Lazy loading

Lazy dependency loading (or on-demand dependency loading) is a method of loading the required objects when they're actually required. The most important effect of lazy loading is that the resolution scope is different for a lazyload dependency. While in a normal dependency, the resolution scope contains immediate dependencies followed by their dependencies sorted in breadth-first order, for a lazy-loaded object, the resolution scope ends with its first-level dependencies. Therefore, all of the lazy-loaded symbols must be satisfied by definitions in its first level dependencies.

Due to this difference, you must carefully consider whether lazy-load dependencies are suitable for your application.

Each dynamic object can have multiple dependencies. Dependencies can be immediate or implicit:

  • Immediate dependencies are those that directly satisfy all external references of the object.
  • Implicit dependencies are those that satisfy dependencies of the object's dependencies.

The ultimate dependent object is the executable binary itself, but we will consider any object that needs to resolve its external symbols to be dependent. When referring to immediate or implicit dependencies, we always view them from the point of view of the dependent object.

Here are some other terms:

Lazy-load dependency
Dependencies that aren't immediately loaded are referred to as lazy-load dependencies.
Lookup scope/resolution scope
A list of objects where a symbol is looked for. The lookup scope is determined at the object's load time.
Immediate and lazy symbol resolution
All symbolic references must be resolved. Some symbol resolutions need to be performed immediately, such as symbolic references to global data. Another type of symbolic references can be resolved on first use: external function calls. The first type of symbolic references are referred to as immediate, and the second as lazy.

To use lazy loading, specify the RTLD_LAZYLOAD flag when you call dlopen() .

The runtime linker creates the link map for the executable in the usual way, by creating links for each DT_NEEDED object. Lazy dependencies are represented by a special link, a placeholder that doesn't refer to actual object yet. It does, however, contain enough information for the runtime linker to look up the object and load it on demand.

The lookup scope for the dependent object and its regular dependencies is the link map, while for each lazy dependency symbol, the lookup scope gets determined on-demand, when the object is actually loaded. Its lookup scope is defined in the same way that we define the lookup scope for an object loaded with dlopen(RTLD_GROUP) (it's important that RTLD_WORLD not be specified, or else we'd be including all RTLD_GLOBAL objects in the lookup scope).

When a call to an external function is made from dependent object, by using the lazy binding mechanism we traverse its scope of resolution in the usual way. If we find the definition, we're done. If, however, we reach a link that refers to a not-yet-loaded dependency, we load the dependency and then look it up for the definition. We repeat this process until either a definition is found, or we've traversed the entire dependency list. We don't traverse any of the implicit dependencies.

The same mechanism applies to resolving immediate relocations. If a dependent object has a reference to global data, and we don't find the definition of it in the currently loaded objects, we proceed to load the lazy dependencies, the same way as described above for resolving a function symbol. The difference is that this happens at the load time of the dependent object, not on first reference.

This approach preserves the symbol-overriding mechanisms provided by LD_PRELOAD.

Another important thing to note is that lazy-loaded dependencies change their own lookup scope; therefore, when resolving a function call from a lazy-loaded dependency, the lookup scope will be different than if the dependency was a normal dependency. As a consequence, lazy loading can't be transparent as, for example, lazy binding is (lazy binding doesn't change the lookup scope, only the time of the symbol lookup).