Re: flock(2): locking against itself?

[RVP <rvp%SDF.ORG@localhost>]

On Sun, 16 Apr 2023, Mouse wrote:

> In particular, I wrote
>
> > [...].  I have a program that uses curses for output but wants to do
> > non-blocking input.  So I set stdin nonblocking and threw fd 0 into
> > the poll() loop.
>
> > But, in normal use, stdin and stdout come from the same open on the
> > session's tty, so setting stdin nonblocking also sets stdout
> > nonblocking, which curses is not prepared to handle, leading to large
> > screen updates getting partially lost.

I think O_NONBLOCK on an input FD would be a problem too for curses.
Otherwise, if only a non-blocking output were the problem, then you
could just re-open the current tty twice and juggle the FDs like this:

```
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int
main(void)
{
	char* tty;
	int fd1, fd2, flg;

	if ((tty = ttyname(STDIN_FILENO)) == NULL)
		err(1, "ttyname failed");

	if ((fd1 = open(tty, O_RDWR)) == -1)
		err(1, "open for FD 0 failed");

	if ((fd2 = open(tty, O_RDWR)) == -1)
		err(1, "open for FDs 1 & 2 failed");

	if (dup2(fd1, 0) == -1)
		err(1, "dup 0 failed");

	if (dup2(fd2, 1) == -1)
		err(1, "dup 1 failed");

	if (dup2(fd2, 2) == -1)
		err(1, "dup 2 failed");

	if (close(fd1) == -1)
		err(1, "close fd1 failed");

	if (close(fd2) == -1)
		err(1, "close fd2 failed");

	if ((flg = fcntl(STDIN_FILENO, F_GETFL, 0)) == -1)
		err(1, "fcntl F_GETFL failed");

	if (fcntl(STDIN_FILENO, F_SETFL, flg | O_NONBLOCK) == -1)
		err(1, "fcntl F_SETFL failed");

	execl("/usr/bin/vi", "vi", NULL);
	err(1, "exec /usr/bin/vi failed");
}
```

But, vi, at least, fails to run like this. So, I think using a pty is
the simplest, not to say standard, solution here...

-RVP