Simple pipe in deadlock?

nutthick · 05-10-2006, 06:05 AM

I'm setting up a pipe as follows:

Code:

mkfifo /tmp/mplayer_cmd

then starting the application with this code

Code:

cat /tmp/mplayer_cmd | mplayer -slave test.mp3

The file starts playing.
I then SSH in and type the following

Code:

echo pause > /tmp/mplayer_cmd

and the track pauses. However, any other commands I issue through echo just hang the prompt. From what I've read this is because the pipe is blocked, as there is nothing to read it. But the track paused, so it is being read. I'm lost. mkfifo(3) apparently solves it, but what does the (3) mean?

Can anyone help?

Thanks

Hko · 05-10-2006, 07:11 AM

Quote:

Originally Posted by nutthick

I'm setting up a pipe as follows:

Code:

mkfifo /tmp/mplayer_cmd

then starting the application with this code

Code:

cat /tmp/mplayer_cmd | mplayer -slave test.mp3

The file starts playing.
I then SSH in and type the following

Code:

echo pause > /tmp/mplayer_cmd

and the track pauses. However, any other commands I issue through echo just hang the prompt.

What happens is that the pipe (fifo) is closed after the first echo. Try:

Code:

cat > /tmp/mplayer_cmd

and start typing multiple commands. Type <enter> after each command, and CTRL-d to close the pipe when you're done.

Quote:

Originally Posted by nutthick

I'm lost. mkfifo(3) apparently solves it, but what does the (3) mean?

"mkfifo(3)" means the man page of the "mkfifo" libc function which is in section 3 of the man-system, rather than the mkfifo command which is in section 1 of the manual.

"(3)" is mentioned because there are two different "mkfifo" man-pages. The command "man -k mkfifo" will show all man pages that deal with some "mkfifo" and the section they are in. The command "man 1 mkfifo" will show you the mkfifo man page of section 1 (executables and shell commands) while "man 3 mkfifo" will show the mkfifo C function which is in section 3 (C-library functions). Just "man mkfifo" will show the man page in the lowest numbered section that is called "mkfifo", so in this case it's equivalent to "man 1 mkfifo".

Even if there's no ambiguity to expect (you never know for sure which man pages people have installed), it considered good practice to mention the section when referring to man pages. It also is a brief way to indicate that a man page is refferred to.

To read about the man page sections, see the man page about "man", or better, see man(1).

ioerror · 05-10-2006, 07:31 AM

Quote:

cat /tmp/mplayer_cmd | mplayer -slave test.mp3

It's blocking because cat will exit after you type the first command (it stops when there is no more data).

You should specify the fifo directly to mplayer, e.g.:

Code:

mplayer -input file=/tmp/mplayer_cmd ...

I thinks that right, can't remember offhand (on my laptop at the moment, so can't check it).

Quote:

I'm lost. mkfifo(3) apparently solves it, but what does the (3) mean?

The 3 refers the section three of the man page system. See man 7 man for a description of the sections.

edit:
arrgh! Hko beat me to it again, I must stop going to make a cup of coffee in the middle of typing a post.

nutthick · 05-10-2006, 07:59 AM

@Hko: Thanks for the reply and for explaining the (3) notation. You never know till you ask

I'm still having a few issues with pipe. I set everything up as I explained before and use the cat > method to control it. But if the track finishes before I type in a command I get

Code:

Broken pipe

and have to delete and recreate the pipe file to get things going again. I managed to get it going though, look below

@ioerror: Thanks for the reply. That worked with echo. However, I have the problem as for Hko, if the track ends, the pipe stops working. The mplayer man says to use -idle to stop mplayer quitting and to keep the pipe, but the command isn't recognised. Any ideas?