An object file contains not only the object code, but also relocation information that the linker uses to assemble multiple object files into an executable or library, program symbols (names of variables and functions), and debugging information.
There are many different object file formats; originally each type of computer had its own unique format, but with the advent of Unix and other portable operating systems, some formats, such as COFF and ELF, have been defined and used on different kinds of systems. It is common for the same file format to be used both as linker input and output, and thus as the library and executable file format.
The design and/or choice of an object file format is a key part of overall system design; it affects the performance of the linker and thus programmer turnaround while developing, and if the format is used for executables, the design also affects the time programs take to begin running, and thus the responsiveness for users. Most object file formats are structured as blocks of data all of the same sort; these blocks can be paged in as needed by the virtual memory system, needing no further processing to be ready to use.
The simplest object file format is the DOS COM format, which is simply a file of raw bytes that is always loaded at a fixed location. Other formats are an elaborate array of structures and substructures whose specification runs to many pages.
Debugging information may either be an integral part of the object file format, as in COFF, or a semi-independent format which may be used with several object formats, such as stabs or DWARF. See debugging format.
Types of data supported by typical object file formats: