Re: Understanding compiler warning options

[Martin Sebor <msebor at gmail dot com>]

On 09/29/2015 05:41 PM, Edward Diener wrote:
> I am trying to understand how the compiler decides which warnings are
> issued when compiling a translation unit.
>
> What is the default warnings that are issued if no compiler warning
> options is set on the command line ?

I don't know of a GCC interface to reliably determine this but GCC
itself decides this by parsing the c.opt files such as the generic

  gcc/common.opt or gcc/c-family/c.opt

or the target-specific

  gcc/config/rs6000/rs6000.opt

An example might be the -Wdiv-by-zero warning that's defined like
this:

  Wdiv-by-zero
  C ObjC C++ ObjC++ Var(warn_div_by_zero) Init(1) Warning
  Warn about compile-time integer division by zero

You may be able to tell (probably less reliably) from the manual
by searching for warning options listed in the negative -Wno-xxx
form. For instance:

  -Wno-endif-labels
      Do not warn whenever an #else or an #endif are followed by text.

means the option is enabled by default. Other options may say
when (e.g., in what language) they're enabled by default.

> If I set a general option, such as -w, -Wall, or -Wextra, do these get
> overridden by more specific compiler options no matter where in the
> command line they are encountered, or does the order of the compiler
> warnings on the command line matter in such a case ? As a specific
> practical example if the command line has "-Wno-unused-local-typedef
> -Wall ..." does the "-Wno-unused-local-typedef" override the "-Wall"
> option ?

Unfortunately, this is neither documented nor necessarily intuitive
(YMMV). IME, the best way to find out is by experimenting.

AFAICS, -w disables all warnings regardless of what follows on the
command line.

-Wno-xxx disables prior -Wxxx on the command line.

-Wxxx or -Wno-xxx overrides -Wall or -Wextra on the command line,
prior or subsequent.

IMO, it would be useful to document this. I'm sure patches would
be welcome.

Martin