stderr for Windows native commands are seemed to be bufferd

Larry Hall (Cygwin) reply-to-list-only-lh@cygwin.com
Fri Mar 1 04:21:00 GMT 2013


On 2/28/2013 10:39 PM, smith@cygwin.akamoz.jp wrote:
> The stderr for Microsoft Windows native commands is undesirably
> buffered when they are executed on Cygwin shell, Like this:
>
> $ uname -a
> CYGWIN_NT-6.1 localhost 1.7.17(0.262/5/3) 2012-10-19 14:39 i686 Cygwin
> $ which nslookup
> /cygdrive/c/Windows/system32/nslookup
> $ nslookup
> ...
>> set q=hogehoge
>> set q=fugafuga
>> exit
> unknown query type: hogehoge
> unknown query type: fugafuga
> $
>
> The error messages "unknown query type" should comes just after
> "set q=" input respectively.
>
> Create a simple program as follows:
>
> #include <stdio.h>
> int main(void) {
> 	fprintf(stderr, "hogehoge\n");
> 	/* fflush(stderr); */
> 	getchar();
> 	fprintf(stderr, "fugafuga\n");
> 	return 0;
> }
>
> Build the program by MSVC:
>
> c:\>cl /?
> Microsoft(R) 32-bit C/C++ Optimizing Compiler Version 14.00.50727.762 for 80x86
> Copyright (C) Microsoft Corporation.  All rights reserved.
>
> On Microsoft's cmd, everything goes fine. On bash in Cygwin,
> first waiting for input, then 2 lines are printed.
> If I insert fflush just before getchar, it works well both environments.
>
> Who is buffering the stderr output? Is there any workaround?

Windows is buffering because it sees the Cygwin ptys as a pipe (and
it buffers on pipes for performance reasons).  The alternative is to
start bash from cmd.exe instead of mintty or other Cygwin terminal.
See the email archives for more discussion of this issue if you're
interested.

-- 
Larry

_____________________________________________________________________

A: Yes.
 > Q: Are you sure?
 >> A: Because it reverses the logical flow of conversation.
 >>> Q: Why is top posting annoying in email?

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