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POPython: garbage collector behavior with ctypes
 

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  1. POPython: garbage collector behavior with ctypes
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    <p>So let's say I C/C++ code that allocates some memory, and returns a pointer to it.</p> <pre><code>#include &lt;stdlib.h&gt; #ifdef __cplusplus extern "C" { #endif void Allocate(void **p) { int N=2048; *p=malloc(N); } #ifdef __cplusplus } #endif </code></pre> <p>I'm expecting that it's my responsibility to free that block of memory, obviously. Now suppose I compile this into a shared library and call it from Python with ctypes, but don't explicitly free that memory.</p> <pre><code>import ctypes from ctypes import cdll, Structure, byref external_lib = cdll.LoadLibrary('libtest.so.1.0') ptr=ctypes.c_void_p(0) external_lib.Allocate(ctypes.byref(ptr)) </code></pre> <p>If I run this script with valgrind, I get a memory leak of 2048 bytes if I compile test.cpp without the '-O3' flag. But if I compile it with the '-O3' flag, then I do not get the memory leak. </p> <p>It's not really a problem - I'll always be careful to explicitly free any memory I allocate. But I'm curious where this behavior comes from.</p> <p>I tested this with the following script in linux.</p> <pre><code>g++ -Wall -c -fPIC -fno-common test.cpp -o libtest1.o g++ -shared -Wl,-soname,libtest1.so.1 -o libtest1.so.1.0 libtest1.o g++ -O3 -Wall -c -fPIC -fno-common test.cpp -o libtest2.o g++ -shared -Wl,-soname,libtest2.so.1 -o libtest2.so.1.0 libtest2.o valgrind python test1.py &amp;&gt; report1 valgrind python test2.py &amp;&gt; report2 </code></pre> <p>with the following output</p> <p>report1:</p> <pre><code>==27875== LEAK SUMMARY: ==27875== definitely lost: 2,048 bytes in 1 blocks ==27875== indirectly lost: 0 bytes in 0 blocks ==27875== possibly lost: 295,735 bytes in 1,194 blocks ==27875== still reachable: 744,633 bytes in 5,025 blocks ==27875== suppressed: 0 bytes in 0 blocks </code></pre> <p>report2:</p> <pre><code>==27878== LEAK SUMMARY: ==27878== definitely lost: 0 bytes in 0 blocks ==27878== indirectly lost: 0 bytes in 0 blocks ==27878== possibly lost: 295,735 bytes in 1,194 blocks ==27878== still reachable: 746,681 bytes in 5,026 blocks ==27878== suppressed: 0 bytes in 0 blocks </code></pre>
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    1. COI've followed your steps and the results are interesting. With `Python 3.3.2` the two reports both give leakage with 2048 bytes, however with `Python 2.7.5` there's no leakage with either report. Tested in `Linux 3.11.4 x86_64` with `gcc 4.8.1 20130725`
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