RTLDNOW

RTLD_NOW is a flag used with the POSIX dynamic linking function dlopen to request immediate resolution of all undefined symbols when a shared library is loaded. It applies to the loaded object and its dependencies, causing the dynamic linker to perform all necessary symbol relocations at load time rather than deferring them.

With RTLD_NOW, if any symbol required by the shared object (or its dependencies) cannot be resolved, dlopen fails, returning NULL and setting an error message that can be retrieved with dlerror. This contrasts with RTLD_LAZY, where undefined symbols may be resolved only when they are first referenced at runtime. RTLD_NOW thus provides early error detection but can introduce longer load times and startup latency.

RTLD_NOW is typically used in conjunction with other flags such as RTLD_GLOBAL or RTLD_LOCAL, which control symbol visibility to subsequently loaded libraries and the calling program. The exact numeric value of RTLD_NOW is platform-dependent and defined in the system’s dlfcn.h; many systems provide both RTLD_NOW and RTLD_LAZY as distinct options.

In practice, RTLD_NOW is chosen when ensuring that all dependencies are valid and all symbols can be resolved at load time is important for correctness or safety. It is commonly used in environments where missing or incompatible symbols would be problematic if discovered later during execution, such as plugins that must fully register their interfaces upfront. For scenarios emphasizing fast startup and on-demand symbol resolution, RTLD_LAZY may be preferred.