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PO__attribute__((weak)) and LD_PRELOAD
 

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    <p>I want a code to be able to run an application <em>with</em> a certain self-written library and <em>without</em> it. So I use the <code>__attribute__ ((weak))</code> and preload the library if needed. I need to be able to do that without recompiling. Everything works fine if I link the library statically, though.</p> <p>Furthermore the library is written in C++ while the applications using it can be C++ or C.</p> <p>I boils down to this:</p> <p>library header <code>test_lib.h</code>:</p> <pre><code>#ifdef __cplusplus extern "C" #endif void test_func() __attribute__ ((weak)); </code></pre> <p>library implementation <code>test_lib.cpp</code>:</p> <pre><code>#include "test_lib.h" #include &lt;iostream&gt; extern "C" void test_func() { std::cout &lt;&lt; "in test_func\n"; } </code></pre> <p>the C test <code>test_main.c</code>:</p> <pre><code>#include &lt;stdio.h&gt; #include "test_lib.h" int main(void) { if (test_func){ printf("+\n"); } else{ printf("-\n"); } return 0; } </code></pre> <p>the C++ test <code>test_main_cpp.cpp</code>:</p> <pre><code>#include &lt;iostream&gt; #include "test_lib.h" int main(void) { if (test_func){ std::cout &lt;&lt; "+\n"; } else{ std::cout &lt;&lt; "-\n"; } return 0; } </code></pre> <p>the compile and run script for your convenience:</p> <pre><code>#!/bin/bash g++ -shared -fPIC test_lib.cpp -o libtest.so gcc test_main.c -o test_c g++ test_main_cpp.cpp -o test_cpp # output should be "-" ./test_c ./test_cpp # output should be "+" LD_PRELOAD=libtest.so ./test_c LD_PRELOAD=libtest.so ./test_cpp </code></pre> <p>The expected output is:</p> <pre><code>- - + + </code></pre> <p>What I get is:</p> <pre><code>- - - - </code></pre> <p>and finally a little extra information:</p> <pre><code>$ uname -a Linux bermuda-iii 3.8.6-1-ARCH #1 SMP PREEMPT Sat Apr 6 07:27:01 CEST 2013 x86_64 GNU/Linux $ gcc --version gcc (GCC) 4.8.0 $ nm libtest.so | grep -i func 0000000000000858 W test_func $ nm test_c | grep -i func w test_func $ nm test_cpp | grep -i func w test_func </code></pre> <p>So: (de)mangling seems to work fine, the symbol <code>test_func</code> is known to the executables. But the `LD_PRELOAD´ doesn't seem to work. </p> <p>What am I missing?</p>
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    1. USsteffen
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      1. USAnish Ramaswamy
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