Capture ESCAPE key using termios under OSX

Question

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.

Works for me on Linux. Have you tried to run it under a debugger? — n. m. could be an AI, Dec 10 '15 at 21:49
Thank you @n.m., yes, it works also for me on Debian, Mavericks comes without gdb nor valgrind, is not my laptop and I can not install it :( — David Ranieri, Dec 10 '15 at 21:56
I hear restoring VMIN to 1 doesn't always work on OSX. https://bitbucket.org/techtonik/python-pager/issues/5/read-doesnt-block-on-mac-os-x. No idea how to cope. — n. m. could be an AI, Dec 11 '15 at 05:46

score 2 · Accepted Answer · answered Dec 12 '15 at 02:14

2

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:

answered Dec 12 '15 at 02:14

Thomas Dickey

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Thank you Thomas, you mean it is preferable to use `read` instead of `getchar`? (regardless of fault) – David Ranieri Dec 12 '15 at 09:52
1

yes: without delving into the source-code for OSX's `getchar`, I could not tell exactly *why* it set the `EOF` condition. These internal details are not well-standardized (and can differ). But using `read`, you can make the prorgram much more predictable and portable. – Thomas Dickey Dec 12 '15 at 12:20
Does `read` puts `stdout` in line buffered mode?, I have to `fflush(stdout);` when I use `read` instead of `getchar()` – David Ranieri Dec 12 '15 at 14:15
`read` does not affect `stdout`. Your program may mix other calls such as `write` and `printf` which could be relevant. – Thomas Dickey Dec 12 '15 at 14:35
Thank you again Thomas, sorry but I don't get you, before switching from `getchar` to `read`, I was able to `printf("something");` without a trailing newline or a call to `fflush(stdout);`. Something has changed. – David Ranieri Dec 12 '15 at 15:34
Perhaps you could add your revised program to the current question (or start a new question). – Thomas Dickey Dec 12 '15 at 15:36
Yes, I ask a new question. – David Ranieri Dec 12 '15 at 15:36

Capture ESCAPE key using termios under OSX

1 Answers1