How to use C89 with certain C99 features

[David Brown]

On 05/04/2011 12:32, richardcavell@mail.com wrote:
> Hi, everyone. I'm building a project in C. My idea is that I want it to
> be mostly C89, except that I will be using certain features of C99. What
> compiler flags do I need to tell GCC what I want?
>

What are you actually trying to achieve?  The only reason I can think of 
for someone to write C89 standard is to be compatible with more limited 
or older compilers.  If that's the case, then you should write C89 - not 
C99.  Many compilers that are basically C89 (or C90, or ANSI) will 
accept a few C99 features, such as // comments.  But they are very 
unlikely to accept more complex C99 features, such as newer ways to 
handle variadic parameters.  Thus your code will only work with a proper 
C99 compiler, and thus might as well use --std=c99.



> At present, if I compile with -std=c99, I get no errors or warnings at
> all. If I compile with -std=c89, I get the following warnings:
>
> (When I use macros with ... and __VA_ARGS__)
> warning: anonymous variadic macros were introduced in C99
>
> (When I use long string literals)
> warning: string length '613' is greater than the length '509' ISO C90
> compilers are required to support
>
> (When I use vsnprintf)
> warning: implicit declaration of function 'vsnprintf'
>
> (When I initialize a struct with run-time data)
> warning: initializer element is not computable at load time
>
> Richard
>
>