I would like to writing a small C program that runs an infinite loop until the user presses a key on the keyboard (ie: there is a char in the stdin buffer). I am running into trouble breaking the loop on user input. I have tried using fgetc
but that does not behave as expected. The code below waits for user input rather then running until user input.
Sample C Code:
while((c=fgetc(stdin) == EOF) {
/* Does stuff for infinite loop here */
printf("Example work in the loop\n");
}
printf("Out of the loop!\n");
How do I write a loop that executes until user intervention? Pressing any key or a specific key could be the intervention trigger.
Note 1: I am writing this for a Unix console in case of platform specific solutions
Note 2: Do not suggest Ctrl + C/X/Z
as the user intervention trigger
This seems to work for me:
#include <fcntl.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
static void set_non_blocking(int fd)
{
int flags = fcntl(fd, F_GETFL, 0 );
flags |= O_NONBLOCK;
flags = fcntl(fd, F_SETFL, flags);
}
int main(int argc, char ** argv)
{
int fd = fileno(stdin);
char buf[10];
set_non_blocking(fd);
while (read(fd, buf, sizeof buf) < 0) {
perror("read");
sleep(1);
}
return 0;
}
or you could use select
:
int main(int argc, char ** argv)
{
int fd = fileno(stdin);
struct timeval tv = {0,0};
fd_set fdset;
int s;
do {
sleep(1);
FD_ZERO(&fdset);
FD_SET(fd, &fdset);
} while ((s = select(fd+1, &fdset, NULL, NULL, &tv)) == 0);
if (s < 0) {
perror("select");
}
return 0;
}
Poll works too :-)
int main(int argc, char ** argv)
{
struct pollfd pfd;
int s;
pfd.fd = fileno(stdin);
pfd.events = POLLRDNORM;
while ((s = poll(&pfd, 1, 0)) == 0) {
perror("polling");
sleep(1);
}
if (s < 0) {
perror("poll");
}
return 0;
}
One last way is to set the terminal in 'raw' mode. Note that this upsets output to the terminal (at least on mine on OS-X) in that \r becomes necessary after \n. Note also that it needs to be undone at the end (the terminating tcsetattr
call). This is the only one that does not need a \n (ie any keypress will do)
#include <poll.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <termios.h>
static void set_non_blocking(int fd)
{
int flags = fcntl(fd, F_GETFL, 0) | O_NONBLOCK;
if (fcntl(fd, F_SETFL, flags) < 0) {
perror("fcntl");
exit(EXIT_FAILURE);
}
}
int main(int argc, char ** argv)
{
struct termios params;
struct termios params_orig;
char buf[10];
int fd = fileno(stdin);
if (tcgetattr(fd, ¶ms) < 0) {
perror("tcgetattr");
exit(EXIT_FAILURE);
}
params_orig = params;
cfmakeraw(¶ms);
if (tcsetattr(fd, TCSANOW, ¶ms) < 0) {
perror("tcsetattr");
exit(EXIT_FAILURE);
}
set_non_blocking(fd);
while (read(fd, buf, sizeof buf) < 0) {
perror("\rread");
sleep(1);
}
(void) tcsetattr(fd, TCSANOW, ¶ms_orig);
return 0;
}