laid

是一个 cgo 的测试用例

使用下面实例 完成 cgo 库的学习

VS Code 配置

工作区 配置

• laid.code-workspace

{
    "folders": [
        {
            "path": "."
        }
    ],
    "settings": {
        "files.associations": {
            "laid.h": "c"
        }
    }
}

C/C++ Extension Pack 配置:

• settings.json
  控制错误提醒

{
    "files.associations": {
        "laid.h": "c"
    },
    "C_Cpp.errorSquiggles": "enabled"
}

• c_cpp_properties.json
  配置代码提示

{
    "configurations": [
        {
            "name": "Mac",
            "includePath": [
                "${workspaceFolder}/**",
                "${workspaceFolder}/include/"
            ],
            "defines": [],
            "macFrameworkPath": [
                "/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/Library/Frameworks"
            ],
            "compilerPath": "/usr/bin/gcc",
            "cStandard": "c17",
            "cppStandard": "c++17",
            "intelliSenseMode": "macos-gcc-x64",
            "mergeConfigurations": true
        }
    ],
    "version": 4
}

• tasks.json
  配置代码编译

{
    "tasks": [
        {
            "type": "cppbuild",
            "label": "c 源码编译",
            "command": "/usr/bin/gcc",
            "args": [
                "-fdiagnostics-color=always",
                "-g",
                "${file}",
                "-o",
                "${workspaceFolder}/builds/${fileBasenameNoExtension}.o",
                "-I", "${workspaceFolder}/include"
            ],
            "options": {
                "cwd": "${fileDirname}"
            },
            "problemMatcher": [
                "$gcc"
            ],
            "group": {
                "kind": "build",
                "isDefault": true
            },
            "detail": "调试器生成的任务。"
        }
    ],
    "version": "2.0.0"
}

• launch.json
  配置 调试工具

{
    // 使用 IntelliSense 了解相关属性。 
    // 悬停以查看现有属性的描述。
    // 欲了解更多信息，请访问: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    
    "configurations": [
        {
            "name": "gdb 启动",
            "type": "cppdbg",
            "request": "launch",
            "program": "${workspaceFolder}/builds/main.out",
            "args": [],
            "stopAtEntry": false,
            "cwd": "${fileDirname}",
            "environment": [],
            "externalConsole": false,
            "MIMode": "lldb",
            "preLaunchTask": "c 源码编译"
        },
    ]
}

preLaunchTask 必须是 tasks.json 里定义的 tasks下的 某一个 label

代码目录结构:

.
├── README.md
├── builds
│   ├── laid.o
│   ├── liblaid.a
│   ├── main.o
│   └── main.o.dSYM
│       └── Contents
│           ├── Info.plist
│           └── Resources
│               └── DWARF
│                   └── main.o
├── include
│   └── laid.h
├── laid.code-workspace
└── src
    ├── laid.c
    └── main.c

include/laid.h 内容:

//
// 定义 laid 头文件
//
#include <stdlib.h>

struct laid
{
    char *name;
    void (* say_hello)(struct laid *l);
    char * (* get_name)(struct laid *l);
};

struct laid *create_laid(char *name);
void say_hello(struct laid *l);
char* get_name(struct laid *l);

src/laid.c 内容:

//
// laid 实现文件
//
#include <stdio.h>
#include "laid.h"

void say_hello(struct laid *l) {
    printf("Hello: %s", l->name);
}

char* get_name(struct laid *l) {
    return l->name;
}

struct laid *create_laid(char *name)
{
    struct laid *l = malloc(sizeof(struct laid));
    l->name = name;
    l->say_hello = say_hello;
    l->get_name = get_name;

    return l;
}

• 编译 并生成 laid.o 对象文件

/usr/bin/gcc -c -fdiagnostics-color=always -g ~/workspace/bangongyi/golang/la/3rd/laid/src/laid.c -o ~/workspace/bangongyi/golang/la/3rd/laid/builds/laid.o -I ~/workspace/bangongyi/golang/la/3rd/laid/include 

• 打包库文件

ar rcs builds/liblaid.a builds/laid.o

golang 里通过cgo 调用

laid-go/linker.go 调用C代码 :

package laid_go

/*
#include "laid.h"
#cgo CFLAGS: -I./../3rd/laid/include
#cgo LDFLAGS: -L./../3rd/laid/builds -llaid
*/
import "C"
import (
    "log"
    "unsafe"
)

func Laid_print_info(name string) {

    l := C.create_laid(C.CString(name))
    defer C.free(unsafe.Pointer(l))

    log.Println(l.name)
    log.Println(C.GoString(l.name))
    log.Println(l.say_hello)
    log.Println(C.say_hello)

    // 这行会报错
    // l.say_hello(l)
    // laid-go/linker.go:28:2: invalid operation: cannot call non-function l.say_hello (variable of type *[0]byte)

    log.Println("'-----'")

    s := C.get_name(l)
    log.Println(C.GoString(s))
}

main.go 调用写好的

package main

import (
    "log"
    laid_go "bangongyi.com/la/laid-go"
)

func main() {
    laid_go.Laid_print_info("王五")
}

执行结果:

2023/05/14 17:19:23 0x1084000b0
2023/05/14 17:19:23 &[]
2023/05/14 17:19:23 0x1002cf5b0
2023/05/14 17:19:23 '-----'
2023/05/14 17:19:23 王五

笔记:

从 laid_go.Laid_print_info 函数体里测试,发现以下问题:

• C.${func}

它只能调用 真正的 function, 对于结构体内部的函数指针不能调用,例如:
// l.say_hello(l) 这行会报错
// laid-go/linker.go:28:2: invalid operation: cannot call non-function l.say_hello (variable of type *[0]byte)

• C.CString(go_string)

将 go_string 转换成 C语言的 字符串指针变量, 所以打印的是地址

• C.GoString(ptr_c_string)

将 C语言的指针字符串(char *) 转换成 golang string

遗憾

目前对于golang 还处于学习阶段, 对 C. 调用函数指针 没有找到有效方法, 目前能想到能实现的方案是 代理函数

类似:

char* laid_get_name(struct laid * l) {
    return l->get_name(l);
}

文末推荐一个有意思的工具: https://revealjs.com/ 网页幻灯片

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