I'm not sure if this is a known issue, or if I just don't understand how to use threads, but I've got writeln statements sometimes printing out twice in some areas of my code. It seems to only happen when I start a thread that checks for input with stdin.byLineCopy (I'm not sure of the internal workings of it so that may be the issue)

The setup is basically this:

void main(){
    auto thread = Thread(&func).start;
    writeln("Output");
    ...
}

void func(){
    foreach (line; stdin.byLineCopy) {
        ...
    }
}

Some of the time it will print out "Output" and some of the time it will print out
"Output
Output"

Does anyone know what is causing this or how I can fix it?

On Wednesday, 30 August 2017 at 10:13:57 UTC, Timothy Foster wrote:
> I'm not sure if this is a known issue, or if I just don't understand how to use threads, but I've got writeln statements sometimes printing out twice in some areas of my code.
> <...>
> Does anyone know what is causing this or how I can fix it?

Difficult to say by what you posted.

You may want to provide a complete example so that others may try to reproduce it.  Additionally, as you gradually simplify your code until it is small enough to post here, or on DPaste, you may find the cause faster yourself.

Ivan Kazmenko.

On Wednesday, 30 August 2017 at 10:44:43 UTC, Ivan Kazmenko wrote:
> On Wednesday, 30 August 2017 at 10:13:57 UTC, Timothy Foster wrote:
>> I'm not sure if this is a known issue, or if I just don't understand how to use threads, but I've got writeln statements sometimes printing out twice in some areas of my code.
>> <...>
>> Does anyone know what is causing this or how I can fix it?
>
> Difficult to say by what you posted.
>
> You may want to provide a complete example so that others may try to reproduce it.  Additionally, as you gradually simplify your code until it is small enough to post here, or on DPaste, you may find the cause faster yourself.
>
> Ivan Kazmenko.

import std.stdio, core.thread;

void main(){	
	auto thread = new Thread(&func).start;
        writeln("Output");
	writeln("Output2");
	writeln("Output3");
	while(true){}
}

void func(){
    foreach(line; stdin.byLineCopy){}
}

The printout from the above typically gives me:
Output
Output2
Output2
Output3

I've narrowed down the issue. I just don't know what to do about it.

On Wednesday, 30 August 2017 at 10:55:20 UTC, Timothy Foster wrote:
> On Wednesday, 30 August 2017 at 10:44:43 UTC, Ivan Kazmenko wrote:
>> On Wednesday, 30 August 2017 at 10:13:57 UTC, Timothy Foster wrote:
>>> I'm not sure if this is a known issue, or if I just don't understand how to use threads, but I've got writeln statements sometimes printing out twice in some areas of my code.
>>> <...>
>>> Does anyone know what is causing this or how I can fix it?
>>
>> Difficult to say by what you posted.
>>
>> You may want to provide a complete example so that others may try to reproduce it.  Additionally, as you gradually simplify your code until it is small enough to post here, or on DPaste, you may find the cause faster yourself.
>>
>> Ivan Kazmenko.
>
> import std.stdio, core.thread;
>
> void main(){	
> 	auto thread = new Thread(&func).start;
>         writeln("Output");
> 	writeln("Output2");
> 	writeln("Output3");
> 	while(true){}
> }
>
> void func(){
>     foreach(line; stdin.byLineCopy){}
> }
>
> The printout from the above typically gives me:
> Output
> Output2
> Output2
> Output3
>
> I've narrowed down the issue. I just don't know what to do about it.

I cannot reproduce this. what os / compiler are you using ?

On Wednesday, 30 August 2017 at 11:28:49 UTC, Stefan Koch wrote:
> On Wednesday, 30 August 2017 at 10:55:20 UTC, Timothy Foster wrote:
>> On Wednesday, 30 August 2017 at 10:44:43 UTC, Ivan Kazmenko wrote:
>>> [...]
>>
>> import std.stdio, core.thread;
>>
>> void main(){	
>> 	auto thread = new Thread(&func).start;
>>         writeln("Output");
>> 	writeln("Output2");
>> 	writeln("Output3");
>> 	while(true){}
>> }
>>
>> void func(){
>>     foreach(line; stdin.byLineCopy){}
>> }
>>
>> The printout from the above typically gives me:
>> Output
>> Output2
>> Output2
>> Output3
>>
>> I've narrowed down the issue. I just don't know what to do about it.
>
> I cannot reproduce this. what os / compiler are you using ?

Windows 7 Ultimate 64-bit
DMD32 D Compiler v2.075.1

The issue is inconsistent. Sometimes it'll print as it should, sometimes it'll double the first line, sometimes the second, sometimes both the first and second.

On Wednesday, 30 August 2017 at 10:55:20 UTC, Timothy Foster wrote:
> import std.stdio, core.thread;
>
> void main(){	
> 	auto thread = new Thread(&func).start;
>         writeln("Output");
> 	writeln("Output2");
> 	writeln("Output3");
> 	while(true){}
> }
>
> void func(){
>     foreach(line; stdin.byLineCopy){}
> }

I also cannot reproduce this.

My current system is Win7 64-bit.  I tried 32-bit dmd 2.072.0 and 2.075.1, with optimizations turned on and off, but it prints correctly tens of times in each case.

Try running the program in a bare console (cmd.exe on Windows, or some *sh on Linux).  If the problem goes away, your usual environment is likely at fault.  If not,.. well, no idea for now.

Ivan Kazmenko.

On Wednesday, 30 August 2017 at 13:24:55 UTC, Ivan Kazmenko wrote:
> On Wednesday, 30 August 2017 at 10:55:20 UTC, Timothy Foster wrote:
>> import std.stdio, core.thread;
>>
>> void main(){	
>> 	auto thread = new Thread(&func).start;
>>         writeln("Output");
>> 	writeln("Output2");
>> 	writeln("Output3");
>> 	while(true){}
>> }
>>
>> void func(){
>>     foreach(line; stdin.byLineCopy){}
>> }
>
> I also cannot reproduce this.
>
> My current system is Win7 64-bit.  I tried 32-bit dmd 2.072.0 and 2.075.1, with optimizations turned on and off, but it prints correctly tens of times in each case.
>
> Try running the program in a bare console (cmd.exe on Windows, or some *sh on Linux).  If the problem goes away, your usual environment is likely at fault.  If not,.. well, no idea for now.

Hey, I followed my own advice and do see the same issue! when:
1. compiling with dmd 2.075.1 (optimization switches seem to be irrelevant but the issue does not reproduce with 2.072.0),
2. running in a bare cmd.exe, and
3. running the program as just "a.exe", so that it waits for console input (previously I redirected some input to it, like "a.exe <a.exe" or "a.exe <a.d", and the issue does not reproduce then).
Interesting.  As to what to do with it, no idea for now.  At the very least we can issue a bug report, now that at least two people can reproduce it, so it is unlikely to be environment-dependent.

Ivan Kazmenko.

On Wednesday, 30 August 2017 at 13:33:06 UTC, Ivan Kazmenko wrote:
> Interesting.  As to what to do with it, no idea for now.  At the very least we can issue a bug report, now that at least two people can reproduce it, so it is unlikely to be environment-dependent.

Reported: https://issues.dlang.org/show_bug.cgi?id=17797 .

On 8/30/17 9:33 AM, Ivan Kazmenko wrote:
> On Wednesday, 30 August 2017 at 13:24:55 UTC, Ivan Kazmenko wrote:
>> On Wednesday, 30 August 2017 at 10:55:20 UTC, Timothy Foster wrote:
>>> import std.stdio, core.thread;
>>>
>>> void main(){
>>>     auto thread = new Thread(&func).start;
>>>         writeln("Output");
>>>     writeln("Output2");
>>>     writeln("Output3");
>>>     while(true){}
>>> }
>>>
>>> void func(){
>>>     foreach(line; stdin.byLineCopy){}
>>> }
>>
>> I also cannot reproduce this.
>>
>> My current system is Win7 64-bit.  I tried 32-bit dmd 2.072.0 and 2.075.1, with optimizations turned on and off, but it prints correctly tens of times in each case.
>>
>> Try running the program in a bare console (cmd.exe on Windows, or some *sh on Linux).  If the problem goes away, your usual environment is likely at fault.  If not,.. well, no idea for now.
> 
> Hey, I followed my own advice and do see the same issue! when:
> 1. compiling with dmd 2.075.1 (optimization switches seem to be irrelevant but the issue does not reproduce with 2.072.0),
> 2. running in a bare cmd.exe, and
> 3. running the program as just "a.exe", so that it waits for console input (previously I redirected some input to it, like "a.exe <a.exe" or "a.exe <a.d", and the issue does not reproduce then).
> Interesting.  As to what to do with it, no idea for now.  At the very least we can issue a bug report, now that at least two people can reproduce it, so it is unlikely to be environment-dependent.

Just a thought, but the "double printing" could be a misunderstanding. It could be printing Output\nOutput2, but not getting the 2 out there.

Note that DMD 32-bit is using DMC libc. It might be that it gets hung up somehow when expecting input, like it locks the console somehow.

I would say that byLineCopy puts the thread to sleep waiting for input, and it doesn't get out of that state. So it could be that the bug only appears when it gets to that state at some point in the output. I'd pepper some sleeps around the outputs to see if you can make the context switches more predictable.

-Steve

On Thursday, 31 August 2017 at 14:43:39 UTC, Steven Schveighoffer wrote:
> Just a thought, but the "double printing" could be a misunderstanding. It could be printing Output\nOutput2, but not getting the 2 out there.

No no, it's four lines instead of three.  If we change the lines to disjoint sets of letters, the problem persists.

> Note that DMD 32-bit is using DMC libc. It might be that it gets hung up somehow when expecting input, like it locks the console somehow.
>
> I would say that byLineCopy puts the thread to sleep waiting for input, and it doesn't get out of that state. So it could be that the bug only appears when it gets to that state at some point in the output. I'd pepper some sleeps around the outputs to see if you can make the context switches more predictable.

Inserting different sleeps into the threads makes the problem go away (cannot reproduce).  Inserting identical sleeps produces the error with roughly the same probability.

Anyway, I've reported it (https://issues.dlang.org/show_bug.cgi?id=17797), along with a more or less exact version (2.073.2) where the problem was introduced.

Ivan Kazmenko.