what are the packages with extension (a)

can anybody explain what are these packages that have extension (.a) that we may add it sometimes to xcode:

1) what exactly are those package开发者_运维问答s ?

2) how do they differ from other packages that have (.framework) extension ?

3) can I customize/change the code inside the (.a) package ?

thank you so much in advance.

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They are a static library (or archive) generated by ar. It contains one or more .o object files, which contain compiled code and symbols needed for linking.

A .framework contains both header files (.h) specifying the interface, along with a .dylib (a shared library).

A static library is already compiled into machine code - it is not easy to modify code in it. Unlike a shared library, a static library is designed to be linked by the linker - the object code will literally be copied into the destination file, whereas a shared library requires that it is present at runtime since only references are made.