RTLDNEXT

RTLD_NEXT is a special handle defined in the dynamic linking interface (dlfcn.h) that can be passed to the symbol resolution function dlsym to locate the next occurrence of a symbol after the current object in the shared object dependency chain. It is primarily used to implement function interposition in shared libraries, allowing a library to wrap or override a symbol provided by lower layers and then delegate to the original version.

Usage and behavior: include <dlfcn.h>. When implementing a replacement for a library function, obtain the address of the next symbol by calling dlsym(RTLD_NEXT, "function_name"). You can then call through to that address, typically after performing any pre- or post-processing. For example, a wrapped open function might resolve the original as int (real_open)(const char, int, mode_t) = dlsym(RTLD_NEXT, "open"); and then call real_open with the appropriate arguments. RTLD_NEXT ensures the search skips the current object, referencing the next definition in the chain rather than the one in the same object.

Comparison with other handles: RTLD_NEXT is distinct from RTLD_DEFAULT, which asks the linker to search all available objects for a symbol, and from using an explicit handle obtained via dlopen. RTLD_NEXT is specifically for walking the link chain in interposition scenarios.

Limitations and portability: RTLD_NEXT is a GNU extension and is available on platforms with glibc or compatible dynamic linkers. Its behavior is not defined for static linking or outside the dynamic loader context, and its availability may vary across non-ELF or non-GNU systems. Developers should check platform documentation when portability is a concern.

See also: dlfcn.h, dlsym, RTLD_DEFAULT.