Re: Bug or not? printf allocates buffer & never frees it

[Konstantin Kharlamov <hi-angel@yandex.ru>]

On Wed, 2021-03-17 at 12:30 -0500, Ian Pilcher via Libc-help wrote:
> Fedora 33 with glibc-2.32-4.fc33.x86_64.
> 
> Simple reproducer:
> 
>   #include <mcheck.h>
>   #include <stdio.h>
> 
>   int main(void)
>   {
>         mtrace();
>         printf("%d\n", 0);
>         muntrace();
>         return 0;
>   }
> 
> Running with MALLOC_TRACE set shows:
> 
>   Memory not freed:
>   -----------------
>              Address     Size     Caller
>   0x0000000000e1d690    0x400  at 0x7f19511d2934
> 
> The trace file shows that the memory was allocated by
> _IO_file_doallocate:
> 
>   = Start
>   @ /lib64/libc.so.6:(_IO_file_doallocate+0x94)[0x7f19511d2934] + 
> 0xe1d690 0x400
>   = End
> 
> And gdb shows that _IO_file_doallocate is called from printf:
> 
>   #0  0x00007ffff7e578a0 in _IO_file_doallocate () from /lib64/libc.so.6
>   #1  0x00007ffff7e662f0 in _IO_doallocbuf () from /lib64/libc.so.6
>   #2  0x00007ffff7e65488 in __GI__IO_file_overflow () from /lib64/libc.so.6
>   #3  0x00007ffff7e64536 in __GI__IO_file_xsputn () from /lib64/libc.so.6
>   #4  0x00007ffff7e4f269 in __vfprintf_internal () from /lib64/libc.so.6
>   #5  0x00007ffff7e3b2af in printf () from /lib64/libc.so.6
>   #6  0x0000000000401163 in main () at leak.c:11
> 
> Multiple calls to printf don't leak additional memory, so this isn't
> really a problem (other than the time wasted by folks hunting memory
> leaks).
> 
> Is this considered a bug?
> 

Hi! I'm not very acknowledged with the output, am I right that printf calls "deallocate", however the actual memory is still being held and not returned to the kernel? If yes, then you're likely stumbled upon this bug https://sourceware.org/bugzilla/show_bug.cgi?id=27103 See also this gigantic thread https://debbugs.gnu.org/cgi/bugreport.cgi?bug=45200