Mail Archives: cygwin/2007/10/24/15:21:53
This is a reply to the message
http://cygwin.com/ml/cygwin/2007-10/msg00509.html:
The linker is supposed to resolve a function-local static variable into
one exactly one instance, constructed on the first call, even if the
function is expanded in-line from multiple compilation units. It seems
that the problem is not just multiple in-line expansions, but the fact
that there is no obvious "home" compilation unit for this header-only
class. The following variant on Brad's example shows this:
-------------- Cut Here test2.sh ---------------
#! /bin/bash
#
echo "A Cygwin g++ -O2 Bug Report:"
echo "RESULT: This slightly modified version works"
echo "SIDE EFFECTS: the files ./bug0.hpp, bug1.cpp, and bug2.cpp are
created."
echo "RUN TEST: ./test2.sh"
#
cat << EOF > bug0.hpp
# include <string>
class Element {
public:
std::string file;
~Element();
static Element *root(void)
{ static Element r;
return &r;
}
};
EOF
cat << EOF > bug0.cpp
# include "bug0.hpp"
Element::~Element(){}
EOF
cat << EOF > bug1.cpp
# include "bug0.hpp"
extern void bug2(void);
int main(void)
{ std::string str("A");
char c = str[0];
Element *r = Element::root();
bug2();
return 0;
}
EOF
cat << EOF > bug2.cpp
# include <cassert>
# include "bug0.hpp"
void bug2(void)
{
Element *r = Element::root();
Element *s = Element::root();
assert( r != 0 );
}
EOF
echo "g++ bug1.cpp bug2.cpp bug0.cpp -O1 -o bug.exe"
g++ bug1.cpp bug2.cpp bug0.cpp -O1 -o bug.exe
echo "./bug"
./bug
echo "g++ bug1.cpp bug2.cpp bug0.cpp -O2 -o bug.exe"
g++ bug1.cpp bug2.cpp bug0.cpp -O2 -o bug.exe
echo "./bug"
./bug
-------------- Cut Here ---------------
Here, the offending function is still in-line and still invoked from two
places, but I have added a destructor and implemented it in one place.
That's enough to make it work.
It's still a bug in the linking process, in that it doesn't conform to
the C++ standard. Brad's original version should have worked.
Header-only libraries are very common, especially where templates are
involved, so this is not unimportant.
By the way, it's not just a g++ problem. Brad's original code (minus
.exe) runs just fine under Linux using an even older version of g++
(3.2.3 on my Linux, vs. 3.4.4 under Cygwin).
Bob Goddard
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