CGO Preliminary Cognition and Basic Data Type Conversion

CGO 是什么？

CGO 是 GO 语言里面的一个特性,CGO 属于 GOLANG 的高级用法,主要是通过使用 GOLANG 调用 CLANG Implemented library

使用

我们可以使用

import "C" 来使用 CGO 这个特性

一个最简单的 CGO 使用

package main


//#include <stdio.h>
import "C"

func main(){
    
	C.puts(C.CString("Hello, Cgo\n"))
}

import "C" The above can write the library that needs to be imported C 库,need to be annotated,CGO The comments here will be treated as such C 的代码,被称为序文（preamble）

Functional explanation of the above code

使用 CGO 包的 C.CString 函数将 Go The language string is converted to C 语言字符串

最后调用 CGO 包的 C.puts The function prints the converted to the standard output window C 字符串

使用 go build -x main.go 编译一下

加上 -x Instructions executed during compilation can be printed out

# go build -x main.go
WORK=/tmp/go-build594331603
mkdir -p $WORK/b001/
cat >$WORK/b001/importcfg.link << 'EOF' # internal packagefile command-line-arguments=/root/.cache/go-build/fb/fbb37eeb6735cb453f6d92e2e3f46f14d9dceb5baa1cdd10aae11d1d47d60e55-d packagefile runtime/cgo=/usr/local/go/pkg/linux_amd64/runtime/cgo.a packagefile syscall=/usr/local/go/pkg/linux_amd64/syscall.a packagefile runtime=/usr/local/go/pkg/linux_amd64/runtime.a packagefile errors=/usr/local/go/pkg/linux_amd64/errors.a packagefile internal/bytealg=/usr/local/go/pkg/linux_amd64/internal/bytealg.a packagefile internal/oserror=/usr/local/go/pkg/linux_amd64/internal/oserror.a packagefile internal/race=/usr/local/go/pkg/linux_amd64/internal/race.a packagefile internal/unsafeheader=/usr/local/go/pkg/linux_amd64/internal/unsafeheader.a packagefile sync=/usr/local/go/pkg/linux_amd64/sync.a packagefile internal/cpu=/usr/local/go/pkg/linux_amd64/internal/cpu.a packagefile runtime/internal/atomic=/usr/local/go/pkg/linux_amd64/runtime/internal/atomic.a packagefile runtime/internal/math=/usr/local/go/pkg/linux_amd64/runtime/internal/math.a packagefile runtime/internal/sys=/usr/local/go/pkg/linux_amd64/runtime/internal/sys.a packagefile internal/reflectlite=/usr/local/go/pkg/linux_amd64/internal/reflectlite.a packagefile sync/atomic=/usr/local/go/pkg/linux_amd64/sync/atomic.a EOF
mkdir -p $WORK/b001/exe/
cd .
/usr/local/go/pkg/tool/linux_amd64/link -o $WORK/b001/exe/a.out -importcfg $WORK/b001/importcfg.link -buildmode=exe -buildid=Vv0to6CWqbWf5_KTN66F/K36AEO-x4qJ_LJbz5wgG/HVbBbLSaW0sTSwlN8TzN/Vv0to6CWqbWf5_KTN66F -extld=gcc /root/.cache/go-build/fb/fbb37eeb6735cb453f6d92e2e3f46f14d9dceb5baa1cdd10aae11d1d47d60e55-d
/usr/local/go/pkg/tool/linux_amd64/buildid -w $WORK/b001/exe/a.out # internal
mv $WORK/b001/exe/a.out main
rm -r $WORK/b001/

尝试自己写一个 C 函数,让 GO 来调用他

Gocall this in the locale SayHello 函数

package main

/*
#include <stdio.h>

static void SayHello(const char* s) {
    
    puts(s);
}
*/
import "C"

func main(){
    
	C.SayHello(C.CString("hello xiaomotong study cgo\n"))
}

尝试自己写一个 C 文件,然后 GO Import and call in

xmtC.h

void SayHi(const char * str);

xmtC.c

(Must be in the same directory .c 文件,cgo 使用 go build 编译的时候,It will be found in the same directory by default.c文件进行编译,If we need to C The file is made into a static library 或者 动态库的方式,Then don't C The source code files are placed in the same directory,避免重名)

#include <stdio.h>
#include "xmtC.h"

void SayHi(const char * str){
    
    puts(str);
}

main.go

package main

//void SayHi(const char * str);
import "C"

func main(){
    
	C.SayHi(C.CString("hello xiaomotong study cgo\n"))
}

直接运行go build进行编译,Just run the executable program

# go build
# ls
cgo  main.go  xmtC.c
# ./cgo
hello xiaomotong study cgo

通过面向CProgramming techniques for language interfaces,We don't just free up implementors of functions,It also simplifies the user of the function.现在我们可以将 SayHi Used as a standard library function（和putsFunctions are used in a similar way）

咱们也可以在 go The file is written like this

package main

//#include <xmtC.h>
import "C"

func main(){
    
	C.SayHi(C.CString("hello xiaomotong study cgo\n"))
}

合并 C 和 GO 的代码

在Go1.10中CGO新增加了一个_GoString_预定义的C语言类型,用来表示Go语言字符串

// +build go1.10

package main

//void SayHi(_GoString_ s);
import "C"

import (
	"fmt"
)

func main() {
    
	C.SayHi("hello xiaomotong study cgo\n")
}

//export SayHi
func SayHi(s string) {
    
	fmt.Print(s)
}

The specific execution logic sequence of the above code is as follows：

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CGO 环境

使用 CGO Some environmental support is required

• linux 下 需要有 gcc/g++ 的编译环境
• windows Need to have MinGW 工具
• 需要把 GO 的环境变量 CGO_ENABLED 置位 1

上述的例子中,We have a few points to note：

• import "C" Statements cannot be combined with other import 语句放在一起,It needs to be placed on a separate line

• 上述我们在GOthe value passed inside,例如 C.CString("hello xiaomotong study cgo\n") 是调用了 C virtual package,将字符串转换成 C The string passed in

• Go是强类型语言

  所以 cgo The parameter type passed in must exactly match the declared type,And it must be used before delivery ”C” The transformation function in is converted into the corresponding C类型,不能直接传入Govariable of type

  通过虚拟的 C 包导入的Clanguage symbolsIt does not need to start with a capital letter,它们不受GoThe language's export rule constraints

#cgo 用法

我们可以使用 #cgo The statement sets the relevant parameters of the compilation phase and the linking phase

• Compile phase parameters

Mainly used to define related macros and specify the header file retrieval path

• Parameters for the linking phase

Mainly specify the library file retrieval path and the library file to be linked

例如我们可以这样

// #cgo CFLAGS: -DPNG_DEBUG=1 -I./include
// #cgo LDFLAGS: -L/usr/local/lib -lpng
// #include <png.h>
import "C"

CFLAGS

• -DPNG_DEBUG

定义宏 PNG_DEBUG ,设置为 1

• -I

The search directory for defining header files is ./include

LDFLAGS

• -L

Specifies the link-time library file retrieval directory ,可以通过写 ${SRCDIR}to represent the absolute path of the current package

• -l

指定链接时需要的库,此处是 png 库

条件编译 build tag

就是在我们 go build 的时候,Add some conditional parameters,Of course, this conditional parameter is required in the corresponding file,

For example above we use Go1.10 的时候,just added in the file // +build go1.10

我们可以这样用：

go build -tags="debug"
go build -tags="debug test"
go build -tags="linux,386"

go build 的时候加上 -tags 参数,If there are more than one, we can write them together,用空格间隔,表示 或,用逗号interval representation与

GO 和 C 数据类型相互转换

cgo The official provides the following data type conversions：

需要注意 3 个点：

• CGO 中,C 语言的int和longTypes are corresponding4个字节的内存大小,size_t 类型可以当作Go语言 uint Unsigned integer types are treated

• CGO 中,C 语言的int固定为4字节的大小 , GO 语言的 int 和 uint 却在32位和64Correspondingly under the bit system 4 个字节和 8 个字节大小

• For example, there are spaces between data types,unsigned int 不能直接通过 C.unsigned int 访问,可以使用typedefThe keyword provides a type of rule named,This is more beneficialCGO中访问

String and slice types

CGO生成的 _cgo_export.h 头文件中有 GO 里面字符串,切片,通道,字典,The corresponding representation of data types such as interfaces,But we generally use strings and slices that are valuable

因为 CGO No helper functions are provided for other data types

typedef struct {
     const char *p; GoInt n; } GoString;

We can write this when we export the function：

使用 _GoString_预定义类型,Writing this way can reduce the cgo Probably right in the code _cgo_export.h Risk of circular dependencies created by header files

_GoString_ 是 Go1.10 针对 Go special characters

extern void helloString(_GoString_ p0);

We can use officially provided functions计算字符串的长度 和 获取字符串的地址：

size_t _GoStringLen(_GoString_ s);
const char *_GoStringPtr(_GoString_ s);

struct ,union,enum

GO 语言中访问 C 语言的 struct ,union,enum,You can view the corresponding relationship in the table below

**对于结构体 struct **

The memory layout of the structure follows C General alignment rules for the language

在32位Go语言环境 C The language structure also follows32Bit alignment rules,在64位GoLocale as per64Bit alignment rules

For structures that specify special alignment rules,无法在 CGO 中访问

GO can be accessed like this C 的结构体

package main

/* struct struct_TEST { int i; float f; }; */
import "C"
import "fmt"

func main() {
    
	var a C.struct_TEST
	a.i = 1
	a.f = 2

	fmt.Println(a.i)
	fmt.Println(a.f)
}

需要注意如下 2 个大点：

• The names and struct members GO The names of the keywords are handled the same way

For example, the above structure member names are like this

struct struct_TEST {
    
    int type;
    float f;
};

那么我们访问 type 的时候,可以这样访问a._type 即可

If the structure is like this？

struct struct_TEST {
    
    int type;
    float _type;
};

It was still the same when we visited, a._type ,But the actual visit is float _type; ,通过 GO There is no way to access int type;

GO also cannot be accessed C 中的 零长数组 和 位字段,例如

struct struct_TEST {
    
	int   size: 10; // Bit fields cannot be accessed
    float arr[];    // Arrays of zero length cannot be accessed
};

• 在 C 语言中,无法直接访问 Go The language-defined structure type

对于枚举 enum

The underlying corresponding to the enumeration typeint类型,Values ​​of negative type are supported , 我们可以直接使用 C.xx 来进行访问

For example the enum type is ：

enum TEST {
    
    ONE,
    TWO,
};

Using this type we can use c C.enum_TEST

When copying this variable,我们可以这样做：c = C.ONE

对于联合体 union

Go 语言中并不支持 C Language union type,They are converted to byte arrays of the corresponding size

例如

union B1 {
    
    int i;
    float f;
};

union B1 会被转换成为 4 个字节大小的 字节数组 [4]uint8

GO The operation union variable has 3 种方式：

• 在CHelper functions are defined in the language
• Go语言的 encoding/binary Manually decode members**(Need to pay attention to the big endian little endian problem)**
• 使用unsafeThe package is cast to the corresponding type

举个例子

package main

/*
#include <stdint.h>

union TEST {
    
    int i;
    float f;
};
*/
import "C"
import (
	"fmt"
	"unsafe"
)

func main() {
    
	var b C.union_TEST

	*(*C.int)(unsafe.Pointer(&b)) = 1

	fmt.Println("b.i:", *(*C.int)(unsafe.Pointer(&b)))
	fmt.Println("b.f:", *(*C.float)(unsafe.Pointer(&b)))
}

When we read and write union variables,使用 unsafe Package performance is the best,通过unsafe 获取指针,Then convert it to a pointer of the corresponding data type

参考资料：

GO 高级编程

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