Re: Discussion about why GNU/Linux system upgrades cause old programs to break

Richard Stallman <[email protected]> writes:

> [[[ To any NSA and FBI agents reading my email: please consider    ]]]
> [[[ whether defending the US Constitution against all enemies,     ]]]
> [[[ foreign or domestic, requires you to follow Snowden's example. ]]]
>
>   > First, Emacs 24 was not a C99 program.
>
> What determines whether a program Foo is a C99 program?  And how?
> [...]
> Is it controlled by command line options?  If so, what and how?
> [...]

GCC assumes what language its input is based on the file extension and
flags the developer passed to it.

For instance, if the developer gives GCC a .c file, it will know it is
C.

Which version of C depends on the default version baked into GCC, which
is the latest version that is considered fully implemented and stable at
the moment of release.  Extensions are enabled by default also.

This version can be changed via the -std= flag.

So, as an example,  gcc -std=c99 foo.c  compiles foo.c as a C99 program
without language extensions.

gcc -std=gnu23 foo.c  or  gcc foo.c  would (at the moment; with GCC 15)
both compile these programs as C23 programs with GNU extensions enabled.

Similar goes for a .cc, .cxx, or .cpp, ... file, which are all
interpreted as C++ files (-std works much the same way).

And so on, and so forth, for the other languages supported by GCC
(details vary more in the other languages).  See (gcc) Overall Options
and (gcc) Standards.

The language guess can be overridden by the '-x' flag.  For instance,
'-x c' tells GCC that the rest of the input files are C.  But most
people just follow the conventions that are already recognized by GCC.
-- 
Arsen Arsenović

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