runtime function overriding for Mac OS X
C C++ Ruby
Latest commit 919148f May 11, 2014 @rentzsch Merge pull request #16 from mark-chromium/patch-1
[FIX] Stop using mach_host_self and host_page_size, fixing a port right leak. (Mark Mentovai)



mach_override enables you to override ("patch") program- and system-supplied functions at runtime. You can fully replace functions with your implementations, or merely head- or tail-patch the original implementations to add functionality and/or work around bugs.

Must Be This Tall To Ride

Dynamic function overriding is tricky business. You should be familiar with assembly and runtime calling conventions (ABIs) before using mach_override.

mach_override is certainly not bug free and patches are extremely welcome, but the onus is on you when things don't work. Please don't file a bug report stating mach_override is crashing for you when you try to use it -- you have to be hard-core enough to debug the problem yourself.


Please base your work off the unstable branch. After making your changes please re-run rake to ensure it compiles cleanly and the tests pass. Then submit your Pull Request.


  • Test the entire matrix, not just the current ARCH that gcc defaults to:
gcc ppc
clang ppc
gcc i386
clang i386
gcc x86_64
clang x86_64

Obviously ppc can only be tested on ppc Macs or on 10.5 and 10.6 (Rosetta-supporting) Intel Macs.

Version History

mach_override 1.2: Aug 9 2012

  • [NEW] i386 and x86_64 support.

  • [CHANGE] Decoupled from mach_star. Most of folks were just using the side or the other of mach_star and this simplifies things (docs, tests).

  • [CHANGE] Threw away Xcode projects. Trying to get them to work across new and old system versions and Xcode versions is a fool's game. Now there's a Rakefile. Type rake. You've built and tested. Ta-da.

mach_star 1.1.1: Dec 18 2005

  • General Xcode 2.2 project cleanup. mach_star now includes .xcodeproj Xcode 2.2 project files for all of its projects. The old .xcode project files have been left in place, but they aren't maintained and may not work. Xcode 2.2 is the recommended mach_star development environment -- Xcode 2.1 had a bug with inter-project dependancies which would cause compilation failure. It works now again in Xcode 2.2.

  • Inter-project dependancies should working under Xcode 2.2. Any project you pick, you should just be able to hit the "Build" button and everything should Just Work™.

  • There was a stray reference to my username in one of the project, which causes compilation headaches for some folks.

  • Bug fix: in mach_inject_bundle.c's mach_inject_bundle_pid() I no longer call CFRelease() on the framework bundle reference. Reported by Scott Kevill.

  • Added some explicit casts now required by gcc 4.

  • Added this document.

mach_star 1.1: Apr 06 2005

  • New package added: mach_inject_bundle. It has a private subproject: mach_inject_bundle_stub. The stub is a generic reusable implementation of the code that gets squirted across the address spaces, which was always tricky to write. mach_inject_bundle is an embeddable framework that wraps mach_inject and the stub with a simple fire-and-forget API.

  • The "DisposeWindowBeeperOverride" example is replaced by "DisposeWindow+Beep".

  • The "FinderDisposeWindowBeeperInjector" is replaced by "DisposeWindow+Beep_Injector".

  • All the text is now wrapped to 80 chars wide. Done to print nicely in Scott Knaster's Hacking Mac OS X Tiger. Probably will undo this word-wrap in the future. We all have widescreens nowadays, right? ;-)

  • Thanks to Jon Gotow for letting me peek at SCPatch, which I used as a guide for mach_inject_bundle. It saved me a bunch of time. Also thanks to Bob Ippolito for CALL_ON_LOAD assistance.

mach_star 1.0: Jun 18 2003

  • Initial release at MacHack 2003.