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< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-06.html" > < strong aria-hidd
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2022-08-24 10:48:15 +00:00
< li > < em > KusonStack一站式可编程配置技术栈(Go): < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / a > < / em > < / li >
< li > < em > KCL 配置编程语言(Rust): < a href = "https://github.com/KusionStack/KCLVM" > https://github.com/KusionStack/KCLVM< / a > < / em > < / li >
< li > < em > 凹语言™: < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2022-08-04 06:59:33 +00:00
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< hr >
< h2 id = "134-通过cgo调用c代码" > < a class = "header" href = "#134-通过cgo调用c代码" > 13.4. 通过cgo调用C代码< / a > < / h2 >
< p > Go程序可能会遇到要访问C语言的某些硬件驱动函数的场景， ， ， ， （ ： ） < / p >
< p > 在本节中， ， < em > foreign-function interfaces< / em > （ ） ， （ < a href = "http://swig.org" > http://swig.org< / a > ） ， ， < / p >
< p > 在标准库的< code > compress/...< / code > 子包有很多流行的压缩算法的编码和解码实现， （ ） （ ） ， ： < / p >
< pre > < code class = "language-Go" > package gzip // compress/gzip
func NewWriter(w io.Writer) io.WriteCloser
func NewReader(r io.Reader) (io.ReadCloser, error)
< / code > < / pre >
< p > bzip2压缩算法， ， ， ， ， < / p >
< p > 如果是比较小的C语言库， ， ， （ ： ） < / p >
< p > 译注： ， ， ， ： < / p >
< pre > < code class = "language-Go" > // The version of this program that appeared in the first and second
// printings did not comply with the proposed rules for passing
// pointers between Go and C, described here:
// https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md
//
// The rules forbid a C function like bz2compress from storing 'in'
// and 'out' (pointers to variables allocated by Go) into the Go
// variable 's', even temporarily.
//
// The version below, which appears in the third printing, has been
// corrected. To comply with the rules, the bz_stream variable must
// be allocated by C code. We have introduced two C functions,
// bz2alloc and bz2free, to allocate and free instances of the
// bz_stream type. Also, we have changed bz2compress so that before
// it returns, it clears the fields of the bz_stream that contain
// pointers to Go variables.
< / code > < / pre >
< p > 要使用libbzip2， ， ： ， ， （ ， ） < / p >
< p > 我们可以在Go代码中直接调用BZ2_bzCompressInit和BZ2_bzCompressEnd， ， ， ， < / p >
< p > < u > < i > gopl.io/ch13/bzip< / i > < / u > < / p >
< pre > < code class = "language-C" > /* This file is gopl.io/ch13/bzip/bzip2.c, */
/* a simple wrapper for libbzip2 suitable for cgo. */
#include < bzlib.h>
int bz2compress(bz_stream *s, int action,
char *in, unsigned *inlen, char *out, unsigned *outlen) {
s-> next_in = in;
s-> avail_in = *inlen;
s-> next_out = out;
s-> avail_out = *outlen;
int r = BZ2_bzCompress(s, action);
*inlen -= s-> avail_in;
*outlen -= s-> avail_out;
s-> next_in = s-> next_out = NULL;
return r;
}
< / code > < / pre >
< p > 现在让我们转到Go语言部分， < code > import " C" < / code > 的语句是比较特别的。其实并没有一个叫C的包， < / p >
< pre > < code class = "language-Go" > // Package bzip provides a writer that uses bzip2 compression (bzip.org).
package bzip
/*
#cgo CFLAGS: -I/usr/include
#cgo LDFLAGS: -L/usr/lib -lbz2
#include < bzlib.h>
#include < stdlib.h>
bz_stream* bz2alloc() { return calloc(1, sizeof(bz_stream)); }
int bz2compress(bz_stream *s, int action,
char *in, unsigned *inlen, char *out, unsigned *outlen);
void bz2free(bz_stream* s) { free(s); }
*/
import " C"
import (
" io"
" unsafe"
)
type writer struct {
w io.Writer // underlying output stream
stream *C.bz_stream
outbuf [64 * 1024]byte
}
// NewWriter returns a writer for bzip2-compressed streams.
func NewWriter(out io.Writer) io.WriteCloser {
const blockSize = 9
const verbosity = 0
const workFactor = 30
w := & writer{w: out, stream: C.bz2alloc()}
C.BZ2_bzCompressInit(w.stream, blockSize, verbosity, workFactor)
return w
}
< / code > < / pre >
< p > 在预处理过程中， （ ： < code > import " C" < / code > 语句前紧挨着的注释是对应cgo的特殊语法， ） < / p >
< p > 在cgo注释中还可以包含#cgo指令， ， ， （ ： ， ， < / p >
< p > NewWriter函数通过调用C语言的BZ2_bzCompressInit函数来初始化stream中的缓存。在writer结构中还包括了另一个buffer， < / p >
< p > 下面是Write方法的实现， ， ， ， ， ， ， < / p >
< pre > < code class = "language-Go" > func (w *writer) Write(data []byte) (int, error) {
if w.stream == nil {
panic(" closed" )
}
var total int // uncompressed bytes written
for len(data) > 0 {
inlen, outlen := C.uint(len(data)), C.uint(cap(w.outbuf))
C.bz2compress(w.stream, C.BZ_RUN,
(*C.char)(unsafe.Pointer(& data[0])), & inlen,
(*C.char)(unsafe.Pointer(& w.outbuf)), & outlen)
total += int(inlen)
data = data[inlen:]
if _, err := w.w.Write(w.outbuf[:outlen]); err != nil {
return total, err
}
}
return total, nil
}
< / code > < / pre >
< p > 在循环的每次迭代中， ， ， < / p >
< p > Close方法和Write方法有着类似的结构， < / p >
< pre > < code class = "language-Go" > // Close flushes the compressed data and closes the stream.
// It does not close the underlying io.Writer.
func (w *writer) Close() error {
if w.stream == nil {
panic(" closed" )
}
defer func() {
C.BZ2_bzCompressEnd(w.stream)
C.bz2free(w.stream)
w.stream = nil
}()
for {
inlen, outlen := C.uint(0), C.uint(cap(w.outbuf))
r := C.bz2compress(w.stream, C.BZ_FINISH, nil, & inlen,
(*C.char)(unsafe.Pointer(& w.outbuf)), & outlen)
if _, err := w.w.Write(w.outbuf[:outlen]); err != nil {
return err
}
if r == C.BZ_STREAM_END {
return nil
}
}
}
< / code > < / pre >
< p > 压缩完成后， ， ， ， < / p >
< p > 上面的实现中， ， < / p >
< p > 下面的bzipper程序， < / p >
< p > < u > < i > gopl.io/ch13/bzipper< / i > < / u > < / p >
< pre > < code class = "language-Go" > // Bzipper reads input, bzip2-compresses it, and writes it out.
package main
import (
" io"
" log"
" os"
" gopl.io/ch13/bzip"
)
func main() {
w := bzip.NewWriter(os.Stdout)
if _, err := io.Copy(w, os.Stdin); err != nil {
log.Fatalf(" bzipper: %v\n" , err)
}
if err := w.Close(); err != nil {
log.Fatalf(" bzipper: close: %v\n" , err)
}
}
< / code > < / pre >
< p > 在上面的场景中， ， ， （ ， < / p >
< pre > < code > $ go build gopl.io/ch13/bzipper
$ wc -c < /usr/share/dict/words
938848
$ sha256sum < /usr/share/dict/words
126a4ef38493313edc50b86f90dfdaf7c59ec6c948451eac228f2f3a8ab1a6ed -
$ ./bzipper < /usr/share/dict/words | wc -c
335405
$ ./bzipper < /usr/share/dict/words | bunzip2 | sha256sum
126a4ef38493313edc50b86f90dfdaf7c59ec6c948451eac228f2f3a8ab1a6ed -
< / code > < / pre >
< p > 我们演示了如何将一个C语言库链接到Go语言程序。相反， ， （ ： ， ， ， < a href = "https://github.com/golang/go/issues/11058" > Issue11058< / a > ） ， ， ， （ ： ， ； ； ， ， ） ， ， （ ： ） ， < / p >
< p > < strong > 练习 13.3： < / strong > 使用sync.Mutex以保证bzip2.writer在多个goroutines中被并发调用是安全的。< / p >
< p > < strong > 练习 13.4： < / strong > 因为C库依赖的限制。 使用os/exec包启动/bin/bzip2命令作为一个子进程， （ ： ， ， ） < / p >
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