Editing Mach-O (section)

== 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 [[Relocation (computer science)|relocation]] 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, first by the object and then the symbol name.<ref name="file-format-reference">{{cite web|url=https://developer.apple.com/library/mac/documentation/developertools/conceptual/MachORuntime/Reference/reference.html |title=OS X ABI Mach-O File Format Reference |publisher=Apple Inc. |date=2009-02-04 |url-status=dead |archive-url=https://web.archive.org/web/20140904004108/https://developer.apple.com/library/mac/documentation/developertools/conceptual/MachORuntime/Reference/reference.html |archive-date=September 4, 2014}}</ref>

The basic structure—a list of variable-length "load commands" that reference pages of data elsewhere in the file<ref>{{cite conference
 |url=http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.52.675
 |title=A Unix Interface for Shared Memory and Memory Mapped Files Under Mach 
 |author1=Avadis Tevanian, Jr. 
 |author2=Richard F. Rashid 
 |author3=Michael W. Young 
 |author4=David B. Golub 
 |author5=Mary R. Thompson 
 |author6=William Bolosky 
 |author7=Richard Sanzi 
 |book-title=Proceedings of the USENIX Summer Conference
 |location=Phoenix, AZ, USA
 |publisher=USENIX Association}
 |date=June 1987
 |pages=53–67 
 }}</ref>—was also used in the executable file format for [[Accent kernel|Accent]].{{citation needed|date=May 2013}}  The Accent file format was in turn, based on an idea from [[Spice Lisp]].{{citation needed|date=May 2013}}

All multi-byte values in all data structures are written in the byte order of the host for which the code was produced.<ref name="mach-o-ref-data-types">{{cite web |work=OS X ABI Mach-O File Format Reference |title=Data Types |date=2009-02-04 |orig-date=2003 |publisher=[[Apple Inc.]] |url=https://developer.apple.com/library/mac/documentation/developertools/conceptual/MachORuntime/Reference/reference.html#//apple_ref/doc/uid/20001298-BAJFFCGF |url-status=dead |archive-url=https://web.archive.org/web/20140904004108mp_/https://developer.apple.com/library/mac/documentation/developertools/conceptual/MachORuntime/Reference/reference.html#//apple_ref/doc/uid/20001298-BAJFFCGF |archive-date=2014-09-04}}</ref>