c程序中的非规范终端模式缓冲区标准输出

Question

I am working a school project (building a very basic shell).

The idea is to be able to do line edition like in bash. For this, I change the terminal mode to non canonical and I stop echo.

I made a very simple code to expose my issue (please note, I do check for functions returns etc... I just made it as short as possible for this post)

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

int main()
{
    int     ret;
    char    buff;
    char    *term_name;
    char    *termcap;
    struct termios  termios_new;
    struct termios  termios_backup;

    /*
    ** Init termcap library
    */
    term_name = getenv("TERM");
    tgetent(NULL, term_name);

    /*
    ** Get the terminal mode to non canonical and shut down echo
    */
    bzero(&termios_new, sizeof(struct termios));
    tcgetattr(STDIN_FILENO, &termios_backup);
    termios_new = termios_backup;

    termios_new.c_lflag &= ~(ICANON);
    termios_new.c_lflag &= ~(ECHO);
    termios_new.c_cc[VMIN] = 1;
    termios_new.c_cc[VTIME] = 0;

    /*
    **  Set the change
    */
    tcsetattr(STDIN_FILENO, TCSAFLUSH, &termios_new);

    /*
    ** Get the termcap for clearing screen on the particular terminal
    */
    termcap = tgetstr("cl", NULL);

    /*
    ** Loop read to get user entries and clear screen for 'c', output char for 'b', break for 'q'
    */
    while((ret = read(STDIN_FILENO, &buff, 1)) > 0)
    {
        if (buff == 'c')
            tputs(termcap, 1, putchar);
        else if (buff == 'b')
            putchar(buff);
        else if (buff == 'q')
            break ;
        buff = 0;
    }

    /*
    ** Put back the terminal mode as found before
    */
    tcsetattr(STDIN_FILENO, TCSAFLUSH, &termios_backup);
    return (0);
}

So basically it's a loop on read to catch user entries. It clears the screen for 'c', output char for 'b', break and restore the original terminal mode for 'q'.

Issue is :

Whenever I type anything, it seems to be buffered because nothing happens until I break the loop with 'q'. At this moment, the output show on screen, if I typed 5 times b, I'll get the 5 b's and if I types 'c', the screen will be cleared. BUT, only after typing 'q'. The behaviour is the same while restoring or not the original terminal mode. (the last line before return )

What I suspect :

After making the code very short and checking all returns, I tend to think there could only be an issue with the way I change the terminal mode ? I tries with the flags TCSAFLUSH and TCSADRAIN for the tcsetattr function with same result.

Thanks ! :)

Answer 1

✅ SOLVED :

Ok so for anyone who encounter this situation, it's quite interesting because it made me learn many stuffs (well it's a school project so...).

Using putchar in the tputs parameters was the issue, because putchar is buffered, juste like printf. I ended up trying two things that led me to the realisation: fflush() after the tputs function call would work so obviously the issue was buffer related. Then tried to output to STDERR_FILENO and it kinda solved it too, and in fact the stderr is the only one _IONBF , so not buffered.

So I ended up creating a putchar function with only write in it, and that was it.

Here are more info about the three buffer modes:

• http://software-dl.ti.com/ccs/esd/documents/sdto_cgt_tips_for_using_printf.html