An object file is a file containing relocatable format machine code that is usually not directly executable. Object files are produced by an assembler, compiler, or other language translator, and used as input to the linker.
Additionally, object files may contain metadata such as information to resolve symbolic cross-references between different modules, relocation information, stack unwinding information, comments, program symbols, debugging or profiling information. A linker is typically used to generate an executable or library by combining parts of object files.
An object file format is a computer file format used for the storage of object code and related data typically produced by a compiler or assembler.
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:
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