COFF
Encyclopedia
The Common Object File Format (COFF) is a specification of a format
for executable
, object code
, and shared library computer file
s used on Unix
systems. It was introduced in Unix System V
, replaced the previously used a.out format, and formed the basis for extended specifications such as XCOFF
and ECOFF
, before being largely replaced by ELF
, introduced with SVR4
.
COFF and its variants continue to be used on some Unix-like
systems, on Microsoft Windows
, and in EFI
environments, and in some embedded development systems.
is unable to adequately support shared libraries, foreign format identification, or explicit address linkage. As development of Unix-like
systems continued both inside and outside AT&T, different solutions to these and other issues emerged.
COFF was introduced in AT&T
's UNIX System V
for non-VAX 32-bit platforms such as the 3B20. Improvements over the existing AT&T a.out format included arbitrary sections, explicit processor declarations, and explicit address linkage.
However, the COFF design was both too limited and incompletely specified: there was a limit on the maximum number of sections, a limit on the length of section names, included source files, and the symbolic debugging information was incapable of supporting real world languages such as C
, much less newer languages like C++
, or new processors. All real world implementations of COFF were necessarily violations of the standard as a result. This led to numerous COFF extensions. IBM
used the XCOFF
format in AIX
. DEC
, SGI
and others used ECOFF
; And numerous SysV ports and tool chains targeting embedded development each created their own, incompatible, variations.
With the release of SVR4, AT&T replaced COFF with ELF
.
While extended versions of COFF continue to be used for some Unix-like platforms, primarily in embedded systems, perhaps the most widespread use of the COFF format today is in Microsoft
's Portable Executable
(PE) format. Developed for Windows NT
, the PE format (sometimes written as PE/COFF) uses a COFF header for object files, and as a component of the PE header for executable files.
Symbolic names are stored in the COFF symbol table. Each symbol table entry includes a name, storage class, type, value and section number. Short names (8 characters or fewer) are stored directly in the symbol table; longer names are stored as an offset into the string table at the end of the COFF object.
Storage classes describe the type entity the symbol represents, and may include external variables (C_EXT), automatic (stack) variables (C_AUTO), register variables (C_REG), functions (C_FCN), and many others. The symbol type describes the interpretation of the symbol entity's value, and includes values for all the C
data types.
When compiled with appropriate options, a COFF object file will contain line number information for each possible break point in the text section of the object file. Line number information takes two forms: in the first, for each possible break point in the code, the line number table entry records the address and its matching line number. In the second form, the entry identifies a symbol table entry representing the start of a function, enabling a breakpoint to be set using the function's name.
Note that COFF was not capable of representing line numbers or debugging symbols for included source as with header files rendering the COFF debugging information virtually useless without incompatible extensions.
where the first byte of the file will be loaded is called image base address
. The rest of the file is not necessarily loaded in a contiguous block, but in different sections.
Relative Virtual Addresses (RVAs) are not to be confused with standard virtual addresses. A relative virtual address is the virtual address
of an object from the file once it is loaded into memory, minus the base address of the file image. If the file were to be mapped literally from disk to memory, the RVA would be the same as that of the offset into the file, but this is actually quite unusual.
Note that the RVA term is only used with objects in the image file. Once loaded into memory, the image base address is added, and ordinary VAs are used.
File format
A file format is a particular way that information is encoded for storage in a computer file.Since a disk drive, or indeed any computer storage, can store only bits, the computer must have some way of converting information to 0s and 1s and vice-versa. There are different kinds of formats for...
for executable
Executable
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...
, object code
Object code
Object code, or sometimes object module, is what a computer compiler produces. In a general sense object code is a sequence of statements in a computer language, usually a machine code language....
, and shared library computer file
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
s used on Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
systems. It was introduced in Unix System V
UNIX System V
Unix System V, commonly abbreviated SysV , is one of the first commercial versions of the Unix operating system. It was originally developed by American Telephone & Telegraph and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4...
, replaced the previously used a.out format, and formed the basis for extended specifications such as XCOFF
XCOFF
XCOFF, for "eXtended COFF", is an improved and expanded version of the COFF object file format defined by IBM and used in AIX. Early versions of the PowerPC Macintosh also supported XCOFF, as did BeOS....
and ECOFF
ECOFF
The Extended Common Object File Format is a file format for executables, object code, and shared libraries.ECOFF was developed for the MIPS platform, and was used by DEC Ultrix and Tru64 , SGI Irix, and Linux/MIPS.- External links :*...
, before being largely replaced by ELF
Executable and Linkable Format
In computing, the Executable and Linkable Format is a common standard file format for executables, object code, shared libraries, and core dumps. First published in the System V Application Binary Interface specification, and later in the Tool Interface Standard, it was quickly accepted among...
, introduced with SVR4
UNIX System V
Unix System V, commonly abbreviated SysV , is one of the first commercial versions of the Unix operating system. It was originally developed by American Telephone & Telegraph and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4...
.
COFF and its variants continue to be used on some Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems, on Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
, and in EFI
Extensible Firmware Interface
The Unified Extensible Firmware Interface is a specification that defines a software interface between an operating system and platform firmware...
environments, and in some embedded development systems.
History
The original Unix object file format a.outA.out (file format)
a.out is a file format used in older versions of Unix-like computer operating systems for executables, object code, and, in later systems, shared libraries...
is unable to adequately support shared libraries, foreign format identification, or explicit address linkage. As development of Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
systems continued both inside and outside AT&T, different solutions to these and other issues emerged.
COFF was introduced in AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
's UNIX System V
UNIX System V
Unix System V, commonly abbreviated SysV , is one of the first commercial versions of the Unix operating system. It was originally developed by American Telephone & Telegraph and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4...
for non-VAX 32-bit platforms such as the 3B20. Improvements over the existing AT&T a.out format included arbitrary sections, explicit processor declarations, and explicit address linkage.
However, the COFF design was both too limited and incompletely specified: there was a limit on the maximum number of sections, a limit on the length of section names, included source files, and the symbolic debugging information was incapable of supporting real world languages such as C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
, much less newer languages like C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
, or new processors. All real world implementations of COFF were necessarily violations of the standard as a result. This led to numerous COFF extensions. IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
used the XCOFF
XCOFF
XCOFF, for "eXtended COFF", is an improved and expanded version of the COFF object file format defined by IBM and used in AIX. Early versions of the PowerPC Macintosh also supported XCOFF, as did BeOS....
format in AIX
AIX operating system
AIX AIX AIX (Advanced Interactive eXecutive, pronounced "a i ex" is a series of proprietary Unix operating systems developed and sold by IBM for several of its computer platforms...
. DEC
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
, SGI
Silicon Graphics
Silicon Graphics, Inc. was a manufacturer of high-performance computing solutions, including computer hardware and software, founded in 1981 by Jim Clark...
and others used ECOFF
ECOFF
The Extended Common Object File Format is a file format for executables, object code, and shared libraries.ECOFF was developed for the MIPS platform, and was used by DEC Ultrix and Tru64 , SGI Irix, and Linux/MIPS.- External links :*...
; And numerous SysV ports and tool chains targeting embedded development each created their own, incompatible, variations.
With the release of SVR4, AT&T replaced COFF with ELF
Executable and Linkable Format
In computing, the Executable and Linkable Format is a common standard file format for executables, object code, shared libraries, and core dumps. First published in the System V Application Binary Interface specification, and later in the Tool Interface Standard, it was quickly accepted among...
.
While extended versions of COFF continue to be used for some Unix-like platforms, primarily in embedded systems, perhaps the most widespread use of the COFF format today is in Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
's Portable Executable
Portable Executable
The Portable Executable format is a file format for executables, object code and DLLs, used in 32-bit and 64-bit versions of Windows operating systems. The term "portable" refers to the format's versatility in numerous environments of operating system software architecture...
(PE) format. Developed for Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
, the PE format (sometimes written as PE/COFF) uses a COFF header for object files, and as a component of the PE header for executable files.
Features
COFF's main improvement over a.out was the introduction of multiple named sections in the object file. Different object files could have different numbers and types of sections.Symbolic Debugging Information
The COFF symbolic debugging information consists of symbolic (string) names for program functions and variables, and line number information, used for setting breakpoints and tracing execution.Symbolic names are stored in the COFF symbol table. Each symbol table entry includes a name, storage class, type, value and section number. Short names (8 characters or fewer) are stored directly in the symbol table; longer names are stored as an offset into the string table at the end of the COFF object.
Storage classes describe the type entity the symbol represents, and may include external variables (C_EXT), automatic (stack) variables (C_AUTO), register variables (C_REG), functions (C_FCN), and many others. The symbol type describes the interpretation of the symbol entity's value, and includes values for all the C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
data types.
When compiled with appropriate options, a COFF object file will contain line number information for each possible break point in the text section of the object file. Line number information takes two forms: in the first, for each possible break point in the code, the line number table entry records the address and its matching line number. In the second form, the entry identifies a symbol table entry representing the start of a function, enabling a breakpoint to be set using the function's name.
Note that COFF was not capable of representing line numbers or debugging symbols for included source as with header files rendering the COFF debugging information virtually useless without incompatible extensions.
Relative Virtual Address
When a COFF file is generated, it is not usually known where in memory it will be loaded. The virtual addressVirtual address
In computer technology, a virtual address is an address identifying a virtual, i.e. non-physical, entity.-Description:The term virtual address is most commonly used for an address pointing to virtual memory or, in networking, when referring to a virtual network address...
where the first byte of the file will be loaded is called image base address
Base address
In computing, a base address is an address serving as a reference point for other addresses.In computers using relative addressing scheme, to obtain an absolute address, the relevant base address is taken and offset is added to it....
. The rest of the file is not necessarily loaded in a contiguous block, but in different sections.
Relative Virtual Addresses (RVAs) are not to be confused with standard virtual addresses. A relative virtual address is the virtual address
Virtual address
In computer technology, a virtual address is an address identifying a virtual, i.e. non-physical, entity.-Description:The term virtual address is most commonly used for an address pointing to virtual memory or, in networking, when referring to a virtual network address...
of an object from the file once it is loaded into memory, minus the base address of the file image. If the file were to be mapped literally from disk to memory, the RVA would be the same as that of the offset into the file, but this is actually quite unusual.
Note that the RVA term is only used with objects in the image file. Once loaded into memory, the image base address is added, and ordinary VAs are used.
External links
- More on the PE Format and Public documentation at Microsoft.com