Bug#761300: libc6: Printf("%c",'x') does not follow stdio

[Sven Joachim]

Am 14.10.2018 um 13:38 schrieb Florian Weimer:

> * Sven Joachim:
>
>> This result is rather surprising.  After all, "putchar('x')" is supposed
>> to do the same as "putc('x', stdout)", but here it does not.
>
> Can you reproduce this with something newer than 2.13-38+rpi2+deb7u3?
> Or on something else besides armhf?

Surely, I tested 2.27-6 on amd64.

Cheers,
   Sven

Bug#761300: libc6: Printf("%c",'x') does not follow stdio

[Florian Weimer]

* Sven Joachim:

> This result is rather surprising.  After all, "putchar('x')" is supposed
> to do the same as "putc('x', stdout)", but here it does not.

Can you reproduce this with something newer than 2.13-38+rpi2+deb7u3?
Or on something else besides armhf?

Bug#761300: libc6: Printf("%c",'x') does not follow stdio

[Sven Joachim]

Control: retitle -1 libc6: putchar does not follow stdio

On 2014-09-12 09:10 -0700, Thomas D. Dean wrote:

> Package: libc6
> Version: 2.13-38+rpi2+deb7u3
> Severity: normal
>
> Dear Maintainer,
>
>* What led up to the situation?
>
> Redirecting stdout in C code does not work for printf("%c",'x')
> I ssh into the system.  I want to redirect all output to stdout to
> the local terminal.  This works as expected for everything except
> when printing a single character.
>
>* What exactly did you do (or not do) that was effective (or
>  ineffective)?
>
>   FILE *display_fp;
>   if ((display_fp = fopen("/dev/tty1","r+")) == NULL) {
>  perror("Open /dev/tty1");
>  return -1;
>   }
>   stdout = display_fp;
>
> Then,
>
> printf("%s","asdfasdf"); /* output to /dev/tty1 */
> printf("%c",'x'); /* output to original terminal */
>
>* What was the outcome of this action?
>
> All the output except for the "%c" case went to /dev/tty1.  The
> output in the "%c" case went to the ssh terminal

It is actually a bit more subtle than that, as gcc has its own printf
builtin function which comes into play.  Compiling the program with
-fno-builtin in fact makes it work as intended (at least with glibc
2.27-6).

Looking closer, I found that with -fno-builtin

printf("%c",'x');   works
putc('x', stdout);  works
putchar('x');   exhibits the bug

This result is rather surprising.  After all, "putchar('x')" is supposed
to do the same as "putc('x', stdout)", but here it does not.

I'm attaching the whole program, so that future researchers have less to
copy and paste.

Cheers,
   Sven

#include 
#include 

int main (int argc, char** argv)
{
  FILE *display_fp;
  if ((display_fp = fopen("/dev/tty1","r+")) == NULL) {
perror("Open /dev/tty1");
return -1;
  }
  stdout = display_fp;
  printf("%s","asdfasdf"); /* output to /dev/tty1 */
  putchar('x'); /* output to original terminal */
}

Processed: Re: Bug#761300: libc6: Printf("%c",'x') does not follow stdio

[Debian Bug Tracking System]

Processing control commands:

> retitle -1 libc6: putchar does not follow stdio
Bug #761300 [libc6] libc6: Printf("%c",'x') does not follow stdio
Changed Bug title to 'libc6: putchar does not follow stdio' from 'libc6: 
Printf("%c",'x') does not follow stdio'.

-- 
761300: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=761300
Debian Bug Tracking System
Contact ow...@bugs.debian.org with problems

Bug#761300: libc6: Printf(%c,'x') does not follow stdio

[Thomas D. Dean]

Package: libc6
Version: 2.13-38+rpi2+deb7u3
Severity: normal

Dear Maintainer,

   * What led up to the situation?

Redirecting stdout in C code does not work for printf(%c,'x')
I ssh into the system.  I want to redirect all output to stdout to
the local terminal.  This works as expected for everything except
when printing a single character.

   * What exactly did you do (or not do) that was effective (or
 ineffective)?

  FILE *display_fp;
  if ((display_fp = fopen(/dev/tty1,r+)) == NULL) {
 perror(Open /dev/tty1);
 return -1;
  }
  stdout = display_fp;

Then,

printf(%s,asdfasdf); /* output to /dev/tty1 */
printf(%c,'x'); /* output to original terminal */

   * What was the outcome of this action?

All the output except for the %c case went to /dev/tty1.  The
output in the %c case went to the ssh terminal

   * What outcome did you expect instead?

I expected all the output to go to /dev/tty1



-- System Information:
Debian Release: 7.6
Architecture: armhf (armv6l)

Kernel: Linux 3.12.22+ (PREEMPT)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=ANSI_X3.4-1968) 
(ignored: L

C_ALL set to C)
Shell: /bin/sh linked to /bin/dash

Versions of packages libc6 depends on:
ii  libc-bin  2.13-38+rpi2+deb7u3
ii  libgcc1   1:4.7.2-5+rpi1

libc6 recommends no packages.

Versions of packages libc6 suggests:
ii  debconf [debconf-2.0]  1.5.49
pn  glibc-doc  none
ii  locales2.13-38+rpi2+deb7u3

-- debconf information:
  glibc/upgrade: true
  glibc/restart-services:
* libraries/restart-without-asking: true
  glibc/disable-screensaver:
  glibc/restart-failed:


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