[ros-users] Linker --as-needed

[Josh Faust]

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 at bvu:~/scratch$ g++ test_as_needed_so.cpp -shared -fPIC -o
libtest_as_needed_so.so
jfaust at bvu:~/scratch$ g++ test_as_needed.cpp -o test_as_needed
-ltest_as_needed_so
jfaust at bvu:~/scratch$ ./test_as_needed
foo init
main
foo deinit

jfaust at bvu:~/scratch$ g++ test_as_needed.cpp -Wl,--as-needed -o
test_as_needed -ltest_as_needed_so
jfaust at 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 at gmail.com> wrote:

>
>
> On 10 August 2010 02:32, Josh Faust <jfaust at 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;
> }
>
>
>
>
>
>
> --
> Phone : +82-10-5400-3296 (010-5400-3296)
> Home: http://snorriheim.dnsdojo.com/
> Yujin Robot: http://www.yujinrobot.com/
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>
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