It seems that while I was off using an old (but stable) version of Boost.Context Oliver has gone and rewritten it (again) and it has been approved (awesome)! I decided I should migrate my code to the latest API but it appears that no one tested it on OS X. I had to update the fcontext_x86_64_sysv_macho_gas.S: 1) @PLT is an ELF concept that generates the error: junk `@PLT' after expression 2) I had to add an '_' before align_stack otherwise things didn't link. + call _align_stack /* align stack */ - call align_stack@PLT /* align stack */ - call _exit@PLT /* exit application */ + call _exit /* exit application */ With that fix, you can add OS X x64 to your tested platforms. What version of boost will include Context? Dan