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digitalmars.D.learn - Reading input from piped stdin.

reply "Thomas" <sitronvask gmail.com> writes:
I'm new to D, and I find it quite enjoyable so far.
I have however stumbled upon a problem which I can't seem to
figure out.

I am trying to make a program that creates a child process,
writes something to the child process stdin and reading from its
stdout. I am going to use it later for testing out process pair
redundancy.

Appearently the child blocks at "s = stdin.readln()". If I remove
all writing to the child, and instead just read its output,
everything works fine. My code is attached below:

import std.process,std.stdio,std.getopt,core.thread;

void main(string[] args){
	bool backup = false;
	getopt(args, "backup", &backup);
	writeln("Something worked!");
	string s = "test";
	if (backup){
		writeln("Backup up & running");
		while(true){
			s = stdin.readln();
			writeln(s);
		}
	}
	auto pipes = pipeProcess(["./pipetest", "--backup"],
Redirect.all);
	for(int j = 0; j<5; j++){
		writeln(j);
		pipes.stdin.writeln(j);
		writeln(pipes.stdout.readln());
		Thread.sleep(500.msecs);
	}
	while(true){}

}



If anyone could spot what rudimentary mistake I have done, I
would greatly appreciate it. Alternatively, suggesting another
way to implement inter-process communication would also be
appreciated :D
Feb 14 2014
next sibling parent reply "Adam D. Ruppe" <destructionator gmail.com> writes:
Just a quick look, but I betcha it has to do with buffering. 
After writing the line to the pipe, call the flush() method on 
the output pipe and see what happens there.

(Pipes buffer differently than regular output so this is a common 
mixup, especially with IDEs which communicate with stdout via 
pipes normally!)

let me know if it works
Feb 14 2014
parent reply "Thomas" <sitronvask gmail.com> writes:
On Friday, 14 February 2014 at 19:08:20 UTC, Adam D. Ruppe wrote:

 Just a quick look, but I betcha it has to do with buffering. 
 After writing the line to the pipe, call the flush() method on 
 the output pipe and see what happens there.

 (Pipes buffer differently than regular output so this is a 
 common mixup, especially with IDEs which communicate with 
 stdout via pipes normally!)

 let me know if it works


Impressive reply speed! :)

However calling pipes.stdin.flush() immediately after writeln did
not seem to work.
Feb 14 2014
parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Fri, 14 Feb 2014 14:16:23 -0500, Thomas <sitronvask gmail.com> wrote:


 On Friday, 14 February 2014 at 19:08:20 UTC, Adam D. Ruppe wrote:

 Just a quick look, but I betcha it has to do with buffering. After  
 writing the line to the pipe, call the flush() method on the output  
 pipe and see what happens there.

 (Pipes buffer differently than regular output so this is a common  
 mixup, especially with IDEs which communicate with stdout via pipes  
 normally!)

 let me know if it works


 Impressive reply speed! :)

 However calling pipes.stdin.flush() immediately after writeln did
 not seem to work.


You must also flush from the child process -- it too is connected to a  
pipe and not a console.

-Steve
Feb 14 2014
prev sibling next sibling parent reply "nazriel" <spam dzfl.pl> writes:
On Friday, 14 February 2014 at 19:05:02 UTC, Thomas wrote:

 I'm new to D, and I find it quite enjoyable so far.
 I have however stumbled upon a problem which I can't seem to
 figure out.

 I am trying to make a program that creates a child process,
 writes something to the child process stdin and reading from its
 stdout. I am going to use it later for testing out process pair
 redundancy.

 Appearently the child blocks at "s = stdin.readln()". If I 
 remove
 all writing to the child, and instead just read its output,
 everything works fine. My code is attached below:

 import std.process,std.stdio,std.getopt,core.thread;

 void main(string[] args){
 	bool backup = false;
 	getopt(args, "backup", &backup);
 	writeln("Something worked!");
 	string s = "test";
 	if (backup){
 		writeln("Backup up & running");
 		while(true){
 			s = stdin.readln();
 			writeln(s);
 		}
 	}
 	auto pipes = pipeProcess(["./pipetest", "--backup"],
 Redirect.all);
 	for(int j = 0; j<5; j++){
 		writeln(j);
 		pipes.stdin.writeln(j);
 		writeln(pipes.stdout.readln());
 		Thread.sleep(500.msecs);
 	}
 	while(true){}

 }



 If anyone could spot what rudimentary mistake I have done, I
 would greatly appreciate it. Alternatively, suggesting another
 way to implement inter-process communication would also be
 appreciated :D


Maybe try closing stdin pipe after you are done writing to child.

pipes.stdin.close();

Or try flushing after writing to childs stdin, IIRC:

pipes.stdin.flush();
Feb 14 2014
parent "nazriel" <spam dzfl.pl> writes:
On Friday, 14 February 2014 at 19:09:06 UTC, nazriel wrote:

 On Friday, 14 February 2014 at 19:05:02 UTC, Thomas wrote:

 I'm new to D, and I find it quite enjoyable so far.
 I have however stumbled upon a problem which I can't seem to
 figure out.

 I am trying to make a program that creates a child process,
 writes something to the child process stdin and reading from 
 its
 stdout. I am going to use it later for testing out process pair
 redundancy.

 Appearently the child blocks at "s = stdin.readln()". If I 
 remove
 all writing to the child, and instead just read its output,
 everything works fine. My code is attached below:

 import std.process,std.stdio,std.getopt,core.thread;

 void main(string[] args){
 	bool backup = false;
 	getopt(args, "backup", &backup);
 	writeln("Something worked!");
 	string s = "test";
 	if (backup){
 		writeln("Backup up & running");
 		while(true){
 			s = stdin.readln();
 			writeln(s);
 		}
 	}
 	auto pipes = pipeProcess(["./pipetest", "--backup"],
 Redirect.all);
 	for(int j = 0; j<5; j++){
 		writeln(j);
 		pipes.stdin.writeln(j);
 		writeln(pipes.stdout.readln());
 		Thread.sleep(500.msecs);
 	}
 	while(true){}

 }



 If anyone could spot what rudimentary mistake I have done, I
 would greatly appreciate it. Alternatively, suggesting another
 way to implement inter-process communication would also be
 appreciated :D


 Maybe try closing stdin pipe after you are done writing to 
 child.

 pipes.stdin.close();


Ok, nvm.
You are reading from child after each write.
So naa, closing pipe won't do it.

So we are back to flushing :)

 Or try flushing after writing to childs stdin, IIRC:

 pipes.stdin.flush();
Feb 14 2014
prev sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Fri, 14 Feb 2014 14:05:01 -0500, Thomas <sitronvask gmail.com> wrote:


 I'm new to D, and I find it quite enjoyable so far.
 I have however stumbled upon a problem which I can't seem to
 figure out.

 I am trying to make a program that creates a child process,
 writes something to the child process stdin and reading from its
 stdout. I am going to use it later for testing out process pair
 redundancy.

 Appearently the child blocks at "s = stdin.readln()". If I remove
 all writing to the child, and instead just read its output,
 everything works fine. My code is attached below:


stdin and stdout are buffered streams. Buffered streams in D only flush on  
newline when they are attached to a console (terminal window). Otherwise,  
they wait until the buffer is full before flushing. Buffer is probably  
about 4096 bytes.

What is likely happening is that you are writing, it's going into the  
buffer, but not flushing, and then you are waiting for the response (which  
likely is also not flushing from the child process).

Try adding flushes after each writeln.

BTW, this is not really a D problem, you would have the same issue in C.

-Steve
Feb 14 2014
parent "Thomas" <sitronvask gmail.com> writes:
On Friday, 14 February 2014 at 19:12:24 UTC, Steven Schveighoffer
wrote:

 On Fri, 14 Feb 2014 14:05:01 -0500, Thomas 
 <sitronvask gmail.com> wrote:


 I'm new to D, and I find it quite enjoyable so far.
 I have however stumbled upon a problem which I can't seem to
 figure out.

 I am trying to make a program that creates a child process,
 writes something to the child process stdin and reading from 
 its
 stdout. I am going to use it later for testing out process pair
 redundancy.

 Appearently the child blocks at "s = stdin.readln()". If I 
 remove
 all writing to the child, and instead just read its output,
 everything works fine. My code is attached below:


 stdin and stdout are buffered streams. Buffered streams in D 
 only flush on newline when they are attached to a console 
 (terminal window). Otherwise, they wait until the buffer is 
 full before flushing. Buffer is probably about 4096 bytes.

 What is likely happening is that you are writing, it's going 
 into the buffer, but not flushing, and then you are waiting for 
 the response (which likely is also not flushing from the child 
 process).

 Try adding flushes after each writeln.

 BTW, this is not really a D problem, you would have the same 
 issue in C.

 -Steve


Ah, had to add after both writelns. Looks like that solved it.
Thanks!
Feb 14 2014