Lazy Loading of Dynamic Dependencies

When a dynamic object is loaded into memory, it is examined for any additional dependencies. By default, if any dependencies exist they are immediately loaded. This cycle continues until the full dependency tree is exhausted. At which point all inter-object references, specified by relocations, are resolved.

Under this default model, all the dependencies of an application are loaded into memory, and all data relocations are performed. 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 a function call's lazy binding, the loading of a dependency is delayed until it is first referenced. In fact, 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-editor itself, which references a debugging library, liblddbg. Because debugging is only called upon infrequently, loading this library every time the link-editor is invoked is unnecessary and expensive. By indicating that this library can be lazily loaded, the expense of processing it can be 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 is ideal if the number of dlsym() references is small, or 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 found following 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 -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         libc.so.1
       [3]  RUNPATH       0x13b         $ORIGIN
       ...
$ elfdump -y prog
 
Syminfo section: .SUNW_syminfo
     index flgs  boundto                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. Because libc.so.1 has no preceding LAZY flag it 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 it has a dependency on libX.so, 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 it available. But, if libA.so declares libX.so to be lazy loaded, it is possible that libX.so may not be loaded when libB.so makes reference to it. A similar failure can occur if libB.so declares libX.so as a dependency but fails to provide a runpath necessary to locate it.

Regardless of lazy loading, it is recommended that dynamic objects declare all their dependencies and how to locate them. With lazy loading, this dependency information becomes even more important.

Lazy loading can be disabled at runtime by setting the environment variable LD_NOLAZYLOAD to a non-null value.