#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 */

#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's    */
/* 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)
{
    /* re-set the signal handler again to catch_int, for next time */
    signal(SIGTSTP, catch_suspend);
    printf("Suspending execution...\n");
    fflush(stdout);
    /* re-enable echo mode, so the user can keep on working.. */
    echo_on(YES);
    kill(getpid(), SIGSTOP);
    /* ... we'll get back here when the process is resumed... */
    printf("Resuming execution, please hold on...\n");
    /* disable echo mode again. */
    echo_on(NO);
    /* reproduce the prompt, so the users will remember */
    /* where they suspended the program.                */
    printf("Password: ");
    fflush(stdout);
}

/* and the main function goes here */
int main(int argc, char* argv[])
{
    char user[30];	/* user name supplied by the user */
    char passwd[30];	/* password supplied by the user  */

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

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

    /* prompt the user for a password */
    printf("Password: ");
    fflush(stdout);
    /* set input to no-echo mode, so what the user types */
    /* won't be shown on screen.                         */
    echo_on(NO);
    /* wait for input */
    gets(passwd);
    /* re-enable echo on input */
    echo_on(YES);
    /* print a new-line, since the ENTER pressed by the user was not echoec */
    printf("\n");
    fflush(stdout);
    /* switch the TSTP signal handler to its default behaviour */
    signal(SIGTSTP, SIG_DFL);

    /* and now, do something with the password */
    if (strcmp(passwd, GOOD_PASSWORD) == 0)
	printf("Access granted.\n");
    else
        printf("Access denied.\n");

    return 0;
}