osx - Mac OS X pthreads fake return address -


in os x pthreads implementation (http://www.opensource.apple.com/source/libc/libc-825.26/pthreads/thread_setup.c?txt) provide fake return address on thread stack (line 140):

    ts->rip = (uintptr_t) routine;      /*     ** need simulate 16-byte aligned stack frame if had     ** executed call instruction. stack should aligned     ** before comes , don't need push arguments,     ** shouldn't need change it.     */      ts->rdi = (uintptr_t) thread;   /* argument function */     *--sp = 0;            /* fake return address */     ts->rsp = (uintptr_t) sp;   /* set stack pointer */

i not understand how not crash illegal instruction/segfault when function thread executing calls 'ret' , pops return address stack. can explain how prevented/handled?

without looking @ rest of code, can venture guess. intuition says, called thread procedure (the user-supplied start_routine parameter) should never return calling function.

think it: if new thread did return, have 2 threads running on same original code path. imagine thread function actually called wrapper calls user-supplied start_routine. when start_routine returns, wrapper calls pthread_exit.

(main thread)      v pthread_create      v thread_setup  (sets stack), , spawns new thread      v                                     | return main thread                      |                                            |                                            |                                            v                                       wrapper_function                                            v                     user-supplied    start_routine                                            |   (returns)                                            v                                 wrapper_function  calls                                            v                                      pthread_exit

again, guess, whole point is, new thread should never return code called pthread_create. purpose of wrapper ensure pthread_exit gets called.

i have see passing routine thread_setup.

my feelings confirmed fact you don't have call pthread_exit.


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