This is xnu-8019. See this file in:
Directory listing for xnu-8019//libkern/libkern
Block.h
Block_private.h
Makefile
OSAtomic.h
OSBase.h
OSByteOrder.h
OSCrossEndian.h
OSDebug.h
OSKextLib.h
OSKextLibPrivate.h
OSMalloc.h
OSReturn.h
OSRuntime.h
OSSerializeBinary.h
OSTypes.h
_OSByteOrder.h
amfi/
apple_encrypted_archive/
arm/
c++/
coreanalytics/
coretrust/
crc.h
crypto/
i386/
img4/
kernel_mach_header.h
kext_panic_report.h
kext_request_keys.h
kxld.h
kxld_types.h
locks.h
machine/
mkext.h
prelink.h
ptrauth_utils.h
section_keywords.h
stack_protector.h
sysctl.h
tree.h
version.h.template
zconf.h
zlib.h