On 13 August 2010 08:41, Josh Faust <jfaust@willowgarage.com> wrote:

test_as_needed_so.cpp:

#include <cstring>
#include <cstdio>

class Foo
{
public:
  Foo()
  {
   printf("foo init\n");

  }

  ~Foo()
  {
   printf("foo deinit\n");
  }
};

Foo foo;

test_as_needed.cpp
#include <cstdio>

int main(int argc, char** argv)
{
  printf("main\n");
}

jfaust@bvu:~/scratch$ g++ test_as_needed_so.cpp -shared -fPIC -o libtest_as_needed_so.so

jfaust@bvu:~/scratch$ g++ test_as_needed.cpp -o test_as_needed -ltest_as_needed_so
jfaust@bvu:~/scratch$ ./test_as_needed
foo init
main
foo deinit

jfaust@bvu:~/scratch$ g++ test_as_needed.cpp -Wl,--as-needed -o test_as_needed -ltest_as_needed_so
jfaust@bvu:~/scratch$ ./test_as_needed
main

If you use the variable it has to link it in... it's when there's some global initialization that causes side effects even if you're not explicitly referencing any symbols. Not necessarily a problem, but something to look out for.

Josh

On Thu, Aug 12, 2010 at 4:13 PM, Daniel Stonier <d.stonier@gmail.com> wrote:

On 10 August 2010 02:32, Josh Faust <jfaust@willowgarage.com> wrote:

Happy to hear that it works in general (would be pretty bad if it
didn't...). I'm more concerned about whether we're currently
specifying everything completely, or unknowingly exploiting
overlinkage.

I guess the only way to find out is to build and test everything with
that option included.

I just did two quick tests:
1) .so that overrides a global symbol (strstr in this case)

2) .so with a global object that has a constructor/destructor

#1 worked, #2 failed (the .so was removed).

If #1 failed I'd say we can't default to it. Not sure how important #2 is, but it's something that could definitely cause problems that would be difficult to track down.

Josh

Josh, how did you produce result 2)? I just tested it here to be aware of when it might break and couldn't reproduce the failure. It correctly excludes/includes the library when I comment/uncomment the relevant usage of the variable. Snippets below:

LIBRARY HPP:

namespace ecl {

class A {
public:
A() : i(3) {}
~A() {}
int i;
};
extern const A a;

}

LIBRARY CPP:

namespace ecl {

const A a;

}

MAIN CPP

int main() {
std::cout << "i: " << ecl::a.i << std::endl;

return 0;
}

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