【iOS】class-dump源码学习（一）_class-dump工具可以直接dump出mach-o源码头

学习版本：class-dump-3.5.dmg

Build

上手打开工程先build一次（Xcode10）

报错缺少openssl的头文件

命令行指令 which openssl 打印 LibreSSL 2.6.5，说明苹果已经将openssl替换成了libressl

使用 brew install openssl安装openssl

安装完毕后会有这样的提示，应该是和现有有冲突，暂不做处理。

openssl@1.1 is keg-only, which means it was not symlinked into /usr/local,
because openssl/libressl is provided by macOS so don't link an incompatible version.
If you need to have openssl@1.1 first in your PATH run:
  echo 'export PATH="/usr/local/opt/openssl@1.1/bin:$PATH"' >> ~/.bash_profile
For compilers to find openssl@1.1 you may need to set:
  export LDFLAGS="-L/usr/local/opt/openssl@1.1/lib"
  export CPPFLAGS="-I/usr/local/opt/openssl@1.1/include"

打开/usr/local/opt/openssl@1.1/1.1.1d/include，把里面的openssl拷贝一份到/Applications/Xcode.app/Contents/DeveloperPlatforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/

/usr/local/opt/openssl@1.1/1.1.1d/lib也拷贝一份放到对应目录下

再次build，success

直接从class-dump.m的main函数入手

处理参数的代码略过

从读取文件入手

CDFile *file = [CDFile fileWithContentsOfFile:executablePath searchPathState:classDump.searchPathState];

CDFile是基本的文件单位，子类有CDFatFiIe、CDMachOFile，对应fat文件（胖二进制文件，可以看作是多个mach-o文件的聚合体）、mach-o文件。

关注CDFile静态方法

+ (id)fileWithContentsOfFile:(NSString *)filename searchPathState:(CDSearchPathState *)searchPathState;

核心逻辑是分别调用两个子类的initWithData:filename:searchPathState:方法。

先看CDFatFile中的initWithData:filename:searchPathState:方法

- (id)initWithData:(NSData *)data filename:(NSString *)filename searchPathState:(CDSearchPathState *)searchPathState;
{
    if ((self = [super initWithData:data filename:filename searchPathState:searchPathState])) {
        CDDataCursor *cursor = [[CDDataCursor alloc] initWithData:data];
 
        struct fat_header header;
        header.magic = [cursor readBigInt32];
        
        //NSLog(@"(testing fat) magic: 0x%x", header.magic);
        if (header.magic != FAT_MAGIC) {
            return nil;
        }
        
        _arches = [[NSMutableArray alloc] init];
        
        header.nfat_arch = [cursor readBigInt32];
        //NSLog(@"nfat_arch: %u", header.nfat_arch);
        for (NSUInteger index = 0; index < header.nfat_arch; index++) {
            CDFatArch *arch = [[CDFatArch alloc] initWithDataCursor:cursor];
            arch.fatFile = self;
            [_arches addObject:arch];
        }
    }
 
    return self;
}

CDDataCursor很好理解，就是一个可以读取定长数据的指针。而fat_header是系统定义的fat文件头结构。

第一步读取魔数magic。加载器通过这个数值来判断当前文件是什么文件，主要区分32位和64位

#define FAT_MAGIC	0xcafebabe
#define FAT_MAGIC_64	0xcafebabf

不过能看到class-dump并没有处理FAT_MAGIC_64的情况（return nil;意味着会走到mach-o文件的逻辑），个人理解fat文件的魔数并不重要，不能作为加载器选择执行文件的参考。

同时这里有个点，如果使用machoview打开