GCC Bugzilla Bug 11196

preprocessor error!

I wrote a simple C++ programing using GCC 3.2-cygwin, MingW, Linux, etc version
as below.
===================
#include<iostream>
using namespace std;

const double M_PI = 3.1415;

int main()
{
    cout << M_PI << endl;
    return 0;
}
===================

There're a parsing error in "M_PI". It has been defined in cmath(math.h) as:
#define M_PI 3.14159265358979323846

But, I didn't include the <cmath> header file. After undefined the M_PI, no
error be found. I think it's an error of preprocessor of GCC. 

For addition, I just included <iostream> header file, I can use all of methods
declared in <cmath> header file. It's no compiling error on 2.95 or older
version of GCC.

Subject: Re:  New: parse error before numeric constant

"is88071@cis.nctu.edu.tw" <gcc-bugzilla@gcc.gnu.org> writes:


| I wrote a simple C++ programing using GCC 3.2-cygwin, MingW, Linux, etc version
| as below.
| ===================
| #include<iostream>
| using namespace std;
| 
| const double M_PI = 3.1415;
| 
| int main()
| {
|     cout << M_PI << endl;
|     return 0;
| }
| ===================
| 
| There're a parsing error in "M_PI". It has been defined in cmath(math.h) as:
| #define M_PI 3.14159265358979323846
| 
| But, I didn't include the <cmath> header file.

Agreed.  However, <iostream> is permitted to include <cmath>.  But
<cmath> is not supposed to define M_PI, that we fail to provide a
clean <cmath> is a bug in libstdc++.  However what the right fix
should be is not obvious.

-- Gaby

This is not a bug as the ISO C++ standard say that any standard header can
include any other 
standard headers so you include iostream which in turns includes cmath.h. The
bug is that 
cmath.h should not define M_PI which is not included in the C++ standard but is
included in 
XOPEN/SVID.

Subject: Re:  parse error before numeric constant

"pinskia@physics.uc.edu" <gcc-bugzilla@gcc.gnu.org> writes:

| This is not a bug as the ISO C++ standard say that any standard header can include any other 
| standard headers so you include iostream which in turns includes cmath.h. The bug is that 
| cmath.h should not define M_PI which is not included in the C++ standard but is included in 
| XOPEN/SVID.

That is untrue.  XOPEN/SVID does not say <cmath> should define M_PI.
The issue isn't not that simple.  We should just resolve this as of
being very low priority bug to fix. 

-- Gaby

Downgrade priorities. This code is just so unnecessary that it can't be
important
to fix it. Also change category to libstdc++.
W.

This is a bug, see mine and gdr's anyalsis.

The bug is that the C++ front end implicitly #defines _GNU_SOURCE. 
 
In glibc (I have version 2.3.2 here) this, amongst others, enables _BSD_SOURCE 
(see /usr/include/features.h). That in turn enables non-standard symbols like 
M_PI in math.h. Hence the failure. 
 
As a result one can argue that this is a C++ front end bug... however:   
 
gcc/config/linux.h tells us that: 
 
/* The GNU C++ standard library requires that these macros be defined.  */ 
#undef CPLUSPLUS_CPP_SPEC 
#define CPLUSPLUS_CPP_SPEC "-D_GNU_SOURCE %(cpp)" 
 
... so this may be a libstdc++ issue after all.  
 
The #define is also present for other configuration files in gcc/config/* and 
subdirectories. 
 
I googled and searched the libstdc++ archives and Changelog files, but did no 
find the reason why _GNU_SOURCE is in fact needed by gnu's libstdc++. I just 
removed the -D_GNU_SOURCE from gcc/config/linux.h (on i686-suse-linux) and 
bootstrapped gcc (HEAD) with languages=c,c++. That went just fine and 
(obviously) fixed the problem reported here. 
 
Could some libstdc++ guru explain why this define is actually needed? 

(In reply to comment #6)
> The bug is that the C++ front end implicitly #defines _GNU_SOURCE. 
> [..]
> Could some libstdc++ guru explain why this define is actually needed? 

I am no libstdc++ guru, but this:

  echo "#include <cstdlib>" | g++ -x c++ -U _GNU_SOURCE -c -

produces:

  /usr/include/c++/4.1.2/cstdlib:162: error: ‘::lldiv_t’ has not been
declared
  /usr/include/c++/4.1.2/cstdlib:168: error: ‘::_Exit’ has not been
declared
  /usr/include/c++/4.1.2/cstdlib:175: error: ‘::llabs’ has not been
declared
  /usr/include/c++/4.1.2/cstdlib:177: error: ‘lldiv_t’ does not name a type
  /usr/include/c++/4.1.2/cstdlib:181: error: ‘::lldiv’ has not been
declared
  /usr/include/c++/4.1.2/cstdlib:196: error: ‘::strtof’ has not been
declared
  /usr/include/c++/4.1.2/cstdlib:197: error: ‘::strtold’ has not been
declared
  /usr/include/c++/4.1.2/cstdlib:203: error: ‘__gnu_cxx::lldiv_t’ has not
been declared
  /usr/include/c++/4.1.2/cstdlib:205: error: ‘__gnu_cxx::_Exit’ has not
been declared
  /usr/include/c++/4.1.2/cstdlib:208: error: ‘__gnu_cxx::llabs’ has not
been declared
  /usr/include/c++/4.1.2/cstdlib:209: error: ‘__gnu_cxx::div’ has not been
declared
  /usr/include/c++/4.1.2/cstdlib:210: error: ‘__gnu_cxx::lldiv’ has not
been declared
  /usr/include/c++/4.1.2/cstdlib:213: error: ‘__gnu_cxx::strtof’ has not
been declared
  /usr/include/c++/4.1.2/cstdlib:216: error: ‘__gnu_cxx::strtold’ has not
been declared

*** Bug 35557 has been marked as a duplicate of this bug. ***

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