Brian Dessent <[EMAIL PROTECTED]> wrote:
> It has been said before in previous threads but I want to re-state that
> this is possible in a very simple and general way with the LD_PRELOAD
> facility:
>
> $ echo '__attribute__((constructor)) void f() { setvbuf (stdout, NULL,
> _IOLBF, 0); }' | \
> gcc -include stdio.h -x c - -fPIC -shared -o linebuf.so
>
> $ ( while true; do echo "foo"; sleep 1; done ) | LD_PRELOAD=./linebuf.so grep
> foo | cat
> foo
> foo
> foo
> ... # immediate output
Nice!
This deserves to be better known.
What we need is a convenient mechanism by which to use this technique.
I.e., we need a new tool, maybe named line-buffer, that sets
LD_PRELOAD as in your example, and then exec's the target program.
If the new program (use env.c as a model) were to come
with documentation and sufficient portability, I'd probably
be happy to add it to coreutils. [hmm... that'd mean
coreutils would end up installing its first shared library. ]
I.e., with a line-buffer program, (C equivalent of this):
$ cat line-buffer
#!/bin/sh
LD_PRELOAD=/path/to/linebuf.so exec "$@"
your example would become:
$ ( while :; do echo foo; sleep 1; done ) | line-buffer grep foo | cat
foo
foo
foo
... # immediate output
However, that does have the down-side of exec'ing grep,
and not running your shell function or alias.
But if the shared library is installed, you can
instead use your own line-buffer function:
line-buffer()
{
LD_PRELOAD=/t/linebuf.so "$@"
}
but that doesn't expand the command name when it's an alias or function.
Hmm... I know you can do that in bash and zsh, but don't remember
how off-hand. Time to read the manuals...
But now, when I see it's so easy to roll your own,
I wonder if it's worth adding a C program to do that for you.
Opinions?
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