linux - Move the cursor in a C program

Question

I'd like to move the cursor forward and backwards in a C program. I'm reading the whole line in a loop, but i would like that if a cursor key gets pressed the cursor on the screen changes position, without blocking the loop. I tried getwch() but it blocks the caller until enter it's pressed. What i'm looking for is a behavior similar to bash prompt. I'm reading with a code similar to this:

while (TRUE) {
   printf("%s", PROMPT);
   fgets(input, 1024, stdin);
   do_something(input);
}

I'm trying that the function above works like readline(PROMPT) on the readline.h library

Answer 1

Using termios and console-codes (VT100 compatible - not portable):

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

#define cursorforward(x) printf("33[%dC", (x))
#define cursorbackward(x) printf("33[%dD", (x))

#define KEY_ESCAPE  0x001b
#define KEY_ENTER   0x000a
#define KEY_UP      0x0105
#define KEY_DOWN    0x0106
#define KEY_LEFT    0x0107
#define KEY_RIGHT   0x0108

static struct termios term, oterm;

static int getch(void);
static int kbhit(void);
static int kbesc(void);
static int kbget(void);

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

    tcgetattr(0, &oterm);
    memcpy(&term, &oterm, sizeof(term));
    term.c_lflag &= ~(ICANON | ECHO);
    term.c_cc[VMIN] = 1;
    term.c_cc[VTIME] = 0;
    tcsetattr(0, TCSANOW, &term);
    c = getchar();
    tcsetattr(0, TCSANOW, &oterm);
    return c;
}

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

    tcgetattr(0, &oterm);
    memcpy(&term, &oterm, sizeof(term));
    term.c_lflag &= ~(ICANON | ECHO);
    term.c_cc[VMIN] = 0;
    term.c_cc[VTIME] = 1;
    tcsetattr(0, TCSANOW, &term);
    c = getchar();
    tcsetattr(0, TCSANOW, &oterm);
    if (c != -1) ungetc(c, stdin);
    return ((c != -1) ? 1 : 0);
}

static int kbesc(void)
{
    int c;

    if (!kbhit()) return KEY_ESCAPE;
    c = getch();
    if (c == '[') {
        switch (getch()) {
            case 'A':
                c = KEY_UP;
                break;
            case 'B':
                c = KEY_DOWN;
                break;
            case 'C':
                c = KEY_LEFT;
                break;
            case 'D':
                c = KEY_RIGHT;
                break;
            default:
                c = 0;
                break;
        }
    } else {
        c = 0;
    }
    if (c == 0) while (kbhit()) getch();
    return c;
}

static int kbget(void)
{
    int c;

    c = getch();
    return (c == KEY_ESCAPE) ? kbesc() : c;
}

int main(void)
{
    int c;

    while (1) {
        c = kbget();
        if (c == KEY_ENTER || c == KEY_ESCAPE || c == KEY_UP || c == KEY_DOWN) {
            break;
        } else
        if (c == KEY_RIGHT) {
            cursorbackward(1);
        } else
        if (c == KEY_LEFT) {
            cursorforward(1);
        } else {
            putchar(c);
        }
    }
    printf("
");
    return 0;
}