Gnu Emacs / Cygwin TTY Problem

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Home > Emacs/Cygwin > TTY Problem

TTY Problem

The Gnu Emacs and Cygwin tty problem has been haunting me since I have started using them the first day. I did not understand the cause of the problem at that time. After sufficient researches, I think I have now understood the cause of it, but I have no solution to it. Hopefully someone brilliant can solve the problem for me.

TTY Problem on Gnu Emacs

For Gnu Emacs running in Windows, if a shell is invoked (regardless of cmd.exe shell, emacs eshell or cygwin bash shell), the shell is not run under a tty terminal. This can be verified by invoking the tty.exe command distributed with cygwin. The output from the tty.exe command simply says "not a tty". (For Gnu Emacs running in Unix, tty command says "/dev/ttyN".)

If an application is not invoked within a tty terminal, usually the application assumes that its output is redirected to a file. The application will buffer the output. This means the output will not be flushed until a sufficient amount of output has accumulated.

Considered the following simple buffer.c program.

#include <stdio.h>

int main(int argc, char** argv)
{
    char buf[1000];

    if( _isatty( _fileno( stdout ) ) )
        printf( "stdout has not been redirected to a file\n" );
    else
        printf( "stdout has been redirected to a file\n");

    printf("Hello World\n");
    scanf("%s",buf);
    return(0);
}

If you run the program under a DOS prompt console, the output will be as follow (assuming xxxx is your input)

stdout has not been redirected to a file
Hello World
xxxxx

Unfortunately, if the program is run in a Gnu Emacs in Windows, since the shell is not a tty terminal, the output will be as follow

xxxxx
stdout has been redirected to a file
Hello World

TTY Problem on Cygwin rxvt

It is fortunate that the tty problem does not happen in Cygwin Bash shell running in a Windows DOS prompt console. If a tty.exe command is invoked, it says "/dev/conin".

However, if the Cygwin Bash shell is run in a Cygwin rxvt terminal, the problem appears (assuming the buffer.c program is compiled using Microsoft Visual C compiler or using Cygwin gcc with -mno-cygwin option). If a tty.exe command is invoked, it says "/dev/ttyN". This suggests that /dev/ttyN used by Cygwin is not recognized as tty by Windows.

If the same buffer.c program is compiled using Cygwin gcc complier against the cygwin.dll (gcc buffer.c), then the problem goes away. This means Cygwin applications recognize /dev/ttyN as a valid terminal.

In short, all Cygwin applications work well in a rxvt terminal. All Windows applications that do not involve user inputs work in a rxvt terminal as well. The problem is with all Windows applications that require user inputs.