When Relocations Are Performed

Having briefly described the relocation process, together with the simplification of relocations into the two types, non-symbolic and symbolic, it is also useful to distinguish relocations by when they are performed. This distinction arises due to the type of reference being made to the relocated offset, and can be either a:

• A data reference.

• A function reference.

A data reference refers to an address that is used as a data item by the application code. The runtime linker has no knowledge of the application code, so does not know when this data item will be referenced. Therefore, all data relocations must be carried out during process initialization, before the application gains control.

A function reference refers to the address of a function that will be called by the application code. During the compilation and link-editing of any dynamic module, calls to global functions are relocated to become calls to a procedure linkage table entry (these entries make up the .plt section).

Procedure linkage table entries are constructed so that, when first called, control is passed to the runtime linker (see "Procedure Linkage Table (Processor-Specific)"). The runtime linker looks up the required symbol and rewrites information in the application so that any future calls to this .plt entry go directly to the function. This mechanism allows relocations of this type to be deferred until the first instance of a function being called, a process that is sometimes referred to as lazy binding.

The runtime linker's default mode of performing lazy binding can be overridden by setting the environment variable LD_BIND_NOW to any non-null value. This environment variable setting causes the runtime linker to perform both data reference and function reference relocations during process initialization, before transferring control to the application. For example:

$ LD_BIND_NOW=yes prog

Here, all relocations within the file prog and within its dependencies will be processed before control is transferred to the application.

Individual objects can also be built using the link-editors' -z now option to indicate that they require complete relocation processing at the time they are loaded. This relocation requirement is also propagated to any dependencies of the marked object at runtime.