Date: Mon, 23 Jun 2014 18:32:22 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: getcontext etc was Re: Re: go support (was: Best place to discuss other lightweight libraries?) On Mon, Jun 23, 2014 at 10:09:29PM +0100, Justin Cormack wrote: > On Wed, Apr 24, 2013 at 2:42 PM, Rich Felker <dalias@...ifal.cx> wrote: > >> i tried to build gcc 4.7.2 with go support (--enable-languages=c,c++,go) > >> and that fails due to a lack of set/getcontext(). > >> (see pkg/gcc472 in sabotage) > >> > >> according to rich, adding that to musl requires a non-trivial amount > >> of arch specific asm. > > > > Yes, but it is a wanted feature, so I wouldn't mind it getting done. > > It was even part of the standard prior to POSIX 2008, and the reason > > for removing it was stupid. (The reason was that the makecontext > > function's calling convention is bogus and impossible to support > > properly, but they could have fixed this by deprecating the use of the > > variadic arguments in any way except passing a single void* argument, > > rather than deprecating the whole set of interfaces.) > > Just to revive this post 1.0, it would be very nice indeed to have > set,get,swapcontext. One approach to setcontext would be using rt_sigreturn, but I've heard there are issues using it for setcontext involving the alternate signal stack. It's the only way to do resuming _async_ (i.e. the ucontext received by a signal handler for the state it interrupted) contexts though, so maybe there's some way to make it work. This issue was discussed recently on libc-alpha (the glibc list). Otherwise, if async contexts don't need to be supported, these functions are essentially just mechanical transformations of sigsetjmp/siglongjmp. I think setcontext could be implemented just as a transformation of the mcontext_t register values into a jmp_buf, followed by a call to siglongjmp. For getcontext, you can't just wrap sigsetjmp since wrapping returns-twice functions is impossible. But maybe there's some way to make it work without code duplication. Rich
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