C Snippet #8(规定时间输入，否则默认跳转的实现)

http://www.ddj.com/cpp/221600722
Q: HOW DO I... put timers with default actions in my C code?
A: Many times, we need to write programs that will only wait a certain specified amount of time for a user to do something. After that time, we need to assume that the user isn't going to do anything and move on to some specified default action. This Snippet works exactly like the get_ch() function in most PC C compiler libraries, except that it will only wait as long as you tell it for the user to hit a key. If no key is pressed within the time limit, the function returns with an EOF. As usual, a test harness has been added to allow for stand-alone testing. To compile the stand-alone test, simply define the TEST macro.
/*
** TIMEGETC.C - waits for a given number of seconds for the user to press
** a key. Returns the key pressed, or EOF if time expires
**
** Public domain by Bob Jarvis
** Test harness added by Bob Stout
**
** Note: This relies on the non-standard kbhit() function, which is available
** in all Windows PC compilers - including MinGW32 (gcc).
*/
#include <stdio.h>
#include <time.h>
#include <conio.h>
int timed_getch(int n_seconds)
{
time_t start, now;
start = time(NULL);
now = start;
while(difftime(now, start) < (double)n_seconds && !kbhit())
{
now = time(NULL);
}
if(kbhit())
return getch();
else return EOF;
}
#ifdef TEST
main()
{
int c;
const char *ctrlchars[] =
{
"NUL", "SOH", "STX", "ETX", "EOT", "ENQ", "ACK", "BEL",
"BS_", "HT_", "LF_", "VT_", "FF_", "CR_", "SO_", "SI_",
"DLE", "DC1", "DC2", "DC3", "DC4", "NAK", "SYN", "ETB",
"CAN", "EM_", "SUB", "ESC", "FS_", "GS_", "RS_", "US_"
};

printf("Starting a 5 second delay...\n");
c = timed_getch(5);
if(c == EOF)
printf("Timer expired\n