[fpc-pascal] Pipe buffering, accessing C-style stdin/stdout

Michael Van Canneyt michael.vancanneyt at wisa.be
Mon Apr 2 08:42:54 CEST 2007

On Sun, 1 Apr 2007, Ingemar Ragnemalm wrote:

> Dear list,
> I have been struggling for quite a while (that is weeks) to get proper pipe
> communication with a sublaunched program, using TProcess or straight
> pipe/fork/exec calls. This is under Mac OSX, but the problem should be similar
> under any Unix.
> The problem is the buffering. Using stdio, the data is stalled until the stdio
> buffers are full. This is a big problem. Two-way communication is impossible,
> and the user feedback is delayed.
> After a lot of googling and experimenting, I think I have managed to make it
> work, but only in C code, and I want it to run in Pascal. The key to make it
> work in C was to call setvbuf at the right time, to set the buffering mode for
> stdin/stdout. The problem with FPC is that I can't find any straightforward
> way to either get PFile-style stdin/stdout from the Text-style input/output
> (surely there is a connection?), or get the C globals.

The only call which comes close is settextbuf(). If you set it to a smaller
value, it will come close to being unbuffered. But performance will suffer
from this. You can also call flush() after every write statement.

> I have searched through the fpc and Lazarus sources after the definitions and
> possible solutions, tried all ways I can find, but have not succeeded yet. But
> surely someone else has ran into this problem?
> Shouldn't simply TProcess have unbuffered or line-buffering as default?

TProcess does not do any buffering. What is more, it's up to the called
application to decide whether it buffers it's output or not, and TProcess
does not have any control over that. 


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