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Date: Sun, 11 Oct 2009 18:56:12 +0100
From: Dave Korn <dave DOT korn DOT cygwin AT googlemail DOT com>
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Subject: Re: [warning: implicit declaration of function] if enable flag  std=c99
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Andy Koppe wrote:
> 2009/10/11 Salvatore Santagati:
>> Nope, but under Linux works as well ( with and without std=c99 ) ,
>> under cygwin if enable std=c99
>> I've these type of warning ( source code here is only an example )
> 
> Seems Linux has got it wrong then, because ftello and fseeko are not
> standard C99 functions. Use -std=gnu99 to enable GNU extensions.

  This is most likely a variant of the same problem as with snprintf:

     http://www.cygwin.com/ml/cygwin/2009-04/threads.html#00435

  Selecting --std=c99 causes __STRICT_ANSI__ to be defined.  These functions
are not in c99, so the declarations are wrapped in #ifndef __STRICT_ANSI__.
As the GCC manual says (under '-ansi'):

>    The macro `__STRICT_ANSI__' is predefined when the `-ansi' option
>    is used.  Some header files may notice this macro and refrain from
>    declaring certain functions or defining certain macros that the
>    ISO standard doesn't call for; this is to avoid interfering with
>    any programs that might use these names for other things.

  If you use --std=c99, that means you want *only* c99 functions, and have
specifically asked to be warned about non-ANSI functions.  I don't understand
why Linux does things differently, it's probably technically wrong but trying
to be helpful, but the fact is that if you write a program strictly according
to the c99 standard, you can't expect there to be a library function called
fseeko and you might in fact expect to be able to use that name for a function
in your own code without any clashes (since the c99 spec doesn't say it is
reserved).

  As a workaround you could use --std=gnu99, which gets you c99 + GCC
extension features but doesn't define __STRICT_ANSI__, or drop it altogether;
what's best depends why you're using the flag in the first place.

  The longterm fix is to mark up the newlib headers, looking for "#ifndef
__STRICT_ANSI__" and deciding where to add "|| __STDC_VERSION__ >= 199901L"


    cheers,
      DaveK

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