Low-level lecture
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32
lectures/09-lowlevel/bzip/bzip2.c
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32
lectures/09-lowlevel/bzip/bzip2.c
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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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// See page 362.
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//
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// The version of this program that appeared in the first and second
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// printings did not comply with the proposed rules for passing
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// pointers between Go and C, described here:
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// https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md
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//
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// The version below, which appears in the third printing,
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// has been corrected. See bzip2.go for explanation.
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//!+
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/* This file is gopl.io/ch13/bzip/bzip2.c, */
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/* a simple wrapper for libbzip2 suitable for cgo. */
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#include <bzlib.h>
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int bz2compress(bz_stream *s, int action,
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char *in, unsigned *inlen, char *out, unsigned *outlen) {
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s->next_in = in;
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s->avail_in = *inlen;
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s->next_out = out;
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s->avail_out = *outlen;
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int r = BZ2_bzCompress(s, action);
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*inlen -= s->avail_in;
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*outlen -= s->avail_out;
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s->next_in = s->next_out = NULL;
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return r;
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}
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//!-
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111
lectures/09-lowlevel/bzip/bzip2.go
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lectures/09-lowlevel/bzip/bzip2.go
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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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// See page 362.
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//
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// The version of this program that appeared in the first and second
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// printings did not comply with the proposed rules for passing
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// pointers between Go and C, described here:
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// https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md
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//
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// The rules forbid a C function like bz2compress from storing 'in'
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// and 'out' (pointers to variables allocated by Go) into the Go
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// variable 's', even temporarily.
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//
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// The version below, which appears in the third printing, has been
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// corrected. To comply with the rules, the bz_stream variable must
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// be allocated by C code. We have introduced two C functions,
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// bz2alloc and bz2free, to allocate and free instances of the
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// bz_stream type. Also, we have changed bz2compress so that before
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// it returns, it clears the fields of the bz_stream that contain
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// pointers to Go variables.
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//!+
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// Package bzip provides a writer that uses bzip2 compression (bzip.org).
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package bzip
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/*
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#cgo CFLAGS: -I/usr/include
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#cgo LDFLAGS: -L/usr/lib -lbz2
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#include <bzlib.h>
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#include <stdlib.h>
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bz_stream* bz2alloc() { return calloc(1, sizeof(bz_stream)); }
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int bz2compress(bz_stream *s, int action,
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char *in, unsigned *inlen, char *out, unsigned *outlen);
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void bz2free(bz_stream* s) { free(s); }
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*/
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import "C"
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import (
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"io"
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"unsafe"
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)
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type writer struct {
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w io.Writer // underlying output stream
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stream *C.bz_stream
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outbuf [64 * 1024]byte
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}
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// NewWriter returns a writer for bzip2-compressed streams.
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func NewWriter(out io.Writer) io.WriteCloser {
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const blockSize = 9
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const verbosity = 0
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const workFactor = 30
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w := &writer{w: out, stream: C.bz2alloc()}
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C.BZ2_bzCompressInit(w.stream, blockSize, verbosity, workFactor)
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return w
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}
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//!-
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//!+write
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func (w *writer) Write(data []byte) (int, error) {
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if w.stream == nil {
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panic("closed")
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}
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var total int // uncompressed bytes written
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for len(data) > 0 {
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inlen, outlen := C.uint(len(data)), C.uint(cap(w.outbuf))
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C.bz2compress(w.stream, C.BZ_RUN,
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(*C.char)(unsafe.Pointer(&data[0])), &inlen,
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(*C.char)(unsafe.Pointer(&w.outbuf)), &outlen)
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total += int(inlen)
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data = data[inlen:]
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if _, err := w.w.Write(w.outbuf[:outlen]); err != nil {
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return total, err
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}
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}
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return total, nil
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}
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//!-write
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//!+close
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// Close flushes the compressed data and closes the stream.
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// It does not close the underlying io.Writer.
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func (w *writer) Close() error {
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if w.stream == nil {
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panic("closed")
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}
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defer func() {
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C.BZ2_bzCompressEnd(w.stream)
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C.bz2free(w.stream)
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w.stream = nil
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}()
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for {
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inlen, outlen := C.uint(0), C.uint(cap(w.outbuf))
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r := C.bz2compress(w.stream, C.BZ_FINISH, nil, &inlen,
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(*C.char)(unsafe.Pointer(&w.outbuf)), &outlen)
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if _, err := w.w.Write(w.outbuf[:outlen]); err != nil {
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return err
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}
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if r == C.BZ_STREAM_END {
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return nil
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}
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}
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}
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//!-close
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40
lectures/09-lowlevel/bzip/bzip2_test.go
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40
lectures/09-lowlevel/bzip/bzip2_test.go
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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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package bzip_test
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import (
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"bytes"
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"compress/bzip2" // reader
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"io"
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"testing"
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"gopl.io/ch13/bzip" // writer
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)
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func TestBzip2(t *testing.T) {
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var compressed, uncompressed bytes.Buffer
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w := bzip.NewWriter(&compressed)
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// Write a repetitive message in a million pieces,
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// compressing one copy but not the other.
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tee := io.MultiWriter(w, &uncompressed)
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for i := 0; i < 1000000; i++ {
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io.WriteString(tee, "hello")
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}
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if err := w.Close(); err != nil {
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t.Fatal(err)
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}
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// Check the size of the compressed stream.
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if got, want := compressed.Len(), 255; got != want {
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t.Errorf("1 million hellos compressed to %d bytes, want %d", got, want)
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}
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// Decompress and compare with original.
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var decompressed bytes.Buffer
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io.Copy(&decompressed, bzip2.NewReader(&compressed))
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if !bytes.Equal(uncompressed.Bytes(), decompressed.Bytes()) {
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t.Error("decompression yielded a different message")
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}
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}
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84
lectures/09-lowlevel/lecture.slide
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84
lectures/09-lowlevel/lecture.slide
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low-level
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Лекция 8
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Короткий Фёдор
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* Low Level Programming
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* package unsafe
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import "unsafe"
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fmt.Println(unsafe.Sizeof(float64(0))) // "8"
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- `unsafe` выглядит как обычный пакет
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- Но реализован в компиляторе
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* package unsafe
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type Pointer *ArbitraryType
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- Два преобразования
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*T -> unsafe.Pointer
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unsafe.Pointer -> *T
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- `unsafe.Pointer` - это _настоящий_ `void*`
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func Float64bits(f float64) uint64 {
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return *(*uint64)(unsafe.Pointer(&f))
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}
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* unsafe.Pointer
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.code unsafeptr/main.go /var/,/42/
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Действия с указателями дожны выполняться *атомарно*.
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.code unsafeptr/main.go /\+wrong/,/\-wrong/
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Иначе сборщик мусора освободит память используемого объекта.
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pT := uintptr(unsafe.Pointer(new(T))) // NOTE: wrong!
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* unsafe tricks
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// A Builder is used to efficiently build a string using Write methods.
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// It minimizes memory copying. The zero value is ready to use.
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// Do not copy a non-zero Builder.
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type Builder struct {
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buf []byte
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}
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// String returns the accumulated string.
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func (b *Builder) String() string {
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return *(*string)(unsafe.Pointer(&b.buf))
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}
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// *-----*-----*-----*
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// * ptr * len * cap * // []byte
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// *-----*-----*-----*
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//
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// *-----*-----*
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// * ptr * len * // string
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// *-----*-----*
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* cgo
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.code bzip/bzip2.c /include/,/^}/
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* cgo
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.code bzip/bzip2.go /package/,/import/
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.code bzip/bzip2.go /type writer/,/^}/
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* cgo
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.code bzip/bzip2.go /type writer/,/^}/
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.code bzip/bzip2.go /func NewWriter/,/^}/
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* cgo
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.code bzip/bzip2.go /func .* Write/,/^}/
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36
lectures/09-lowlevel/unsafeptr/main.go
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36
lectures/09-lowlevel/unsafeptr/main.go
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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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// See page 357.
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// Package unsafeptr demonstrates basic use of unsafe.Pointer.
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package main
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import (
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"fmt"
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"unsafe"
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)
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func main() {
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var x struct {
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a bool
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b int16
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c []int
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}
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// equivalent to pb := &x.b
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pb := (*int16)(unsafe.Pointer(
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uintptr(unsafe.Pointer(&x)) + unsafe.Offsetof(x.b)))
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*pb = 42
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fmt.Println(x.b) // "42"
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}
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/*
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//!+wrong OMIT
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// NOTE: subtly incorrect!
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tmp := uintptr(unsafe.Pointer(&x)) + unsafe.Offsetof(x.b)
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pb := (*int16)(unsafe.Pointer(tmp))
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*pb = 42
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//!-wrong OMIT
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*/
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