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cmacosterminalxtermtermios

Capture ESCAPE key using termios under OSX

I am trying to capture ESC key (ASCII 27) on a OSX terminal or xterm using kbhit to distinguish a real Escape from Arrow keys:

#include <stdio.h>
#include <termios.h>
#include <unistd.h>

static struct termios newt;
static struct termios oldt;

static void kb_fini(void)
{
    tcsetattr(STDIN_FILENO, TCSANOW, &oldt);
}

void kb_init(void)
{
    tcgetattr(STDIN_FILENO, &oldt);
    newt = oldt;
    newt.c_lflag &= (tcflag_t)~(ICANON | ECHO | ISIG);
    newt.c_cc[VMIN] = 1;
    newt.c_cc[VTIME] = 0;
    tcsetattr(STDIN_FILENO, TCSANOW, &newt);
    atexit(kb_fini);
}

static int kb_hit(void)
{
    int c = 0;

    newt.c_cc[VMIN] = 0;
    tcsetattr(STDIN_FILENO, TCSANOW, &newt);
    c = getc(stdin);
    newt.c_cc[VMIN] = 1;
    tcsetattr(STDIN_FILENO, TCSANOW, &newt);
    if (c != -1) {
        ungetc(c, stdin);
        return 1;
    }
    return 0;
}

int main(void)
{
    int c;

    kb_init();
    printf("Press ESC several times\n");
    while (1) {
        c = getchar();
        if ((c == 27) && (kb_hit() == 0)) {
            printf("You pressed ESC\n");    
        } else
        if (c == '\n') {
            break;
        }
    }
    return 0;
}

But it only works the first time, the second time I press the escape key, the terminal doesn't admit more data. It doesn't freeze completely, as the prompt keeps blinking, but pressing more keys doesn't change anything.

Solution

  • Manipulating VTIME and VMIN like that would be useful if you were reading the standard input directly using read. However, you are reading it via the C stream input stdin, which means that you are relying on a particular behavior for that, rather than the low-level termios feature.

    The program loops because getchar has decided it has detected an end-of-file condition, and continues to return -1, never returning to kb_hit. You can amend that by calling

    clearerr(stdin);

    after the call to getchar (since it resets the end-of-file condition), though relying upon any particular behavior or interaction between stream and low-level I/O is nonportable.

    For instance, the Linux manual pages for getchar and gets advise

    It is not advisable to mix calls to input functions from the stdio library with low-level calls to read(2) for the file descriptor associated with the input stream; the results will be undefined and very probably not what you want.

    For reference: