }
}


int main(int argc, char *argv[], char *envp[])
{
    char buffer[BUFFER_SIZE];
    char *args[ARR_SIZE];
    char bin_fn[BUFFER_SIZE];
    struct sigaction signal_handler_struct;
    char *s;    

    int *ret_status = NULL;
    size_t nargs;
    pid_t pid;
    int background;
    
    char *username;
    char *default_username = "UNKNOWN";

    char *path;
    char *default_path = "/usr/bin:/bin";

    memset (&signal_handler_struct, 0, sizeof(signal_handler_struct));
    signal_handler_struct.sa_handler = signal_handler;
    signal_handler_struct.sa_flags = SA_RESTART;
    
    if (sigaction(SIGCHLD, &signal_handler_struct, NULL)) {
            fprintf(stderr, "Couldn't register SIGCHLD handler.\n");
    }

    if (sigaction(SIGHUP, &signal_handler_struct, NULL)) {
            fprintf(stderr, "Couldn't register SIGHUP handler.\n");
    }
    
    username = find_env("USER", default_username, envp);
    path = find_env("PATH", default_path, envp);
        
    while(1){
            printf("%s $ ", username);
            s = fgets(buffer, BUFFER_SIZE, stdin);
            
            if (s == NULL) {
                    /* we reached EOF */
                    printf("\n");
                    exit(0);
            }
        
            parse_args(buffer, args, ARR_SIZE, &nargs); 
            
            if (nargs==0) continue;
            
            if (!strcmp(args[0], "exit")) {
                    exit(0);
            }
            
            if (!strcmp(args[0], "plist")) {
                    plist();
                    continue;
            }

            background = 0;            
            if (strcmp(args[nargs-1], "&") == 0) {
                    background = 1;
                    args[nargs-1] = NULL;
                    nargs--;
            }
            
            pid = fork();
            if (pid) {
                    if (background) {
                            fprintf(stderr,
                                    "Process %d running in the background.\n",
                                   pid);
                    } else {
                            pid = wait(ret_status);
                    }
            } else {
                    find_binary(args[0], path, bin_fn, BUFFER_SIZE);
                    
                    if (execve(bin_fn, args, envp)) {
                            puts(strerror(errno));
                            exit(127);
                    }
            }
    }    
    return 0;
}