Since OpenBSD 6.4 (https://www.openbsd.org/64.html), the stack pointer
must point to MAP_STACK memory, or the kernel may kill the process
with a signal. All stack allocators must pass MAP_STACK to mmap(2).
Define BOOST_CONTEXT_USE_MAP_STACK on OpenBSD, and don't define it on
other systems. This doesn't check for old versions of OpenBSD without
MAP_STACK; but OpenBSD has stopped maintaining versions before 6.4.
If BOOST_CONTEXT_USE_MAP_STACK is defined, then cause the stack
allocators to pass MAP_STACK to mmap(2):
- fixedsize_stack uses mmap/munmap instead of malloc/free. This
comes from a patch in OpenBSD Ports.
- protected_fixedsize_stack adds MAP_STACK to the mmap flags (as it
does in OpenBSD Ports). Assume that systems with MAP_STACK also
have MAP_ANON; this is true on OpenBSD. Delete POSIX comment,
because I can't find MAP_ANON nor MAP_ANONYMOUS in POSIX, so these
mmap calls don't conform to POSIX.
- pooled_fixedsize_stack can't call munmap, because the pool's free
doesn't know the allocation's size. Instead use posix_memalign to
allocate memory, then mmap to replace the pages with MAP_STACK
pages, so the pool's free may call std::free.
OpenBSD has no <ucontext.h>, so edit test/Jamfile.v2 to skip ucontext
tests on OpenBSD.
This commit and https://github.com/boostorg/test/pull/231 causes
libs/context/test `b2 full` to pass on OpenBSD 6.5 for 64-bit x86.
`b2 fc` fails because the allocator in test_fcontext.cpp does not use
MAP_STACK. The tests seem not to cover pooled_fixedsize_stack nor
protected_fixedsize_stack, but they still pass when I temporarily hack
callcc to use those allocators instead of fixedsize_stack.
