Subject: Re: bug#32767: glibc shadows gcc's C++ headers
From: Maxim Cournoyer <[email-masked]>
To: Robin Templeton <[email-masked]>
Cc: [email-masked]
Date: Tue, 09 Mar 2021 16:35:19 -0500
List-Id: <debbugs-submit.debbugs.gnu.org>

Hello,

Robin Templeton <[email-masked]> writes:

> When compiling C++ programs, glibc's include directory takes precedence
> over gcc's for standard C headers like math.h, but glibc's headers
> aren't completely compatible with C++. For example, isnan from math.h is
> supposed to be a function, but glibc defines it as a macro.
>
> This program demonstrates the problem:
>
> #include <math.h>
> int main(void) { int isnan(0); return isnan; }
>
> It works as expected when gcc's built-in C++ headers are used, but not
> if glibc is installed:
>
> % guix environment --pure --ad-hoc gcc -- g++ -E isnan.cpp | tail -n1 
> int main (void) { int isnan (0); return isnan; }
> % guix environment --pure --ad-hoc gcc glibc -- g++ -E isnan.cpp | tail -n1
> int main (void) { int __builtin_isnan (0); return isnan; }
>
> As a temporary workaround, I'm using the following package as a
> replacement for glibc, to keep the glibc headers out of $CPATH. If it's
> installed along with gcc, ld-wrapper and binutils, the test program
> compiles without errors.
>
> (use-modules (guix) (gnu))
> (use-package-modules base)
> (package
>   (inherit glibc)
>   (name "my-glibc")
>   (arguments
>    (substitute-keyword-arguments (package-arguments glibc)
>      ((#:phases phases)
>       `(modify-phases ,phases
>          (add-after 'install 'move-include
>            (lambda _
>              (rename-file (string-append %output "/include")
>                           (string-append %output "/include-glibc"))
>              #t)))))))

Thanks for the report.  Nowadays the recommended way to install the
GCC-based toolchain is to use the 'gcc-toolchain' package.  The 'gcc'
package is now even made hidden in Guix, which doesn't seem to have this
particular issue:

$ guix environment --pure --ad-hoc gcc-toolchain -- g++ -E /tmp/isnan.cpp | tail -n1
int main(void) { int isnan(0); return isnan; }

$ guix environment --pure --ad-hoc gcc-toolchain glibc -- g++ -E /tmp/isnan.cpp | tail -n1
int main(void) { int isnan(0); return isnan; }

I'm closing this issue, but feel free to open a fresh one if you still
encounter problems.

Thank you,

Maxim