dlsym

dlsym is a function provided by the dynamic linker on most Unix-like systems. It retrieves the address of a symbol from a loaded shared object. The function signature is void *dlsym(void *handle, const char *symbol); handle is obtained from dlopen or is one of the special handles RTLD_DEFAULT or RTLD_NEXT, and symbol is the name of the symbol to locate. The return value is a pointer to the symbol, or NULL if the symbol cannot be found or an error occurred. When an error occurs, additional information is available through dlerror.

dlsym is commonly used after loading a library with dlopen to resolve functions or data objects at runtime. Once a symbol is found, its address can be converted to the appropriate function or data pointer type before use. This enables dynamic plugins and optional components that may not be linked at build time. If the symbol is a function, you typically cast the obtained void * to a function pointer with the correct signature before invoking it.

The symbol resolution facilities are declared in <dlfcn.h>, and linking typically requires -ldl on many systems (though the exact requirements may vary by platform). The RTLD_DEFAULT and RTLD_NEXT handles provide flexible search behavior within the process’s global symbol space or the dynamic linker's search order.

Error handling and portability considerations are important. Before calling dlsym, it is common to clear any existing error by invoking dlerror(), then call dlsym, and finally call dlerror() again to check for errors. The per-thread error string retrieved by dlerror helps diagnose issues in multi-threaded programs. In C++, access to C symbols may require extern "C" to prevent name mangling.