Understanding rpath, runpath, and LD_LIBRARY_PATH

1. rpath (Runtime Path):
   • rpath is a flag within an executable file used to specify the search path for shared libraries at runtime.
   • When a program runs, the system searches for shared libraries based on the rpath setting. If the corresponding shared libraries are found, the program can run successfully.
   • rpath is set during compilation or linking of the program and can be specified using the rpath option or in a Makefile.
2. runpath (Dynamic Run-time Linking):
   • runpath is an alternative mechanism to rpath used to specify the search path for shared libraries at runtime.
   • Unlike rpath, runpath can be modified dynamically at runtime, meaning its value can be changed as needed without recompiling or relinking the program.
   • runpath settings can also be specified during compilation or linking, typically using the runpath option or corresponding linker options.
3. LD_LIBRARY_PATH:
   • LD_LIBRARY_PATH is an environment variable used to specify the search path for shared libraries.
   • When a program runs, the system searches for shared libraries in the paths specified in the LD_LIBRARY_PATH variable.
   • Unlike rpath and runpath, LD_LIBRARY_PATH is a global environment variable that affects all running programs in the system. Therefore, it should be used with caution.
   • LD_LIBRARY_PATH can be set in the command line, for example: export LD_LIBRARY_PATH=/path/to/lib.

In summary, these mechanisms address the issue of how shared libraries are located and loaded during program runtime. rpath and runpath are primarily specified by program developers during compilation or linking, whereas LD_LIBRARY_PATH is a global environment variable that can be set by system administrators or users. rpath and runpath have higher search priority and are generally considered more reliable, while LD_LIBRARY_PATH is a global configuration option that can potentially impact all programs on the system and should be used with caution.