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Date: Fri, 08 Jan 2010 01:40:33 -0600
From: "Yaakov (Cygwin/X)" <yselkowitz AT users DOT sourceforge DOT net>
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To: cygwin AT cygwin DOT com
Subject: gcc4: OpenMP vs. <math.h>
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POSIX allows for <math.h> functions to also be defined as macros. 
Currently, only log2 and log2f are so defined.

These macros pose problems with a few projects which define their own 
static/inline/template log2() (off the top of my head, I can think of 2: 
the CRAN rgl module, and OpenCV; both are C++).  Of course, those can be 
fixed with an #undef log2 after the #include's.

However, I just encountered tonight a much larger conflict: OpenMP/C++ 
and <math.h> are incompatible.  STC attached:

$ g++ -D_GLIBCXX_PARALLEL -fopenmp openmp.cxx -lgomp
In file included from 
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/parallel/algobase.h:46,
                  from 
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/bits/stl_algobase.h:1137,
                  from 
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/bits/char_traits.h:46,
                  from /usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/ios:46,
                  from 
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/ostream:45,
                  from 
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/iostream:45,
                  from openmp.cxx:7:
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/parallel/base.h:112: 
error: expected `)' before ‘/’ token

It gets even worse if you #include <algorithm> as well, and no, it 
doesn't help to #include <cmath>, although moving the math include after 
all other includes does work.

I see two possible solutions:

1) Make the log2 macros dependent on #ifndef __cplusplus;
2) Make the parallel/* headers #undef log2 and log2f.

Thoughts?


Yaakov

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// http://en.wikipedia.org/wiki/OpenMP#Hello_World
// with an #include <math.h>
// if you also #include <algorithm>, this *really* blows up

#include <omp.h>
#include <iostream>
#include <cmath>

int main (int argc, char *argv[]) {
	int th_id, nthreads;
#pragma omp parallel private(th_id)
	{
		th_id = omp_get_thread_num();
		std::cout << "Hello World from thread" << th_id << "\n";
#pragma omp barrier
		if ( th_id == 0 ) {
			nthreads = omp_get_num_threads();
			std::cout << "There are " << nthreads << " threads\n";
		}
	}
	return 0;
}


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