return b, nil
}

func (m *mmapper) Munmap(data []byte) (err error) {
	if len(data) == 0 || len(data) != cap(data) {
		return EINVAL
	}

	// Find the base of the mapping.
	p := &data[cap(data)-1]
	m.Lock()
	defer m.Unlock()
	b := m.active[p]
	if b == nil || &b[0] != &data[0] {
		return EINVAL
	}

	// Unmap the memory and update m.

	if r == ^uintptr(0) {
		return nil, int(err0)
	}
	return unsafe.Pointer(r), int(err0)
}

//go:nosplit
func munmap(addr unsafe.Pointer, n uintptr) {
	r, err := syscall2(&libc_munmap, uintptr(addr), uintptr(n))
	if int32(r) == -1 {
		println("syscall munmap failed: ", hex(err))
		throw("syscall munmap")
	}
}

//go:nosplit
func madvise(addr unsafe.Pointer, n uintptr, flags int32) {

//   - Write the content
//   - possibly apply any edits in the output buffer
//   - possibly write more content to the file. These writes take place in a heap
//     backed buffer that will get synced to disk.
//   - Munmap the output file
//
// And finally, it provides a mechanism by which you can multithread the
// writing of output files. This mechanism is accomplished by copying a OutBuf,
// and using it in the thread/goroutine.
//

}

// Don't split the stack as this function may be invoked without a valid G,
// which prevents us from allocating more stack.
//
//go:nosplit
func sysFreeOS(v unsafe.Pointer, n uintptr) {
	munmap(v, n)
}

func sysFaultOS(v unsafe.Pointer, n uintptr) {
	mprotect(v, n, _PROT_NONE)
	madvise(v, n, _MADV_DONTNEED)
}

func sysReserveOS(v unsafe.Pointer, n uintptr) unsafe.Pointer {

		// bump start to next page
		start = round(limit, pageSize)
	}
	return start, start + n
}

var memmap = mmap.Mmap
var munmap = mmap.Munmap

func (m *mappedFile) close() {
	m.closeOnce.Do(func() {
		if m.mapping != nil {
			munmap(m.mapping)
			m.mapping = nil
		}
		if m.f != nil {
			m.f.Close() // best effort
			m.f = nil
		}
	})
}

//go:cgo_import_dynamic libc_kill kill "libc.so"
//go:cgo_import_dynamic libc_madvise madvise "libc.so"
//go:cgo_import_dynamic libc_malloc malloc "libc.so"
//go:cgo_import_dynamic libc_mmap mmap "libc.so"
//go:cgo_import_dynamic libc_munmap munmap "libc.so"
//go:cgo_import_dynamic libc_open open "libc.so"
//go:cgo_import_dynamic libc_pthread_attr_destroy pthread_attr_destroy "libc.so"
//go:cgo_import_dynamic libc_pthread_attr_getstack pthread_attr_getstack "libc.so"

//sys	munmap(addr uintptr, length uintptr) (err error)

var mapper = &mmapper{
	active: make(map[*byte][]byte),
	mmap:   mmap,
	munmap: munmap,
}

func Mmap(fd int, offset int64, length int, prot int, flags int) (data []byte, err error) {
	return mapper.Mmap(fd, offset, length, prot, flags)
}

func Munmap(b []byte) (err error) {
	return mapper.Munmap(b)
}

//sys	write(fd int, p []byte) (n int, err error)
//sys	writev(fd int, iovecs []Iovec) (n uintptr, err error)
//sys	mmap(addr uintptr, length uintptr, prot int, flag int, fd int, pos int64) (ret uintptr, err error)
//sys	munmap(addr uintptr, length uintptr) (err error)
//sys	getfsstat(stat *Statfs_t, bufsize uintptr, flags int) (n int, err error)

//sys	Unmount(path string, flags int) (err error)
//sys	write(fd int, p []byte) (n int, err error)
//sys	mmap(addr uintptr, length uintptr, prot int, flag int, fd int, pos int64) (ret uintptr, err error)
//sys	munmap(addr uintptr, length uintptr) (err error)
//sys	readlen(fd int, buf *byte, nbuf int) (n int, err error) = SYS_READ
//sys	utimensat(dirfd int, path string, times *[2]Timespec, flag int) (err error)

	libcCall(unsafe.Pointer(abi.FuncPCABI0(mmap_trampoline)), unsafe.Pointer(&args))
	return args.ret1, args.ret2
}
func mmap_trampoline()

//go:nosplit
//go:cgo_unsafe_args
func munmap(addr unsafe.Pointer, n uintptr) {
	libcCall(unsafe.Pointer(abi.FuncPCABI0(munmap_trampoline)), unsafe.Pointer(&addr))
	KeepAlive(addr) // Just for consistency. Hopefully addr is not a Go address.
}
func munmap_trampoline()