Added an option to use libc backtrace function from execinfo.h

[ivanarh]

• Added new definition BOOST_STACKTRACE_USE_LIBC_BACKTRACE_FUNCTION
• Set on iOS 32-bit platforms. See stacktrace does not work on iOS armv7/armv7s #46

[coveralls]

Coverage increased (+0.02%) to 90.265% when pulling 839a1a1 on ivanarh:use_execinfo_backtrace into d708d17 on boostorg:develop.

[apolukhin]

Many thanks for the PR!

I'm worried about async signal safety of backtrace() on Ios. Are there any guarantees?

[ivanarh]

I'm worried about async signal safety of backtrace() on Ios. Are there any guarantees?

It may depend on libc implementation and (or) on platform. For example, let's take a look to it's source in glibc:
https://github.com/lattera/glibc/blob/master/debug/backtrace.c
It's a simple wrapper around _Unwind_Backtrace. But it's loaded by dlopen/dlsym calls. So if backtrace() function is called for a first time from signal handler it's may be unsafe. But a workaround may be used: backtrace() should be called once before any signal handlers.

What about apple libc implementation, it's completely different:
https://opensource.apple.com/source/Libc/Libc-1272.200.26/gen/backtrace.c.auto.html
https://opensource.apple.com/source/Libc/Libc-1272.200.26/gen/thread_stack_pcs.c.auto.html
It relies on Apple ABI where an old frame pointer is saved to the stack. All it does is jumping from current frame pointer to previous in a cycle. But couple of external calls exist:

pthread_t self = pthread_self();
void *stacktop = pthread_get_stackaddr_np(self);
void *stackbot = stacktop - pthread_get_stacksize_np(self);

pthread_self is signal safe, pthread_get_stackaddr_np is not properly documented.
https://opensource.apple.com/source/libpthread/libpthread-218.60.3/src/pthread.c.auto.html
I see some explicit locks in its source code:

void *
pthread_get_stackaddr_np(pthread_t t)
{
	int ret;
	void *addr = NULL;

	if (t == NULL) {
		return (void *)(uintptr_t)ESRCH; // XXX bug?
	}
	
	// since the main thread will not get de-allocated from underneath us
	if (t == pthread_self() || t == &_thread) {
		return t->stackaddr;
	}

	_PTHREAD_LOCK(_pthread_list_lock);

So it may be unsafe. But this code isn't reached when we work with a current thread.

Therefore, signal safety of backtrace function is an arguable question.

[apolukhin]

Looks safe to me. Many thanks!