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POAnsi C patch using dlsym compiles OK under linux but fails on Mac Os X
 

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  1. POAnsi C patch using dlsym compiles OK under linux but fails on Mac Os X
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    <p>I have build a little patch to append to a certain application and trace the invocations of some functions. Among them, malloc() and open(). I am using dlsym to store the pointer to the original symbol and replace the function name with my own. It compiles -and works- perfectly under linux. Here's the code:</p> <pre><code>#define _GNU_SOURCE #include &lt;stdint.h&gt; #include &lt;stdio.h&gt; #include &lt;string.h&gt; #include &lt;stdarg.h&gt; #include &lt;dlfcn.h&gt; /** * Interponemos nuestra funcion open * * @param char* filename * @param int flags **/ int open(char * filename, int flags) { static int (*real_open)(char*, int) = NULL; if (!real_open) real_open = dlsym(RTLD_NEXT, "open"); // Entero int p = real_open(filename, flags); fprintf(stderr, "Abrimos %s) = %i\n", filename, flags); // Devolvemos return p; } //-------------------------------------------------------- /** * Interponemos nuestra funcion malloc * * @param size_t size */ void* malloc(size_t size) { static void* (*real_malloc)(size_t) = NULL; if (!real_malloc) real_malloc = dlsym(RTLD_NEXT, "malloc"); void *p = real_malloc(size); // Memoria reservada fprintf(stderr, "Reserva de memoria (%d) = %p\n", size, p); // Devolvemos return p; } </code></pre> <p>Then, I compile it with the following instruction, creating a pi.so.</p> <pre><code>gcc -Wall -O2 -fPIC -shared -ldl -o pi.so pi.c </code></pre> <p>And then, I use the LD_PRELOAD directive to inject it into any app.</p> <pre><code>LD_PRELOAD=/home/.../injection/pi.so &lt;binary&gt; </code></pre> <p>And it worjks marvellous under Linux! But when I get back home and try to compile it using GCC under Mac, it fails to compile and the LD_PRELOAd directive does not work. What should I change? Thank you very much.</p>
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    1. COJust so you know, there was a technical corrigendum published to work around the fact that C does not define behaviour for converting a void pointer to a function pointer. See the example on the POSIX page for [dlsym](http://www.opengroup.org/onlinepubs/009695399/functions/dlsym.html) to see how to use it in a compliant manner.
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