OS1 - Cv 1

// Operating Systems: sample code
// Signals
#include <stdio.h>  // standard I/O functions
#include <unistd.h> // standard Unix functions, like getpid()
#include <signal.h> // signal name macros, and the signal() prototype
#include <stdlib.h> // system()
#include <string.h> // strcmp()

#define GOOD_PASSWORD   "marvelous" // password we're expecting to get

// define values for 'YES' and 'NO'
#ifndef YES
#  define YES   1
#  define NO    0
#endif


// This function switches echo mode on or off. In off mode, nothing
// the user types is echoed on the screen (e.g. when typing a password).
// Note: We're using the 'stty' command to switch the mode. There are
//       cleaner ways, but we don't want to complicate things now.
void echo_on(int is_it)
{
    if (is_it == YES)
        system("stty echo");
    else
        system("stty -echo");
}

// Here is the signal handler. It switches the input mode to echo-on,
// then suspends the process using a STOP signal. Once the process'
// operation is resumed, the function continues right after the call to
// the kill() function, sets echo mode off, and reproduces the password
// prompt, so the user will know that we're still expecting a password.
void catch_suspend(int sig_num)
{
    signal(sig_num, catch_suspend); // re-set the signal handler

    printf("\nSuspending execution...\n");
    fflush(stdout);

    echo_on(YES);           // re-enable echo mode

    kill(getpid(), SIGSTOP);    // stop self

    // we'll get back here when the process is resumed
    printf("Resuming execution...\n");

    echo_on(NO);            // disable echo mode again

    printf("Password: ");       // reproduce the prompt
    fflush(stdout);
}


// -------- and the main function goes here ---------

int main(int argc, char *argv[])
{
#define MAX_SIZE 30
    char user[MAX_SIZE];    // user name supplied by the user
    char passwd[MAX_SIZE];  // password supplied by the user

    printf("Username: ");       // prompt the user for a user name
    fflush(stdout);

    fgets(user, MAX_SIZE, stdin);   // wait for input

    signal(SIGTSTP, catch_suspend); // set the TSTP (Ctrl-Z) signal handler

    printf("Password: ");       // prompt the user for a password
    fflush(stdout);

    echo_on(NO);            // set input to no-echo mode
    fgets(passwd, MAX_SIZE, stdin); // get the user input
    echo_on(YES);           // re-enable echo on input

    printf("\n");           // the Enter pressed by the user was not echoed
    fflush(stdout);

    signal(SIGTSTP, SIG_DFL);   // switch the TSTP signal handler to its default behaviour

    // verify the password (\n is stored, don't compare it)
    if (strlen(passwd) - 1 == strlen(GOOD_PASSWORD) &&
        strncmp(passwd, GOOD_PASSWORD, strlen(passwd) - 1) == 0)
        printf("Access granted.\n");
    else
        printf("Access denied.\n");

    return 0;
}