/*
 * ISA 563, Spring 2011
 * Copyleft by Muhammad Abdulla
 */

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>
#include <stdlib.h>

/*
 * alarm.c -- demonstrates an practical use of alarm function.
 */

enum { FAILURE, SUCCESS } ;

void alarm_handler(int signo)
{
    printf("Operation timed out. Exiting...\n\n");
    exit(0);
}


int check_user(char *uname, int umax)
{
    int ulen;

    char *valid_chars = "abcdefghijklmnopqrstuvwxyz1234567890";

    fprintf(stdout, "Enter your user name: ");
    fflush(stdout);

    alarm(20);   // set off alarm so that we will receive SIGALRM in 20 seconds

    if( fgets(uname, umax, stdin) == NULL ) {
        // alarm(0);  // turn off alert before we return (try commenting off this)
        return FAILURE;
    }

    alarm(0);  // turn off alert before we return

    ulen = strlen(uname);

    if ( uname[ulen - 1] == '\n' ) { // chop off the newline character
       uname[--ulen] = '\0';
    }

    // username characters should be alphanumeric, and should not start with a digit
    if ( (ulen == 0) || isdigit(uname[0]) || (ulen != strspn(uname, valid_chars)) ) { 
       return FAILURE;
    }

    return SUCCESS;
}

int main ( int argc, char *argv[] )
{
    char uname[64];

    signal(SIGALRM, alarm_handler);

    if ( check_user(uname, sizeof(uname)) == FAILURE ) {
       printf("User name is invalid.\n");
    } else {
       printf("User name is valid.\n");
    }

    printf("Waiting something to happen ...\n");
    pause();
}