Source file src/pkg/cmd/compile/internal/gc/mapfile_mmap.go

     1	// Copyright 2018 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	// +build darwin dragonfly freebsd linux netbsd openbsd
     6	
     7	package gc
     8	
     9	import (
    10		"os"
    11		"reflect"
    12		"syscall"
    13		"unsafe"
    14	)
    15	
    16	// TODO(mdempsky): Is there a higher-level abstraction that still
    17	// works well for iimport?
    18	
    19	// mapFile returns length bytes from the file starting at the
    20	// specified offset as a string.
    21	func mapFile(f *os.File, offset, length int64) (string, error) {
    22		// POSIX mmap: "The implementation may require that off is a
    23		// multiple of the page size."
    24		x := offset & int64(os.Getpagesize()-1)
    25		offset -= x
    26		length += x
    27	
    28		buf, err := syscall.Mmap(int(f.Fd()), offset, int(length), syscall.PROT_READ, syscall.MAP_SHARED)
    29		keepAlive(f)
    30		if err != nil {
    31			return "", err
    32		}
    33	
    34		buf = buf[x:]
    35		pSlice := (*reflect.SliceHeader)(unsafe.Pointer(&buf))
    36	
    37		var res string
    38		pString := (*reflect.StringHeader)(unsafe.Pointer(&res))
    39	
    40		pString.Data = pSlice.Data
    41		pString.Len = pSlice.Len
    42	
    43		return res, nil
    44	}
    45	
    46	// keepAlive is a reimplementation of runtime.KeepAlive, which wasn't
    47	// added until Go 1.7, whereas we need to compile with Go 1.4.
    48	var keepAlive = func(interface{}) {}
    49	

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