From 21b522df4167f57195f6b4b40a49632905cc7fbb Mon Sep 17 00:00:00 2001
From: jdl <johnl@crumpgington.com>
Date: Wed, 31 Mar 2021 10:33:16 +0200
Subject: [PATCH] Added mmap.

---
 mmap/mmap.go | 95 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)
 create mode 100644 mmap/mmap.go

diff --git a/mmap/mmap.go b/mmap/mmap.go
new file mode 100644
index 0000000..b14e753
--- /dev/null
+++ b/mmap/mmap.go
@@ -0,0 +1,95 @@
+package mt
+
+import (
+	"fmt"
+	"os"
+	"reflect"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+// Map creates a memory map of the given file.
+func Map(
+	prot int, // Memory protection bitmask. See syscall.PROT_* flags.
+	flags int, // See syscall.MAP_* flags. MAP_SHARED is usually fine.
+	length int64,
+	f *os.File,
+) (
+	[]byte,
+	error,
+) {
+
+	addr, _, errno := syscall.Syscall6(
+		syscall.SYS_MMAP,
+		0, // addr: 0 => allow kernel to choose
+		uintptr(length),
+		uintptr(prot),
+		uintptr(flags),
+		f.Fd(),
+		0) // offset: 0 => start of file
+	if errno != 0 {
+		return nil, syscall.Errno(errno)
+	}
+
+	var b []byte
+	hdr := (*reflect.SliceHeader)(unsafe.Pointer(&b))
+	hdr.Data = addr
+	hdr.Cap = int(length)
+	hdr.Len = 0
+
+	return b, nil
+}
+
+// Unmap unmaps the data obtained by Map.
+func Unmap(data []byte) error {
+	_, _, errno := syscall.Syscall(
+		syscall.SYS_MUNMAP,
+		uintptr(unsafe.Pointer(&data[:1][0])),
+		uintptr(cap(data)),
+		0)
+	if errno != 0 {
+		return syscall.Errno(errno)
+	}
+	return nil
+}
+
+// Remap truncates the file to `newSize` and returns a new memory map for the
+// file.
+func Remap(
+	f *os.File,
+	data []byte,
+	newSize int64,
+) ([]byte, error) {
+	const mRemapMayMove = 0x1
+
+	if err := f.Truncate(newSize); err != nil {
+		return data, err
+	}
+
+	addr, size, errno := unix.Syscall6(
+		unix.SYS_MREMAP,
+		uintptr(unsafe.Pointer(&data[:1][0])),
+		uintptr(cap(data)),
+		uintptr(newSize),
+		uintptr(mRemapMayMove),
+		0,
+		0)
+
+	if errno != 0 {
+		return data, syscall.Errno(errno)
+	}
+
+	hdr := (*reflect.SliceHeader)(unsafe.Pointer(&data))
+	hdr.Data = addr
+	hdr.Cap = int(newSize)
+	hdr.Len = int(newSize)
+
+	if int64(size) != newSize {
+		panic(fmt.Sprintf("mremap returned incorrect size %d != %d",
+			size, newSize))
+	}
+
+	return data, nil
+}