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Computer programs typically are composed of several parts or modules; these parts/modules need not all be contained within a single object file, and in such cases refer to each other by means of symbols as addresses into other modules, which are mapped into memory addresses when linked for execution. Typically, an object file can contain three kinds of symbols: defined "external" symbols, sometimes called "public" or "entry" symbols, which allow it to be called by other modules, undefined "external" symbols, which reference other modules where these symbols are defined, and local symbols, used internally within the object file to facilitate relocation. For most compilers, each object file is the result of compiling one input source code file. When a program comprises multiple object files, the linker combines these files into a unified executable program, resolving the symbols as it goes along. Linkers can take objects from a collection called a library or runtime library. Most linkers do not include the whole library in the output; they include only the files that are referenced by other object files or libraries. Library linking may thus be an iterative process, with some modules included requiring additional modules to be linked, and so on. Libraries exist for diverse purposes, and one or more system libraries are usually linked in by default. The linker also takes care of arranging the objects in a program's address space. This may involve relocating code that assumes a specific base address into another base. Since a compiler seldom knows where an object will reside, it often assumes a fixed base location (for example, zero). Relocating machine code may involve re-targeting of absolute jumps, loads and stores. The executable output by the linker may need another relocation pass when it is finally loaded into memory (just before execution). This pass is usually omitted on hardware ing virtual memory: every program is put into its own address space, so there is no conflict even if all programs load at the same base address. This pass may also be omitted if the executable is a position independent executable. On some Unix variants, such as SINTRAN III, the process performed by a linker (assembling object files into a program) was called loading (as in loading executable code onto a file). Additionally, in some operating systems, the same program handles both the jobs of linking and loading a program