Object File Formats
An object file format is a computer file format used for the storage of object code and related data.
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 possible 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. 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, each block containing a certain type of data (see Memory segmentation). These blocks can be paged in as needed by the virtual memory system, needing no further processing to be ready to use.
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.
The GNU Project's Binary File Descriptor library (BFD library) provides a common API for the manipulation of object files in a variety of formats.
Types of data supported by typical object file formats:
- Header (descriptive and control information)
- Text segment (executable code)
- Data segment (static data)
- BSS segment (uninitialized data)
- External definitions and references for linking
- Relocation information
- Dynamic linking information
- Debugging information
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—Henri-Frédéric Amiel (1821–1881)
“A common and natural result of an undue respect for law is, that you may see a file of soldiers, colonel, captain, corporal, privates, powder-monkeys, and all, marching in admirable order over hill and dale to the wars, against their wills, ay, against their common sense and consciences, which makes it very steep marching indeed, and produces a palpitation of the heart.”
—Henry David Thoreau (1817–1862)