printf prints to wrong file after assignment to stdout

[Per Cederqvist]

I got a bounce when I tried to use the glibcbug script, so I'm
resending this to address@hidden instead.  (Postmaster gets a copy,
since this seems to be a mail-related problem -- see the attached
bounced message).

Description:

If you assign to stdout, as the FAQ of glibc says that you can do, not
all output will be affected by the assignment.  Some output will
continue to be sent to the old stream.

If you compile with -O2, a printf with a constant single-character
string will end up in the wrong place.  If you compile without
optimization, calls to putchar() will end up in the wrong place.

How-to-repeat:

$ cat demo.c
#include <stdio.h>

int main(void)
{
  FILE *fp = fopen("demo.txt", "w");
  if (!fp)
    {
      perror("fopen");
      exit(1);
    }

  stdout = fp;

  printf("Dem");
  printf("o");
  printf("\n");
  putchar('t');
  putchar('e');
  putchar('s');
  putchar('t');
  putchar('\n');
}
$ gcc -O2 demo.c
$ ./a.out
o
$ cat demo.txt
Demtest
$ gcc demo.c
$ ./a.out
test
$ cat demo.txt
Demo
$ exit

For more details, please see the attached bounced mail.

    /ceder

> --- Begin Message ---
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> Subject: 
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> Date: 
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> Mon, 03 Nov 2003 10:42:39 -0500
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> ------ This is a copy of the message, including all the headers. ------
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> Return-path: <address@hidden>
> Received: from monty-python.gnu.org ([199.232.76.173])
>         by fencepost.gnu.org with esmtp (Exim 4.24)
>         id 1AGgql-0006W0-Ex
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>         for address@hidden; Mon, 03 Nov 2003 10:23:23 -0500
> Received: from yagi.ingate.se (IDENT:40Aeev5jviMEvLV048/address@hidden 
> [193.180.23.16])
>         by usagi.ingate.se (8.12.8/8.11.6) with ESMTP id hA3FNFs7032014;
>         Mon, 3 Nov 2003 16:23:15 +0100
> Received: from yagi.ingate.se (IDENT:address@hidden [127.0.0.1])
>         by yagi.ingate.se (8.12.8/8.11.6) with ESMTP id hA3FNFGL027355;
>         Mon, 3 Nov 2003 16:23:15 +0100
> Received: (from address@hidden)
>         by yagi.ingate.se (8.12.8/8.12.8/Submit) id hA3FNFHF027353;
>         Mon, 3 Nov 2003 16:23:15 +0100
> Date: Mon, 3 Nov 2003 16:23:15 +0100
> Message-Id: <address@hidden>
> From: address@hidden
> CC: address@hidden
> To: address@hidden
> Subject: Assigning stdout confuses printf
> X-Spam-Status: No, hits=0.8 required=5.0
>         tests=NO_REAL_NAME
>         version=2.55
> X-Spam-Level: 
> X-Spam-Checker-Version: SpamAssassin 2.55 (1.174.2.19-2003-05-19-exp)
>
> >Submitter-Id:  net
> >Originator:    
> >Organization:
>   Per Cederqvist <address@hidden>, Director Development, Ingate Systems AB
> >
> >Confidential:  no
> >Synopsis:      printf prints to wrong file after assignment to stdout
> >Severity:      serious
> >Priority:      medium
> >Category:      libc
> >Class:         sw-bug
> >Release:       libc-2.3.2
> >Environment:
>         
> Host type: i386-redhat-linux-gnu
> System: Linux yagi.ingate.se 2.4.20-20.9 #1 Mon Aug 18 11:45:58 EDT 2003 i686 
> i686 i386 GNU/Linux
> Architecture: i686
>
> Also tested on a home-grown Linux box running a pristine copy of
> libc-2.3.1 compiled using a pristine copy of gcc-3.2.1.
>
> Addons: linuxthreads c_stubs glibc-compat
> Build CFLAGS: -march=i386 -DNDEBUG=1 -finline-limit=2000 -g -O3
> Build CC: gcc
> Compiler version: 3.2.2 20030222 (Red Hat Linux 3.2.2-5)
> Kernel headers: 2.4.22
> Symbol versioning: yes
> Build static: yes
> Build shared: yes
> Build pic-default: no
> Build profile: yes
> Build omitfp: no
> Build bounded: no
> Build static-nss: no
>
> >Description:
>
> If you assign to stdout, as the FAQ of glibc says that you can do, not
> all output will be affected by the assignment.  Some output will
> continue to be sent to the old stream.
>
> If you compile with -O2, a printf with a constant single-character
> string will end up in the wrong place.  If you compile without
> optimization, calls to putchar() will end up in the wrong place.
>
> >How-To-Repeat:
>
> See this transcript:
>
> $ cat demo.c
> #include <stdio.h>
>
> int main(void)
> {
>   FILE *fp = fopen("demo.txt", "w");
>   if (!fp)
>     {
>       perror("fopen");
>       exit(1);
>     }
>
>   stdout = fp;
>
>   printf("Dem");
>   printf("o");
>   printf("\n");
>   putchar('t');
>   putchar('e');
>   putchar('s');
>   putchar('t');
>   putchar('\n');
> }
> $ gcc -O2 demo.c
> $ ./a.out
> o
> $ cat demo.txt
> Demtest
> $ gcc demo.c
> $ ./a.out
> test
> $ cat demo.txt
> Demo
> $ exit
>
> Script done on Mon Nov  3 16:20:32 2003
>
> >Fix:
>
> Unknown.
>
>
> --- End Message ---

-- 
Per Cederqvist <address@hidden>, Director Development, Ingate Systems AB