View Full Version : Simple Checking Keyboard Input Program in C/C++ but with Strange Behavior!!

July 27th, 2009, 10:05 AM
Hi all,

I have a small function for checking whether the keyboard has been pressed without blocking. I tried to compile the program using g++ and another time with gcc. But nothing works correctly.

This is function

int checkKeyboard() {
fd_set readvec;
struct timeval timeout;
int ret, stdin_fd;

timeout.tv_sec = 0;
timeout.tv_usec = 0;
stdin_fd = 0;
FD_SET(stdin_fd, &readvec);

ret = select(1, &readvec, NULL, NULL, &timeout);

if (ret == -1) { // got an error
if (errno != EINTR)
printf("select() error while attempting to read text input.\n");
return FALSE;
} else if (ret == 0) {
return FALSE; // select timed out
return TRUE;

This is the whole testing program :

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#define TRUE 1
#define FALSE 0

//The previous code segment
int checkKeyboard() {
int main(int argc, char* argv[]) {
int i;
for (i=0; i<100000; i++) {
if (vmd_check_stdin()>0) {
printf("Keyboard was pressed. Terminating...\n");
return 0;
} else {
printf("i = %d\n", i);
return 0;

The program loops and exits when the user presses any key in the keyboard. When I run it, program loops and never exits although I pressed the keyboard multiple times while its counting!!After the program finish, the characters are printed to the console as if they all have been buffered while the program ran:confused:

Does anyone have an idea about this strange problem??