Auto Link

Auto-Link

• 简介
• 探索

简介

LC_LINKER_OPTION: 链接器的特性 Auto-Link

启用这个特性后，当我们import <模块>，不需要我们再去往链接器去配置链接参数。

比如import <framework>我们在代码里使用这个是framework格式的库文件，那么在生成目标文件时，会自动在目标文件的Mach-O中，插入（编译阶段）一个 load command，格式是LC_LINKER_OPTION，存储这样一个链接器参数-framework <framework>

探索

• 准备代码

  #import <Foundation/Foundation.h>
  
  int main() {
  
      return 0;
  }
  
  

• 编译 .o 脚本, 增加了一个 -fmodules 参数

  clang -x objective-c -fmodules \
  -target x86_64-apple-macos11.1 \
  -fobjc-arc \
  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.1.sdk \
  -I./Frameworks/TestExample.framework/Headers \
  -c test.m -o test.o

• otool查看 .o

  otool -l /Users/shenyj/Documents/CodeForTest/lib/staticLib/test.o

  输出结果:

  /Users/shenyj/Documents/CodeForTest/lib/staticLib/test.o:
  Load command 0
      cmd LC_SEGMENT_64
  cmdsize 392
  segname
  vmaddr 0x0000000000000000
  vmsize 0x0000000000000078
  fileoff 856
  filesize 120
  maxprot 0x00000007
  initprot 0x00000007
  nsects 4
      flags 0x0
  Section
  sectname __text
  segname __TEXT
      addr 0x0000000000000000
      size 0x000000000000000f
      offset 856
      align 2^4 (16)
      reloff 0
      nreloc 0
      flags 0x80000400
  reserved1 0
  reserved2 0
  Section
  sectname __objc_imageinfo
  segname __DATA
      addr 0x000000000000000f
      size 0x0000000000000008
      offset 871
      align 2^0 (1)
      reloff 0
      nreloc 0
      flags 0x10000000
  reserved1 0
  reserved2 0
  Section
  sectname __compact_unwind
  segname __LD
      addr 0x0000000000000018
      size 0x0000000000000020
      offset 880
      align 2^3 (8)
      reloff 976
      nreloc 1
      flags 0x02000000
  reserved1 0
  reserved2 0
  Section
  sectname __eh_frame
  segname __TEXT
      addr 0x0000000000000038
      size 0x0000000000000040
      offset 912
      align 2^3 (8)
      reloff 0
      nreloc 0
      flags 0x6800000b
  reserved1 0
  reserved2 0
  Load command 1
      cmd LC_BUILD_VERSION
  cmdsize 24
  platform 1
      minos 11.1
      sdk 12.1
  ntools 0
  Load command 2
      cmd LC_SYMTAB
  cmdsize 24
  symoff 984
  nsyms 1
  stroff 1000
  strsize 8
  Load command 3
              cmd LC_DYSYMTAB
          cmdsize 80
      ilocalsym 0
      nlocalsym 0
      iextdefsym 0
      nextdefsym 1
      iundefsym 1
      nundefsym 0
          tocoff 0
          ntoc 0
      modtaboff 0
          nmodtab 0
  extrefsymoff 0
      nextrefsyms 0
  indirectsymoff 0
  nindirectsyms 0
      extreloff 0
          nextrel 0
      locreloff 0
          nlocrel 0
  Load command 4
      cmd LC_LINKER_OPTION
  cmdsize 40
  count 2
  string #1 -framework
  string #2 Foundation
  Load command 5
      cmd LC_LINKER_OPTION
  cmdsize 40
  count 2
  string #1 -framework
  string #2 CoreGraphics
  Load command 6
      cmd LC_LINKER_OPTION
  cmdsize 40
  count 2
  string #1 -framework
  string #2 CoreServices
  Load command 7
      cmd LC_LINKER_OPTION
  cmdsize 32
  count 2
  string #1 -framework
  string #2 IOKit
  Load command 8
      cmd LC_LINKER_OPTION
  cmdsize 40
  count 2
  string #1 -framework
  string #2 DiskArbitration
  Load command 9
      cmd LC_LINKER_OPTION
  cmdsize 40
  count 2
  string #1 -framework
  string #2 CFNetwork
  Load command 10
      cmd LC_LINKER_OPTION
  cmdsize 32
  count 2
  string #1 -framework
  string #2 Security
  Load command 11
      cmd LC_LINKER_OPTION
  cmdsize 40
  count 2
  string #1 -framework
  string #2 CoreFoundation

可以看到多出很多LC_LINKER_OPTION 的 load command

这也是 xcode默认执行的操作, Xcode 设置路径: