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// Copyright 2011 Evan Shaw. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This file defines the common package interface and contains a little bit of
// factored out logic.
// Package mmap allows mapping files into memory. It tries to provide a simple, reasonably portable interface,
// but doesn't go out of its way to abstract away every little platform detail.
// This specifically means:
// * forked processes may or may not inherit mappings
// * a file's timestamp may or may not be updated by writes through mappings
// * specifying a size larger than the file's actual size can increase the file's size
// * If the mapped file is being modified by another process while your program's running, don't expect consistent results between platforms
package mmap
import (
"errors"
"os"
"reflect"
"unsafe"
)
const (
// RDONLY maps the memory read-only.
// Attempts to write to the MMap object will result in undefined behavior.
RDONLY = 0
// RDWR maps the memory as read-write. Writes to the MMap object will update the
// underlying file.
RDWR = 1 << iota
// COPY maps the memory as copy-on-write. Writes to the MMap object will affect
// memory, but the underlying file will remain unchanged.
COPY
// If EXEC is set, the mapped memory is marked as executable.
EXEC
)
const (
// If the ANON flag is set, the mapped memory will not be backed by a file.
ANON = 1 << iota
)
// MMap represents a file mapped into memory.
type MMap []byte
// Map maps an entire file into memory.
// If ANON is set in flags, f is ignored.
func Map(f *os.File, prot, flags int) (MMap, error) {
return MapRegion(f, -1, prot, flags, 0)
}
// MapRegion maps part of a file into memory.
// The offset parameter must be a multiple of the system's page size.
// If length < 0, the entire file will be mapped.
// If ANON is set in flags, f is ignored.
func MapRegion(f *os.File, length int, prot, flags int, offset int64) (MMap, error) {
var fd uintptr
if flags&ANON == 0 {
fd = uintptr(f.Fd())
if length < 0 {
fi, err := f.Stat()
if err != nil {
return nil, err
}
length = int(fi.Size())
}
} else {
if length <= 0 {
return nil, errors.New("anonymous mapping requires non-zero length")
}
fd = ^uintptr(0)
}
return mmap(length, uintptr(prot), uintptr(flags), fd, offset)
}
func (m *MMap) header() *reflect.SliceHeader {
return (*reflect.SliceHeader)(unsafe.Pointer(m))
}
// Lock keeps the mapped region in physical memory, ensuring that it will not be
// swapped out.
func (m MMap) Lock() error {
dh := m.header()
return lock(dh.Data, uintptr(dh.Len))
}
// Unlock reverses the effect of Lock, allowing the mapped region to potentially
// be swapped out.
// If m is already unlocked, aan error will result.
func (m MMap) Unlock() error {
dh := m.header()
return unlock(dh.Data, uintptr(dh.Len))
}
// Flush synchronizes the mapping's contents to the file's contents on disk.
func (m MMap) Flush() error {
dh := m.header()
return flush(dh.Data, uintptr(dh.Len))
}
// Unmap deletes the memory mapped region, flushes any remaining changes, and sets
// m to nil.
// Trying to read or write any remaining references to m after Unmap is called will
// result in undefined behavior.
// Unmap should only be called on the slice value that was originally returned from
// a call to Map. Calling Unmap on a derived slice may cause errors.
func (m *MMap) Unmap() error {
dh := m.header()
err := unmap(dh.Data, uintptr(dh.Len))
*m = nil
return err
}
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