http://d.puremagic.com/issues/show_bug.cgi?id=4600

           Summary: writeln() is not thread-safe
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: jmdavisProg@gmail.com


--- Comment #0 from Jonathan M Davis <jmdavisProg@gmail.com> 2010-08-08 20:53:43 PDT ---
Okay. Maybe I misunderstood something somewhere, but as I understand it, writeln() is supposed to be thread-safe. Multiple threads can interleave lines that they're printing, but each line that a thread prints with writeln() is supposed to be completely printed before a line from another thread can be printed. However, at least some of the time, the output from one thread is mixed with another on the same line. For instance, take this program:

import std.concurrency;
import std.stdio;

void main(string[] args)
{
    spawn(&func1, thisTid, args.idup);
    spawn(&func2, thisTid, args.idup);
    writeln("main() 1");
    writeln("main() 2");
    writeln("main() 3");
}

void func1(Tid parentTid, immutable string[] args)
{
    writefln("func1() begin");
    writefln("func1(): %s", args);
    writefln("func1() end");
}

void func2(Tid parentTid, immutable string[] args)
{
    writefln("func2() begin");
    writefln("func2(): %s", args);
    writefln("func2() end");
}


If I run it like so

./d hello world


I get:

main() 1
main() 2
main() 3
func1() beginfunc2() begin
func1() beginfunc2() begin

[./d, hello, world]
func2() end


or

main() 1
func1() beginfunc2() begin
main() 2
func1() beginfunc2() begin
main() 2

main() 3
eginfunc2() begin
main() 3


or in this case, the line printing out the args array doesn't even print the whole beginning of the line:

main() 1
main() 2
main() 3
func1() begin
func1() begin
func2() begin
, hello, world]
func1() end


In some cases, the lines are properly printed (naturally with whole lines potentially interleaved due to the multiple threads running), but often they aren't. So, if writeln() is really supposed to be thread-safe, it's buggy. If it's not, then programs intending to use writeln() (or its sibling write functions) are going to need to wrap them with locks or some other concurrency mechanism to being able to safely print, which certainly wouldn't play well with the whole message-passing for concurrency paradigm.

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http://d.puremagic.com/issues/show_bug.cgi?id=4600


Jameson <beatgammit@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |beatgammit@gmail.com


--- Comment #1 from Jameson <beatgammit@gmail.com> 2013-05-22 23:59:00 PDT ---
(In reply to comment #0)
> Okay. Maybe I misunderstood something somewhere, but as I understand it, writeln() is supposed to be thread-safe. Multiple threads can interleave lines that they're printing, but each line that a thread prints with writeln() is supposed to be completely printed before a line from another thread can be printed. However, at least some of the time, the output from one thread is mixed with another on the same line. For instance, take this program:
> 
> import std.concurrency;
> import std.stdio;
> 
> void main(string[] args)
> {
>     spawn(&func1, thisTid, args.idup);
>     spawn(&func2, thisTid, args.idup);
>     writeln("main() 1");
>     writeln("main() 2");
>     writeln("main() 3");
> }
> 
> void func1(Tid parentTid, immutable string[] args)
> {
>     writefln("func1() begin");
>     writefln("func1(): %s", args);
>     writefln("func1() end");
> }
> 
> void func2(Tid parentTid, immutable string[] args)
> {
>     writefln("func2() begin");
>     writefln("func2(): %s", args);
>     writefln("func2() end");
> }

I couldn't reproduce this with the current DMD. This issue is quite old, so it's possibly invalid now.

I'm running Linux x86_64 with DMD 2.062. Also tried ldc2 (targeting DMD 2.061),
no problems.

Can this be closed?

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