ordering of printed lines changes when redirecting

[walter harms]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc -I/home/abuild/rpmbuild/BUILD/bash-4.2 
-L/home/abuild/rpmbuild/BUILD/bash-4.2/../readline-6.2
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-suse-linux-gnu'
-DCONF_VENDOR='suse' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib
-fmessage-length=0 -grecord-gcc-switches -O2 -Wall -D_FORTIFY_SOURCE=2 
-fstack-protector -funwind-tables -fasynchronous-unwind-tables -g
-D_GNU_SOURCE -DRECYCLES_PIDS -Wall -g -Wuninitialized -Wextra 
-Wno-unprototyped-calls -Wno-switch-enum -Wno-unused-variable
-Wno-unused-parameter -ftree-loop-linear -pipe -DBNC382214=0 -fprofile-use
uname output: Linux omnfr121 4.5.0-4.g3d86af7-default #1 SMP PREEMPT Fri Mar 18 
13:03:45 UTC 2016 (3d86af7) x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-suse-linux-gnu

Bash Version: 4.2
Patch Level: 47
Release Status: release

Description:   ordering of printed lines changes when merging stdin/stdout and 
redirecting


Repeat-By:

#include <stdio.h>

int main()
{
        printf("hallo %d\n",__LINE__);
        fprintf(stderr,"hallo %d\n",__LINE__);
        printf("hallo %d\n",__LINE__);
        fprintf(stderr,"hallo %d\n",__LINE__);
        printf("hallo %d\n",__LINE__);

        return 0;
}

compile: gcc prg.c

then:
 ( ./a.out 2>&1 )
hallo 5
hallo 6
hallo 7
hallo 8
hallo 9

( ./a.out 2>&1 ) >xx
cat xx
hallo 6
hallo 8
hallo 5
hallo 7
hallo 9

note: the same happens with
./a.out  >xx 2>&1
 cat xx
hallo 6
hallo 8
hallo 5
hallo 7
hallo 9

I am not sure if that is an intended behavior. at least it is annoying
when trying to analyze bugs.
If this is intended behavior it would be nice if that would be included
into the manual (i have found no hint, maybe wrong keyword ?) and a way
to fix that.

re,
 wh