Mach-O
Encyclopedia
Mach-O, short for Mach object
file format, is a file format
for executable
s, object code
, shared libraries, dynamically-loaded code, and core dump
s. A replacement for the a.out format, Mach-O offered more extensibility
and faster access to information in the symbol table
.
Mach-O was once used by most systems based on the Mach kernel. NeXTSTEP
, iOS
and Mac OS X
are examples of systems that have used this format for native executables, libraries and object code. GNU Hurd
, which uses GNU Mach
as its microkernel
, uses ELF
, and not Mach-O, as its standard binary format.
format to handle references to symbols. When looking up symbols Mach-O uses a two-level namespace that encodes each symbol into an 'object/symbol name' pair that is then linearly searched for by first the object and then the symbol name.
The basic structure—a list of variable-length "load commands" that reference pages of data elsewhere in the file—was also used in the executable file format for Accent
. The Accent file format was in turn, based on an idea from Spice Lisp
.
, OPENSTEP
and Mac OS X
, multiple Mach-O files can be combined in a multi-architecture binary. This allows a single binary file to contain code to support multiple instruction set architectures. For example, a multi-architecture binary for Mac OS X could contain both 32-bit
and 64-bit
PowerPC
code, or could contain both 32-bit PowerPC or x86 code, or could contain 32-bit PowerPC code, 64-bit PowerPC code, 32-bit x86 code, and 64-bit x86 (x86-64
) code.
10.6 platform the Mach-O file has undergone a significant modification that causes binaries compiled on a 10.6 computer to be by default only able to run on a 10.6 computer. The difference stems from load commands that Mac OS X's linker (dyld) can not understand on previous Mac OS X versions. Another significant change to the Mach-O format is the change in how the Link Edit tables (found in the __LINKEDIT section) function. In 10.6 these new Link Edit tables are compressed by removing unused and unneeded bits of information, however Mac OS X 10.5 and earlier cannot read this new Link Edit table format. To resolve this issue, the linker flag "-mmacosx-version-min=" is heavily used and depended on. Apple, current maintainer of the Mach-O format, recommends that all developers now use this flag along with the appropriate SDK headers when creating an application/binary.
have had Mach-O support added as part of an implementation of binary compatibility, which allowed some Mac OS 10.3 binaries to be executed.
For Linux a Mach-O loader was written by that can load 10.6 binaries.
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....
file format, is a file format
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...
s, 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....
, shared libraries, dynamically-loaded code, and core dump
Core dump
In computing, a core dump consists of the recorded state of the working memory of a computer program at a specific time, generally when the program has terminated abnormally...
s. A replacement for the a.out format, Mach-O offered more extensibility
Extensibility
In software engineering, extensibility is a system design principle where the implementation takes into consideration future growth. It is a systemic measure of the ability to extend a system and the level of effort required to implement the extension...
and faster access to information in the symbol table
Symbol table
In computer science, a symbol table is a data structure used by a language translator such as a compiler or interpreter, where each identifier in a program's source code is associated with information relating to its declaration or appearance in the source, such as its type, scope level and...
.
Mach-O was once used by most systems based on the Mach kernel. NeXTSTEP
NEXTSTEP
NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...
, iOS
IOS
iOS is an operating system for iPad, iPhone, iPod Touch, and Apple TV.IOS may also refer to:-Companies and organisations:* Illinois Ornithological Society, American state-based bird club...
and Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
are examples of systems that have used this format for native executables, libraries and object code. GNU Hurd
GNU Hurd
GNU Hurd is a free software Unix-like replacement for the Unix kernel, released under the GNU General Public License. It has been under development since 1990 by the GNU Project of the Free Software Foundation...
, which uses GNU Mach
GNU Mach
GNU Mach is an implementation of the Mach microkernel. It is the default microkernel in the GNU Hurd operating system. GNU Mach runs on IA-32 machines. GNU Mach is maintained by developers on the GNU project...
as its microkernel
Microkernel
In computer science, a microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system . These mechanisms include low-level address space management, thread management, and inter-process communication...
, uses 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...
, and not Mach-O, as its standard binary format.
Mach-O file layout
Each Mach-O file is made up of one Mach-O header, followed by a series of load commands, followed by one or more segments, each of which contains between 0 and 255 sections. Mach-O uses the REL relocationRelocation (computer science)
"Relocation is the process of assigning load addresses to various parts of [a] program and adjusting the code and data in the program to reflect the assigned addresses."...
format to handle references to symbols. When looking up symbols Mach-O uses a two-level namespace that encodes each symbol into an 'object/symbol name' pair that is then linearly searched for by first the object and then the symbol name.
The basic structure—a list of variable-length "load commands" that reference pages of data elsewhere in the file—was also used in the executable file format for Accent
Accent kernel
Accent was an operating system kernel developed at Carnegie Mellon University . Accent was developed as a follow-on to the Aleph kernel developed at the University of Rochester, fixing several of its problems and re-targeting its hardware support for networks of workstation machines instead of...
. The Accent file format was in turn, based on an idea from Spice Lisp
Spice Lisp
Spice Lisp is a Lisp dialect and its implementation originally written by CMU's Spice Lisp Group which targeted the microcode of the 16-bit PERQ workstation and its Accent operating system; it used that workstation's microcode abilities to implement a stack architecture to store its data...
.
Multi-architecture binaries
Under NeXTSTEPNEXTSTEP
NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...
, OPENSTEP
OpenStep
OpenStep was an object-oriented application programming interface specification for an object-oriented operating system that used a non-NeXTSTEP operating system as its core, principally developed by NeXT with Sun Microsystems. OPENSTEP was a specific implementation of the OpenStep API developed...
and Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
, multiple Mach-O files can be combined in a multi-architecture binary. This allows a single binary file to contain code to support multiple instruction set architectures. For example, a multi-architecture binary for Mac OS X could contain both 32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
and 64-bit
64-bit
64-bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs, and by extension the software that runs on them. 64-bit CPUs have existed in supercomputers since the 1970s and in RISC-based workstations and servers since the early 1990s...
PowerPC
PowerPC
PowerPC is a RISC architecture created by the 1991 Apple–IBM–Motorola alliance, known as AIM...
code, or could contain both 32-bit PowerPC or x86 code, or could contain 32-bit PowerPC code, 64-bit PowerPC code, 32-bit x86 code, and 64-bit x86 (x86-64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
) code.
Mach-O future
With the introduction of Apple's Mac OS XMac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
10.6 platform the Mach-O file has undergone a significant modification that causes binaries compiled on a 10.6 computer to be by default only able to run on a 10.6 computer. The difference stems from load commands that Mac OS X's linker (dyld) can not understand on previous Mac OS X versions. Another significant change to the Mach-O format is the change in how the Link Edit tables (found in the __LINKEDIT section) function. In 10.6 these new Link Edit tables are compressed by removing unused and unneeded bits of information, however Mac OS X 10.5 and earlier cannot read this new Link Edit table format. To resolve this issue, the linker flag "-mmacosx-version-min=" is heavily used and depended on. Apple, current maintainer of the Mach-O format, recommends that all developers now use this flag along with the appropriate SDK headers when creating an application/binary.
Other implementations
Some versions of NetBSDNetBSD
NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...
have had Mach-O support added as part of an implementation of binary compatibility, which allowed some Mac OS 10.3 binaries to be executed.
For Linux a Mach-O loader was written by that can load 10.6 binaries.
See also
- Fat binaryFat binaryA fat binary is a computer program with code native to multiple Instruction sets which can consequently be run on multiple processor types. The usual method of implementation is to include a version of the machine code for each instruction set, preceded by code compatible with all operating...
- Universal binaryUniversal binaryA universal binary is, in Apple parlance, an executable file or application bundle that runs natively on either PowerPC or Intel-manufactured IA-32 or Intel 64-based Macintosh computers; it is an implementation of the concept more generally known as a fat binary.With the release of Mac OS X Snow...
- Apple–Intel transition
- XcodeXcodeXcode is a suite of tools, developed by Apple, for developing software for Mac OS X and iOS. Xcode 4.2, the latest major version, is available on the Mac App Store for free for Mac OS X 10.7 , and on the Apple Developer Connection website for free to registered developers Xcode is a suite of tools,...
- Comparison of executable file formatsComparison of executable file formatsThis is a comparison of executable file formats.Among the above formats, the ones in most common use are PE , ELF and Mach-O .- References :...