When a dynamic object is loaded into memory, the object is examined for any additional dependencies. By default, any dependencies that exist are immediately loaded. This cycle continues until the full dependency tree is exhausted. Finally, all inter-object data references that are specified by relocations, are resolved. These operations are performed regardless of whether the code in these dependencies is referenced by the application during its execution.
Under a lazy loading model, any dependencies that are labeled for lazy loading are loaded only when explicitly referenced. By taking advantage of the lazy binding of a function call, the loading of a dependency is delayed until the function is first referenced. As a result, objects that are never referenced are never loaded.
A relocation reference can be immediate or lazy. Because immediate references must be resolved when an object is initialized, any dependency that satisfies this reference must be immediately loaded. Therefore, identifying such a dependency as lazy loadable has little effect. See When Relocations Are Performed. Immediate references between dynamic objects are generally discouraged.
Lazy loading is used by the link-editors reference to a debugging library, liblddbg. As debugging is only called upon infrequently, loading this library every time that the link-editor is invoked is unnecessary and expensive. By indicating that this library can be lazily loaded, the expense of processing the library is moved to those invocations that ask for debugging output.
The alternate method of achieving a lazy loading model is to use dlopen() and dlsym() to load and bind to a dependency when needed. This model is ideal if the number of dlsym() references is small. This model also works well if the dependency name or location is not known at link-edit time. For more complex interactions with known dependencies, coding to normal symbol references and designating the dependency to be lazily loaded is simpler.
An object is designated as lazily or normally loaded through the link-editor options –z lazyload and –z nolazyload respectfully. These options are position-dependent on the link-edit command line. Any dependency that follows the option takes on the loading attribute specified by the option. By default, the –z nolazyload option is in effect.
The following simple program has a dependency on libdebug.so.1. The dynamic section, .dynamic, shows libdebug.so.1 is marked for lazy loading. The symbol information section, .SUNW_syminfo, shows the symbol reference that triggers libdebug.so.1 loading.
$ cc -o prog prog.c -L. -zlazyload -ldebug -znolazyload -lelf -R'$ORIGIN'
$ elfdump -d prog
Dynamic Section: .dynamic
index tag value
[0] POSFLAG_1 0x1 [ LAZY ]
[1] NEEDED 0x123 libdebug.so.1
[2] NEEDED 0x131 libelf.so.1
[3] NEEDED 0x13d libc.so.1
[4] RUNPATH 0x147 $ORIGIN
...
$ elfdump -y prog
Syminfo section: .SUNW_syminfo
index flgs bound to symbol
....
[52] DL [1] libdebug.so.1 debug
The POSFLAG_1 with the value of LAZY designates that the following NEEDED entry, libdebug.so.1, should be lazily loaded. As libelf.so.1 has no preceding LAZY flag, this library is loaded at the initial startup of the program.
The use of lazy loading can require a precise declaration of dependencies and runpaths through out the objects used by an application. For example, suppose two objects, libA.so and libB.so, both make reference to symbols in libX.so. libA.so declares libX.so as a dependency, but libB.so does not. Typically, when libA.so and libB.so are used together, libB.so can reference libX.so because libA.so made this dependency available. But, if libA.so declares libX.so to be lazy loaded, it is possible that libX.so might not be loaded when libB.so makes reference to this dependency. A similar failure can occur if libB.so declares libX.so as a dependency but fails to provide a runpath necessary to locate the dependency.
Regardless of lazy loading, dynamic objects should declare all their dependencies and how to locate the dependencies. With lazy loading, this dependency information becomes even more important.