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