在最近的iOS开发中遇到类似以下描述的报错，不知道是什么原因导致的，后来上网搜索了下，看到别人的解决方法时，顿时豁然开朗，写下了以备以后忘记。
问题描述：
I am getting the following error, which seems to be coming from something in the bottom to do with "_main".
Ld /Users/jianglin/Library/Developer/Xcode/DerivedData/TownHall_iPhone-bdlgipvgaapgjhglhromfvcubbxz/Build/Products/Debug-iphonesimulator/TownHall\ iPhone.app/TownHall\ iPhone normal i386
cd "/Users/jianglin/Desktop/TownHall iPhone"
export IPHONEOS_DEPLOYMENT_TARGET=8.1
export PATH="/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Applications/Xcode.app/Contents/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch i386 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.1.sdk -L/Users/jianglin/Library/Developer/Xcode/DerivedData/TownHall_iPhone-bdlgipvgaapgjhglhromfvcubbxz/Build/Products/Debug-iphonesimulator -F/Users/jianglin/Library/Developer/Xcode/DerivedData/TownHall_iPhone-bdlgipvgaapgjhglhromfvcubbxz/Build/Products/Debug-iphonesimulator -filelist /Users/jianglin/Library/Developer/Xcode/DerivedData/TownHall_iPhone-bdlgipvgaapgjhglhromfvcubbxz/Build/Intermediates/TownHall\ iPhone.build/Debug-iphonesimulator/TownHall\ iPhone.build/Objects-normal/i386/TownHall\ iPhone.LinkFileList -Xlinker -rpath -Xlinker @executable_path/Frameworks -Xlinker -objc_abi_version -Xlinker 2 -ObjC -lPods-AFNetworking -lPods-Masonry -lPods-SDWebImage -framework CoreGraphics -framework Foundation -framework ImageIO -framework MobileCoreServices -framework Security -framework SystemConfiguration -framework UIKit -fobjc-arc -fobjc-link-runtime -Xlinker -no_implicit_dylibs -mios-simulator-version-min=8.1 -lPods -Xlinker -dependency_info -Xlinker /Users/jianglin/Library/Developer/Xcode/DerivedData/TownHall_iPhone-bdlgipvgaapgjhglhromfvcubbxz/Build/Intermediates/TownHall\ iPhone.build/Debug-iphonesimulator/TownHall\ iPhone.build/Objects-normal/i386/TownHall\ iPhone_dependency_info.dat -o /Users/jianglin/Library/Developer/Xcode/DerivedData/TownHall_iPhone-bdlgipvgaapgjhglhromfvcubbxz/Build/Products/Debug-iphonesimulator/TownHall\ iPhone.app/TownHall\ iPhone
Undefined symbols for architecture i386:
"_main", referenced from:
implicit entry/start for main executable
(maybe you meant: _OBJC_IVAR_$_PostTableViewCell._mainTextView, _OBJC_IVAR_$_ConfirmationViewController._mainLabel , _OBJC_IVAR_$_SignInViewController._mainLabel , _OBJC_IVAR_$_SignUpViewController._mainLabel )
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)
解决方法：
如果多个target的话，看下对应的.m文件是否加入到target中。
