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copied!<p>First, you should generally use <code>-fPIC</code> flag when building shared libraries.</p> <p>Not using it "works" on 32-bit Linux, but would fail on 64-bit one with an error similar to:</p> <pre><code>/usr/bin/ld: /tmp/ccUUrz9c.o: relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC </code></pre> <p>Second, your program will work as you expect after you add <code>-rdynamic</code> to the link line for the main executable:</p> <pre><code>singleton.num in main : 100 singleton.num in hello.so : 100 singleton.num in hello.so after ++ : 101 singleton.num in main : 101 </code></pre> <p>In order to understand why <code>-rdynamic</code> is required, you need to know about the way dynamic linker resolves symbols, and about the <em>dynamic</em> symbol table.</p> <p>First, let's look at the dynamic symbol table for <code>hello.so</code>:</p> <pre><code>$ nm -C -D hello.so | grep singleton 0000000000000b8c W singleton::instance() 0000000000201068 B singleton::pInstance 0000000000000b78 W singleton::singleton() </code></pre> <p>This tells us that there are two weak function definitions, and one global variable <code>singleton::pInstance</code> that are visible to the dynamic linker.</p> <p>Now let's look at the static and dynamic symbol table for the original <code>example1</code> (linked without <code>-rdynamic</code>):</p> <pre><code>$ nm -C example1 | grep singleton 0000000000400d0f t global constructors keyed to singleton::pInstance 0000000000400d38 W singleton::instance() 00000000006022e0 B singleton::pInstance 0000000000400d24 W singleton::singleton() $ nm -C -D example1 | grep singleton $ </code></pre> <p>That's right: even though the <code>singleton::pInstance</code> is present in the executable as a global variable, that symbol is not present in the <em>dynamic</em> symbol table, and therefore "invisible" to the dynamic linker.</p> <p>Because the dynamic linker "doesn't know" that <code>example1</code> already contains a definition of <code>singleton::pInstance</code>, it doesn't bind that variable inside <code>hello.so</code> to the existing definition (which is what you really want).</p> <p>When we add <code>-rdynamic</code> to the link line:</p> <pre><code>$ nm -C example1-rdynamic | grep singleton 0000000000400fdf t global constructors keyed to singleton::pInstance 0000000000401008 W singleton::instance() 00000000006022e0 B singleton::pInstance 0000000000400ff4 W singleton::singleton() $ nm -C -D example1-rdynamic | grep singleton 0000000000401008 W singleton::instance() 00000000006022e0 B singleton::pInstance 0000000000400ff4 W singleton::singleton() </code></pre> <p>Now the definition of <code>singleton::pInstance</code> inside the main executable is <em>visible</em> to the dynamic linker, and so it will "reuse" that definition when loading <code>hello.so</code>:</p> <pre><code>LD_DEBUG=bindings ./example1-rdynamic |& grep pInstance 31972: binding file ./hello.so [0] to ./example1-rdynamic [0]: normal symbol `_ZN9singleton9pInstanceE' </code></pre>

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