Intercept malloc and free with LD_PRELOAD

LD_PRELOAD Function Interception

LD_PRELOAD is a Linux mechanism that loads a specified shared library before all others, allowing you to intercept and wrap dynamic library functions without modifying the original program.

How It Works

Set the environment variable before running a program:

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Your interceptor defines a function with the same signature as the target (for example malloc), calls the real version via dlsym(RTLD_NEXT, "malloc"), and adds custom logging or counting.

Common use cases:

• Memory tracking: intercept malloc/free to detect leaks
• File system virtualization: intercept open/read/write
• Testing: simulate allocation failures

For This Exercise

You will document how LD_PRELOAD can intercept malloc and free calls to add logging. This technique is widely used in debugging, testing, and security research to observe program behavior without source code access.

Building an Interceptor

Compile your interceptor as a shared library with -shared -fPIC and link -ldl. Export a malloc wrapper that logs the size and caller address, then forwards to the real malloc via dlsym. This pattern appears in debugging tools like Valgrind's interceptors and in security research to monitor API usage without recompiling the target. The interceptor must be thread-safe if the target uses pthreads.