can't open named pipe with a tfilestream

[hakelm]

I have a named pipe made with mkfifo /dev/shm/pip and I want to write to it using a filestream, str:tfilestream;.
My program however hangs on str:=tfilestream.Create(fn,fmOpenWrite); without any message or exception.
I have no problem writing to the pipe if I open it as a textfile for writing or with fpsystem('echo "abc" > /dev/shm/pip'); and reading it with for instance cat /dev/shm/pip.
What is it that I haven't understood?
Håkan
on Lazarus 4.6 on FPC 3.2.2 on Ubuntu 22.04

[dsiders]

I have a named pipe made with mkfifo /dev/shm/pip and I want to write to it using a filestream, str:tfilestream;.
My program however hangs on str:=tfilestream.Create(fn,fmOpenWrite); without any message or exception.
I have no problem writing to the pipe if I open it as a textfile for writing or with fpsystem('echo "abc" > /dev/shm/pip'); and reading it with for instance cat /dev/shm/pip.
What is it that I haven't understood?
Håkan
on Lazarus 4.6 on FPC 3.2.2 on Ubuntu 22.04

The fcl-process package has both input and output pipe streams in pipes.pp. Those should do the trick.

[hakelm]

Thanks, but the pipe is created by another program.
H

[Warfley]

The reason for that has nothing to do with your Pascal code but with named pipes in general. Opening of a named pipe will block until the pipe is connected, i.e. it will wait until the other process opens the pipe (for reading in that case) as well.

Example:
Create a pipe with:

Code: Bash  [Select][+][-]

1. > mkfifo pipe.fifo

And run the following program:

Code: Pascal  [Select][+][-]

1. {$mode objfpc}{$H+}
2.  
3. uses
4.   Classes;
5.  
6. var
7.   s: TFileStream;
8. begin
9.   s:=TFileStream.Create('pipe.fifo',fmOpenWrite);
10.   try
11.     s.Write('Test'#10, 5);
12.   finally
13.     s.Free;
14.   end;
15. end.

The program will be stuck and not finish as you describe. Now open a second terminal and read from the pipe:

Code: Bash  [Select][+][-]

1. > cat pipe.fifo
2. Test
3. >

And the Pascal program has finished

[hakelm]

Mea culpa.
I thought I could test all pieces i one program. Now I realise I need two programs or at least two threads.
Sorry about bothering you.
Håkan