I'm trying to write a program in C (on Linux) that loops until the user presses a key, but shouldn't require a keypress to continue each loop.
Is there a simple way to do this? I figure I could possibly do it with select() but that seems like a lot of work.
Alternatively, is there a way to catch a ctrl-c keypress to do cleanup before the program closes instead of non-blocking io?
This question is related to
c
linux
asynchronous
input
nonblocking
Here's a function to do this for you. You need termios.h which comes with POSIX systems.
#include <termios.h>
void stdin_set(int cmd)
{
struct termios t;
tcgetattr(1,&t);
switch (cmd) {
case 1:
t.c_lflag &= ~ICANON;
break;
default:
t.c_lflag |= ICANON;
break;
}
tcsetattr(1,0,&t);
}
Breaking this down: tcgetattr gets the current terminal information and stores it in t. If cmd is 1, the local input flag in t is set to non-blocking input. Otherwise it is reset. Then tcsetattr changes standard input to t.
If you don't reset standard input at the end of your program you will have problems in your shell.
select() is a bit too low-level for convenience. I suggest you use the ncurses library to put the terminal in cbreak mode and delay mode, then call getch(), which will return ERR if no character is ready:
WINDOW *w = initscr();
cbreak();
nodelay(w, TRUE);
At that point you can call getch without blocking.
The curses library can be used for this purpose. Of course, select() and signal handlers can be used too to a certain extent.
Here's a function to do this for you. You need termios.h which comes with POSIX systems.
#include <termios.h>
void stdin_set(int cmd)
{
struct termios t;
tcgetattr(1,&t);
switch (cmd) {
case 1:
t.c_lflag &= ~ICANON;
break;
default:
t.c_lflag |= ICANON;
break;
}
tcsetattr(1,0,&t);
}
Breaking this down: tcgetattr gets the current terminal information and stores it in t. If cmd is 1, the local input flag in t is set to non-blocking input. Otherwise it is reset. Then tcsetattr changes standard input to t.
If you don't reset standard input at the end of your program you will have problems in your shell.
On UNIX systems, you can use sigaction call to register a signal handler for SIGINT signal which represents the Control+C key sequence. The signal handler can set a flag which will be checked in the loop making it to break appropriately.
The curses library can be used for this purpose. Of course, select() and signal handlers can be used too to a certain extent.
Another way to get non-blocking keyboard input is to open the device file and read it!
You have to know the device file you are looking for, one of /dev/input/event*. You can run cat /proc/bus/input/devices to find the device you want.
This code works for me (run as an administrator).
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <linux/input.h>
int main(int argc, char** argv)
{
int fd, bytes;
struct input_event data;
const char *pDevice = "/dev/input/event2";
// Open Keyboard
fd = open(pDevice, O_RDONLY | O_NONBLOCK);
if(fd == -1)
{
printf("ERROR Opening %s\n", pDevice);
return -1;
}
while(1)
{
// Read Keyboard Data
bytes = read(fd, &data, sizeof(data));
if(bytes > 0)
{
printf("Keypress value=%x, type=%x, code=%x\n", data.value, data.type, data.code);
}
else
{
// Nothing read
sleep(1);
}
}
return 0;
}
On UNIX systems, you can use sigaction call to register a signal handler for SIGINT signal which represents the Control+C key sequence. The signal handler can set a flag which will be checked in the loop making it to break appropriately.
You can do that using select as follow:
int nfds = 0;
fd_set readfds;
FD_ZERO(&readfds);
FD_SET(0, &readfds); /* set the stdin in the set of file descriptors to be selected */
while(1)
{
/* Do what you want */
int count = select(nfds, &readfds, NULL, NULL, NULL);
if (count > 0) {
if (FD_ISSET(0, &readfds)) {
/* If a character was pressed then we get it and exit */
getchar();
break;
}
}
}
Not too much work :D
select() is a bit too low-level for convenience. I suggest you use the ncurses library to put the terminal in cbreak mode and delay mode, then call getch(), which will return ERR if no character is ready:
WINDOW *w = initscr();
cbreak();
nodelay(w, TRUE);
At that point you can call getch without blocking.
If you are happy just catching Control-C, it's a done deal. If you really want non-blocking I/O but you don't want the curses library, another alternative is to move lock, stock, and barrel to the AT&T sfio library. It's nice library patterned on C stdio but more flexible, thread-safe, and performs better. (sfio stands for safe, fast I/O.)
There is no portable way to do this, but select() might be a good way. See http://c-faq.com/osdep/readavail.html for more possible solutions.
You probably want kbhit();
//Example will loop until a key is pressed
#include <conio.h>
#include <iostream>
using namespace std;
int main()
{
while(1)
{
if(kbhit())
{
break;
}
}
}
this may not work on all environments. A portable way would be to create a monitoring thread and set some flag on getch();
On UNIX systems, you can use sigaction call to register a signal handler for SIGINT signal which represents the Control+C key sequence. The signal handler can set a flag which will be checked in the loop making it to break appropriately.
Another way to get non-blocking keyboard input is to open the device file and read it!
You have to know the device file you are looking for, one of /dev/input/event*. You can run cat /proc/bus/input/devices to find the device you want.
This code works for me (run as an administrator).
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <linux/input.h>
int main(int argc, char** argv)
{
int fd, bytes;
struct input_event data;
const char *pDevice = "/dev/input/event2";
// Open Keyboard
fd = open(pDevice, O_RDONLY | O_NONBLOCK);
if(fd == -1)
{
printf("ERROR Opening %s\n", pDevice);
return -1;
}
while(1)
{
// Read Keyboard Data
bytes = read(fd, &data, sizeof(data));
if(bytes > 0)
{
printf("Keypress value=%x, type=%x, code=%x\n", data.value, data.type, data.code);
}
else
{
// Nothing read
sleep(1);
}
}
return 0;
}
There is no portable way to do this, but select() might be a good way. See http://c-faq.com/osdep/readavail.html for more possible solutions.
You probably want kbhit();
//Example will loop until a key is pressed
#include <conio.h>
#include <iostream>
using namespace std;
int main()
{
while(1)
{
if(kbhit())
{
break;
}
}
}
this may not work on all environments. A portable way would be to create a monitoring thread and set some flag on getch();
There is no portable way to do this, but select() might be a good way. See http://c-faq.com/osdep/readavail.html for more possible solutions.
If you are happy just catching Control-C, it's a done deal. If you really want non-blocking I/O but you don't want the curses library, another alternative is to move lock, stock, and barrel to the AT&T sfio library. It's nice library patterned on C stdio but more flexible, thread-safe, and performs better. (sfio stands for safe, fast I/O.)
There is no portable way to do this, but select() might be a good way. See http://c-faq.com/osdep/readavail.html for more possible solutions.
If you are happy just catching Control-C, it's a done deal. If you really want non-blocking I/O but you don't want the curses library, another alternative is to move lock, stock, and barrel to the AT&T sfio library. It's nice library patterned on C stdio but more flexible, thread-safe, and performs better. (sfio stands for safe, fast I/O.)
select() is a bit too low-level for convenience. I suggest you use the ncurses library to put the terminal in cbreak mode and delay mode, then call getch(), which will return ERR if no character is ready:
WINDOW *w = initscr();
cbreak();
nodelay(w, TRUE);
At that point you can call getch without blocking.
You can do that using select as follow:
int nfds = 0;
fd_set readfds;
FD_ZERO(&readfds);
FD_SET(0, &readfds); /* set the stdin in the set of file descriptors to be selected */
while(1)
{
/* Do what you want */
int count = select(nfds, &readfds, NULL, NULL, NULL);
if (count > 0) {
if (FD_ISSET(0, &readfds)) {
/* If a character was pressed then we get it and exit */
getchar();
break;
}
}
}
Not too much work :D
The curses library can be used for this purpose. Of course, select() and signal handlers can be used too to a certain extent.
You probably want kbhit();
//Example will loop until a key is pressed
#include <conio.h>
#include <iostream>
using namespace std;
int main()
{
while(1)
{
if(kbhit())
{
break;
}
}
}
this may not work on all environments. A portable way would be to create a monitoring thread and set some flag on getch();
You probably want kbhit();
//Example will loop until a key is pressed
#include <conio.h>
#include <iostream>
using namespace std;
int main()
{
while(1)
{
if(kbhit())
{
break;
}
}
}
this may not work on all environments. A portable way would be to create a monitoring thread and set some flag on getch();
If you are happy just catching Control-C, it's a done deal. If you really want non-blocking I/O but you don't want the curses library, another alternative is to move lock, stock, and barrel to the AT&T sfio library. It's nice library patterned on C stdio but more flexible, thread-safe, and performs better. (sfio stands for safe, fast I/O.)
Source: Stackoverflow.com