Re: [FFmpeg-devel] [PATCH v17 1/5] libavutil: Add wchartoutf8(), wchartoansi(), utf8toansi() and getenv_utf8()

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Soft Works:
> 
> 
>> -----Original Message-----
>> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
>> Andreas Rheinhardt
>> Sent: Sunday, June 19, 2022 6:59 AM
>> To: ffmpeg-devel@ffmpeg.org
>> Subject: Re: [FFmpeg-devel] [PATCH v17 1/5] libavutil: Add
>> wchartoutf8(), wchartoansi(), utf8toansi() and getenv_utf8()
>>
>> Nil Admirari:
>>> wchartoutf8() converts strings returned by WinAPI into UTF-8,
>>> which is FFmpeg's preffered encoding.
>>>
>>> Some external dependencies, such as AviSynth, are still
>>> not Unicode-enabled. utf8toansi() converts UTF-8 strings
>>> into ANSI in two steps: UTF-8 -> wchar_t -> ANSI.
>>> wchartoansi() is responsible for the second step of the conversion.
>>> Conversion in just one step is not supported by WinAPI.
>>>
>>> Since these character converting functions allocate the buffer
>>> of necessary size, they also facilitate the removal of MAX_PATH
>> limit
>>> in places where fixed-size ANSI/WCHAR strings were used
>>> as filename buffers.
>>>
>>> getenv_utf8() wraps _wgetenv() converting its input from
>>> and its output to UTF-8. Compared to plain getenv(),
>>> getenv_utf8() requires a cleanup.
>>>
>>> Because of that, in places that only test the existence of
>>> an environment variable or compare its value with a string
>>> consisting entirely of ASCII characters, the use of plain getenv()
>>> is still preferred. (libavutil/log.c check_color_terminal()
>>> is an example of such a place.)
>>>
>>> Plain getenv() is also preffered in UNIX-only code,
>>> such as bktr.c, fbdev_common.c, oss.c in libavdevice
>>> or af_ladspa.c in libavfilter.
>>> ---
>>>  configure                  |  1 +
>>>  libavutil/getenv_utf8.h    | 71
>> ++++++++++++++++++++++++++++++++++++++
>>>  libavutil/wchar_filename.h | 51 +++++++++++++++++++++++++++
>>>  3 files changed, 123 insertions(+)
>>>  create mode 100644 libavutil/getenv_utf8.h
>>>
>>> diff --git a/configure b/configure
>>> index 3dca1c4bd3..fa37a74531 100755
>>> --- a/configure
>>> +++ b/configure
>>> @@ -2272,6 +2272,7 @@ SYSTEM_FUNCS="
>>>      fcntl
>>>      getaddrinfo
>>>      getauxval
>>> +    getenv
>>>      gethrtime
>>>      getopt
>>>      GetModuleHandle
>>> diff --git a/libavutil/getenv_utf8.h b/libavutil/getenv_utf8.h
>>> new file mode 100644
>>> index 0000000000..161e3e6202
>>> --- /dev/null
>>> +++ b/libavutil/getenv_utf8.h
>>> @@ -0,0 +1,71 @@
>>> +/*
>>> + * This file is part of FFmpeg.
>>> + *
>>> + * FFmpeg is free software; you can redistribute it and/or
>>> + * modify it under the terms of the GNU Lesser General Public
>>> + * License as published by the Free Software Foundation; either
>>> + * version 2.1 of the License, or (at your option) any later
>> version.
>>> + *
>>> + * FFmpeg is distributed in the hope that it will be useful,
>>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> GNU
>>> + * Lesser General Public License for more details.
>>> + *
>>> + * You should have received a copy of the GNU Lesser General
>> Public
>>> + * License along with FFmpeg; if not, write to the Free Software
>>> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
>> 02110-1301 USA
>>> + */
>>> +
>>> +#ifndef AVUTIL_GETENV_UTF8_H
>>> +#define AVUTIL_GETENV_UTF8_H
>>> +
>>> +#include <stdlib.h>
>>> +
>>> +#include "mem.h"
>>> +
>>> +#ifdef HAVE_GETENV
>>> +
>>> +#ifdef _WIN32
>>> +
>>> +#include "libavutil/wchar_filename.h"
>>> +
>>> +static inline char *getenv_utf8(const char *varname)
>>> +{
>>> +    wchar_t *varname_w, *var_w;
>>> +    char *var;
>>> +
>>> +    if (utf8towchar(varname, &varname_w))
>>> +        return NULL;
>>> +    if (!varname_w)
>>> +        return NULL;
>>> +
>>> +    var_w = _wgetenv(varname_w);
>>> +    av_free(varname_w);
>>> +
>>> +    if (!var_w)
>>> +        return NULL;
>>> +    if (wchartoutf8(var_w, &var))
>>> +        return NULL;
>>> +
>>> +    return var;
>>> +
>>> +    // No CP_ACP fallback compared to other *_utf8() functions:
>>> +    // non UTF-8 strings must not be returned.
>>> +}
>>> +
>>> +#else
>>> +
>>> +static inline char *getenv_utf8(const char *varname)
>>> +{
>>> +    return av_strdup(getenv(varname));
>>
>> This forces allocations and frees in scenarios where this is wholly
>> unnecessary. 
> 
> Why do you think this is unnecessary? At least on Windows, there is
> no guarantee regarding the lifetime of strings returned from
> getenv(). In case when some other code would call _putenv to set the
> env variable, this can cause the previously returned string to become 
> invalid without the caller being able to know.
> 
> 
>> This can be avoided by adding a custom deallocator for
>> strings returned via getenv_utf8: Namely a define/wrapper around
>> av_free in the _WIN32 and a no-op else.
> 
> I don't think I really understand what you mean, by the above 😉
> 

The "scenarios where this is wholly unnecessary" are the scenarios in
which one is in the above #else branch, i.e. not on Windows (I thought
this was clear from the placement of my comment). In this case, the
above av_strdup() can be avoided by adding a custom deallocator that
boils down to av_free on Windows and a no-op in all other cases.

- Andreas
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