Editing Fat binary (section)

====NeXTSTEP Multi-Architecture Binaries====
Fat binaries were a feature of [[NeXT]]'s [[NeXTSTEP]]/[[OPENSTEP]] operating system, starting with NeXTSTEP 3.1. In NeXTSTEP, they were called "Multi-Architecture Binaries". Multi-Architecture Binaries were originally intended to allow software to be compiled to run both on NeXT's Motorola 68k-based hardware and on Intel [[IA-32]]-based [[IBM PC compatible|PC]]s running NeXTSTEP, with a single binary file for both platforms.<ref name="Tevanian-DeMoney-Enderby_Wiebe_Snyder_1995"/> It was later used to allow OPENSTEP applications to run on PCs and the various [[RISC]] platforms OPENSTEP supported. Multi-Architecture Binary files are in a special archive format, in which a single file stores one or more [[Mach-O]] subfiles for each architecture supported by the Multi-Architecture Binary. Every Multi-Architecture Binary starts with a structure ({{code|struct fat_header}}) containing two unsigned integers. The first integer ("magic") is used as a [[File format#Magic number|magic number]] to identify this file as a Fat Binary. The second integer ({{mono|nfat_arch}}) defines how many Mach-O Files the archive contains (how many instances of the same program for different architectures). After this header, there are ''{{mono|nfat_arch}}'' number of fat_arch structures ({{code|struct fat_arch}}). This structure defines the offset (from the start of the file) at which to find the file, the alignment, the size and the CPU type and subtype which the Mach-O binary (within the archive) is targeted at.

The version of the [[GNU Compiler Collection]] shipped with the Developer Tools was able to [[cross-compilation|cross-compile]] source code for the different architectures on which [[NeXTStep]] was able to run. For example, it was possible to choose the target architectures with multiple '-arch' options (with the architecture as argument). This was a convenient way to distribute a program for NeXTStep running on different architectures.

It was also possible to create libraries (e.g. using NeXTStep's {{mono|libtool}}) with different targeted object files.