需要澄清在iOS中链接库时发生了什么

Question

This is probably a totally noob question but I have missing links in my mind when thinking about linking libraries in iOS. I usually just add a new library that's been cross compiled and set the build and linker paths without really know what I'm doing. I'm hoping someone can help me fill in some gaps.

Let's take the OpenCV library for instance. I have this totally working btw because of a really well written tutorial( http://niw.at/articles/2009/03/14/using-opencv-on-iphone/en ), but I'm just wanting to know what is exactly going on.

What I'm thinking is happening is that when I build OpenCV for iOS is that your creating object code that gets placed in the .a files. This object code is just the implementation files( .m ) compiled. One reason you would want to do this is to make it hard to see the source code and so that you don't have to compile that source code every time.

The .h files won't be put in the library ( .a ). You include the .h in your source files and these header files communicate with the object code library ( .a ) in some way.

You also have to include the header files for your library in the Build Path and the Library itself in the Linker Path.

So, is the way I view linking libraries correct? If , not can someone correct me on this ?

Answer 1

Basically, you are correct.

Compiling the source code of a library produces one object file for each of the source files (in more than one, if compiled multiply times against different architectures). Then all the object files are archived (or packaged) into one .a file (or .lib on Windows). The code is not yet linked at this stage.

The .h files provide an interface for the functionality exposed by the library. They contain constants, function prototypes, possibly global declarations (eg extern int bad_global; ), etc. -- basically, everything that is required to compile the code which is using the library.

.h files do not 'communicate' with object code in any way. They simply provide clues for the compiler. Consider this header file:

// library.h

extern int bad_global;

int public_func(int, const void*);

By including this file in your own code, you're simply telling the compiler to copy and paste these declarations into your source file. You could have written declarations for OpenCV library and not use the headers provided with it. In other words, you're asking the compiler to not issue errors about undefined symbols, saying "I have those symbols elsewhere, ok? Here are their declarations, now leave me alone!".

The header files need to be included in the search path in order for compiler to find them. You could simply include them via the full path, eg #include "path/to/file.h" , or supply an -I option for your compiler, telling him where to look for additional headers, and use #include <file.h> instead.

When your code is compiled, the declarations in header files serve as an indication that symbols your code is using are defined somewhere. Note the difference between the words declaration and definition . Header files contain only declarations most of the time.

Now, when your code is compiled, it must be linked in order to produce the final executable. This is where the actual object code stored in the library comes into play. The linker will look at each symbol, function call, etc. in your object code and then try to find the corresponding definition for each such symbol. If it doesn't find one in the object code of your program, it will look the standard library and any other library you've provided it with.

Thus, it is important to understand that compilation and linkage are two separate stages. You could write any function prototypes at all and use them in your code, it will compile cleanly. However, when it comes to the linking stage, you have to provide implementation for symbols used in your code, or you won't get your executable.

Hope that makes sense!

Answer 2

The .a is the compiled version of the code.

The header files provided with a library are its public interface. They show what classes, methods, properties are available. They do not "communicate" with the binary code.

The compiler needs the headers to know that a symbol (a method name for example) is defined somewhere else. They are associated with the right "piece of code" in the library binary later during the "link" step.