Relocation (computing)

In software development, relocation is the process of assigning load addresses for position-dependent code and data of a program and adjusting the code and data to reflect the assigned addresses.

A linker usually performs relocation in conjunction with symbol resolution, the process of searching files and libraries to replace symbolic references or names of libraries with actual usable addresses in memory before running a program.

Relocation is typically done by the linker at link time, but it can also be done at load time by a relocating loader, or at run time by the running program itself.

Segmentation

Object files are typically segmented into various memory segments or section types. Example segment types include code segment (.text), initialized data segment (.data), uninitialized data segment (.bss), or others as established by the programmer, such as common segments or named static segments.

Relocation table

The relocation table is a list of addresses created by a compiler or assembler and stored in the object or executable file. Each entry in the table references an absolute address in the object code that must be changed when the loader relocates the program so that it will refer to the correct location. Entries in the relocation table are known as fixups and are designed to support relocation of the program as a complete unit. In some cases, each fixup in the table is itself relative to a base address of zero, so the fixups themselves must be changed as the loader moves through the table.

In some architectures, a fixup that crosses certain boundaries (such as a segment boundary) or that is not aligned on a word boundary is illegal and flagged as an error by the linker.