On Mon, 20 May 2019 at 08:58, Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote:
>
> On Mon, 20 May 2019 at 04:07, Wx L <lwxiang1994@xxxxxxxxx> wrote:
> >
> > hi, I used gcc5.5.0(configure only with --prefix), glibc2.17 to compile
> > some code with *conversion-null *warning in it.
> > it is every simple so I put it here:
> >
> > #include <stdio.h>
> > int main()
> > {
> > return NULL;
> > }
> >
> > *What I Got*
> > I did compile with:
> > g++ -Wall -S test.c
> > as expected, here came out the warning "warning: converting to non-pointer
> > type 'int' from NULL [-Wconversion-null]"
> >
> > then I did compile step by step:
> > g++ -Wall -E test.c
> > g++ -Wall -S test.i
> >
> > the warning just disappeared, nothing came out.
> >
> > *What I have Tried*
> > I tried to figure out if all warning will be disabled, so I compile some
> > code with different warning in it, just *conversion-null *will be disabled
> > when separate-step compiling.
> > I tried to figure out if all gcc version behave the same way, and I found
> > gcc5.5.0,gcc7.1.0,gcc7.3.0 did, but gcc4.8.5 works when separate-step
> > compiling.
> > After comparing the preprocessing files(*.i) from gcc4.8.5 and gcc5.5.0, I
> > found that gcc5.5.0 put some linemarkers in it while gcc4.8.5 don't.
> > here is the line markers:
> >
> > int main()
> > {
> > int a =
> > # 4 "a.c" 3 4
> > __null
> > # 4 "a.c"
> > ;
> > return a;
> > }
> >
> > if I do preprocess with gcc4.8.5, and do compile with gcc5.5.0, it warns
> > well.
> > if I do preprocess with gcc5.5.0, and do compile with gcc4.8.5, it can not
> > warn *conversion-null.*
> > if I do preprocess with gcc5.5.0 and add -P option(remove linemarkers), and
> > do compile with gcc5.5.0, it warns well.
> >
> > Now I am sure that it is the linemarkers added by gcc5.5.0 preprocessor
> > that cause the *conversion-null *warning not work as expected.
> > But why one-step compile works? So I add -save-temps to one-step compile to
> > see if it got a different preprocessing file.
> > just like:
> >
> > g++ -Wall -save-temps -S test.c
> >
> > and now it can not warns too! the preprocessing file is exactly same as the
> > one with "g++ -Wall -E test.c"
> >
> > *What Help I Need*
> > After all the test, I confuse that if there is a bug with warning analyzer
> > when analyzing the *conversion-null *warning code which around by
> > linemarkers?
>
> I think the problem is that the definition of NULL comes from a system
> header, and GCC gets confused when issuing diagnostics related to
> macros defined in system headers.
>
> If you use -Wsystem-headers then you get a -Wconversion-null warning
> even when preprocessing and compiling separately. This is related to
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77513
>
> Please report a bug.
I think it's related to PR 57201 so I added your example there:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57201#c15
> > Is that -P a workaround to solve this problem when separate-step compiling?
> > Will -P leads to some other problems?
>
> Yes, removing all line directives means the compiler doesn't know
> which code comes from system headers, and so some standard library
> features will no longer compile cleanly. The -P flag should be used
> when the output of the preprocessor is not going to be consumed by the
> compiler, it shouldn't be used for C or C++ code.
