fat 表示包含多个架构，又叫Universal Binary，通用二进制文件
如果是fat的.a：
lipo -info xxx.a
Architectures in the fat file: xxx.a are: armv7 i386 x86_64 arm64
file xxx.a
xxx.a: Mach-O universal binary with 4 architectures: [arm_v7:current ar archive] [i386:current ar archive] [x86_64:current ar archive] [arm64:current ar archive]
如果不是fat的.a：
lipo -info xxx.a
Non-fat file: xxx.a is architecture: x86_64
file xxx.a
xxx.a: current ar archive
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