[FFmpeg-devel] compile issues under mingw-cross-env

[John Calcote]

On 7/10/2010 11:11 PM, David Conrad wrote:
> On Jul 11, 2010, at 12:55 AM, John Calcote wrote:
>
>   
>> On 7/10/2010 3:10 PM, Eli Friedman wrote:
>>     
>>> On Sat, Jul 10, 2010 at 1:57 PM, Ramiro Polla <ramiro.polla at gmail.com> wrote:
>>>
>>>       
>>>> On Sat, Jul 10, 2010 at 5:53 PM, John Calcote <john.calcote at gmail.com> wrote:
>>>>
>>>>         
>>>>> On 7/10/2010 1:25 PM, M?ns Rullg?rd wrote:
>>>>>
>>>>>           
>>>>>> John Calcote <john.calcote at gmail.com> writes:
>>>>>>
>>>>>>             
>>>>>>> Anyone ever tried compiling ffmpeg under mingw-cross-env
>>>>>>> (http://www.nongnu.org/mingw-cross-env/)? I've been playing with it and
>>>>>>> I've found the following issues with CPPFLAGS in the generated config.mak:
>>>>>>>
>>>>>>> 1) I had to change -std=c99 to -std=gnu99 because _STRICT_ANSI is
>>>>>>> enabled if -std=c99 is used which means functions like strcasecmp aren't
>>>>>>> available (at least the prototypes aren't).
>>>>>>>
>>>>>>>
>>>>>>>               
>>>>>> Wrong.  strcasecmp() is part of C99 and on any conforming system is
>>>>>> declared when using those flags.
>>>>>>
>>>>>>
>>>>>>             
>>>>> I'm not seeing strcasecmp as being part of C99. I'm seeing it as being
>>>>> part of POSIX 2001... Are you sure you're correct on this one? When I
>>>>> google search strcasecmp and c99, I get very little information on the
>>>>> pair. However, most of what I do get appear to be comments about how
>>>>> ffmpeg doesn't compile under mingw.
>>>>>
>>>>>           
>>>> 'man standards' says:
>>>> "POSIX.1-2001 is aligned with C99, so that all of the library
>>>> functions standardised in C99 are also standardised in POSIX.1-1001."
>>>>
>>>>         
>>> strcasecmp is defined in strings.h, which is not part of C99.  So
>>> -std=c99 vs. -std=gnu99 shouldn't have any affect.
>>>
>>>       
>> I don't think this is an accurate assessment of the way these flags
>> work. In the first place, gnu99 is technically less restrictive than
>> c99. However, the real issue, in my experience, with compiler flags that
>> ask for compliance with a standard is that they tend to be
>> compound-restrictive, not compound-additive. That is to say these flags
>> are designed to ensure that only the subset of library routines that
>> comply with the requested standard are available, and that uses of all
>> other routines are flagged as errors.
>>
>> To put it another way, they're not designed to provide access to a
>> specific set of functions, but to disallow anything outside the desired
>> set. The reason for this approach is that the flags are designed to help
>> developers write portable code. If you know that three target platform
>> tool sets all conform to C99, and you use the --std=c99 flag on the gcc
>> command line, then you're telling gcc that you don't want to allow
>> anything _except_ C99-compliant functions, not that you want to ensure
>> that you have access to all C99 functions. That way, you can be sure
>> your code will compile on your other two platforms.
>>
>> Thus, if you supply flags that say you want C99 _and_ POSIX 2001, you're
>> likely (on some platforms, at least) to get the intersection of the two
>> sets, not the union of them. Often this situation goes unnoticed because
>> the these two sets overlap to a high degree (in fact C99 is completely
>> contained within POSIX 2001).
>>     
> strcasecmp is required to be in POSIX 2001 [1], and if you define _POSIX_C_SOURCE to 200112L then #include <strings.h> is required to make said symbol visible [2].
>
> So in this case it's mingw that's wrong.
>
> [1] http://www.opengroup.org/onlinepubs/009695399/basedefs/strings.h.html
> [2] http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_02.html
>   

David, your statement is 100 percent true but you missed my point
entirely. Basically, -std=c99 and _POSIX_C_SOURCE=200112L are
conflicting flags that, when used together, give undefined results. It's
cool that the results you've gotten so far are what you wanted, but you
can't count on those results on all platforms because the results of
using the two together is not defined by any standard.

I'm suggesting that what the ffmpeg community really wants to use is the
POSIX flag, and that using the c99 flag in conjunction with it is
problematic.

John