## Contents

Contents

In this post, I would show the way to create static library and dynamic library and how they link to the executable files. I would not dig into the details of comparison, advantages, or disadvantages between static and dynamic library, please make sure you already have a whole picture before starting on this post.

## Create static library

First, create a source file without main function as our library. Attention, filename should start from lib so that compiler can identify it as library file.

 1 2 3 4 5  /* filename: lib_lib.c */ #include void func(void) { printf("hello"); } 

Create header file for lib_lib.c so that we can include it in main program later.

 1 2  /* filename: lib_lib.h */ void func(void); 

Compile the source program with gcc -c lib_lib.c -o lib_lib.o. After we get object file for static libraries, we can finally create static library with ar rcs lib_lib.a lib_lib.o, which would bundle several object file into single static library.(If we use command ar -t lib_lib.a, then we can find that which object file it depends on)
I know that object file would link to static libraries at compilation time. Create a main program, and let compiler do it for us with gcc -static -o main main.o -L. -l_lib (-L. means static lib exists in current folder; -lxxx means link to the lib with name: xxx).

In picture above, nm command would show us the symbol in the executable, and you can see the symbol of func. We also know that compiler will copy the code of lib into executable at compilation time; therefore, I should see the whole code of func() if I dump the assembly code of executable.

In picture above, I dump the assembly code of main with objdump -d main and can find that the code of func() has been copied to main.

## Create dynamic library

We can compile the same source file as dynamic library with gcc -fPIC -c *.c. This command would generate object file for each source file, and the flag fPIC makes sure the code is position-independent. In this step, we would get a file: foo.o. Also, I would like to create a header file for later use: foo.h.
Next, create dynamic library with gcc *.o -shared -o libfoo.so, then we can get a dynamic library: libfoo.so. Attention, filename should also start from lib and end with .so.
Finally, we can link with shared library. To tell program where to search for the libraries, we can use gcc -L$PWD -Wall -o main main.c -lfoo, or just export LD_LIBRARY_PATH=$PWD:\$LD_LIBRARY_PATH.

Same result with nm command. After reading the command manual about nm, we can compare the result in last section with this section. In previous result of nm command, func() showed with label T, which means text section. In this result, func() showed with U, which means undefined, definition exists in another file.
Here is the difference from static library that we cannot see the copied code of func(), but only the func@plt which would be replaced with the real address with lazy binding later.

## Conclusion

Given an executable, we can use command file to check whether it use dynamic-linking or static-linking. In addition to try from the perspective of compilation, it is also interesting to think about the way to trace the source of function if we get an executable. Welcome to correct me or leave your comments to discuss with me.