Creating a Static Library in C

A static library in C is a collection of object files that are linked into the program at compile-time. The final executable contains all the code from the static library, making it self-contained.

Steps to Create a Static Library:

1. Write the Source Files:

   • Write your C source code (.c) and corresponding header files (.h).

   Example:

   // file: add.c
   int add(int a, int b) {
       return a + b;
   }
   
   // file: sub.c
   int sub(int a, int b) {
       return a - b;
   }
   
   // file: mathlib.h
   int add(int a, int b);
   int sub(int a, int b);
   
   

2. Compile the Source Files into Object Files: Use the gcc compiler to create .o object files.

   gcc -c add.c sub.c
   
   

   • c: Tells gcc to only compile the files to object files (add.o, sub.o), not linking them.

3. Create the Static Library Archive: Use the ar (archive) command to bundle the object files into a static library (.a file).

   ar rcs libmathlib.a add.o sub.o
   
   

   • r: Insert or replace object files in the library.
   • c: Create the library if it doesn’t exist.
   • s: Index the object files for faster access.

4. Use the Static Library in a Program: To use the static library in a program, link it during the compilation of the main program.

   Example:

   // file: main.c
   #include "mathlib.h"
   #include <stdio.h>
   
   int main() {
       printf("Sum: %d\\\\n", add(3, 4));
       printf("Difference: %d\\\\n", sub(10, 5));
       return 0;
   }
   
   

   Compile and link the program with the static library:

   gcc main.c -L. -lmathlib -o main
   
   

   • L.: Specifies the directory containing the static library.
   • lmathlib: Links the libmathlib.a library.
   • o main: Generates the final executable.

5. Run the Program: After compiling and linking, run the program:

   ./main
   
   

Advantages of Static Libraries:

• Self-contained executables: All necessary code is embedded in the executable.
• Faster execution: No need for dynamic linking at runtime.

Disadvantages:

• Larger executable: Since the entire library is included, the executable can be large.
• No code sharing: Each program has its own copy of the library, which can waste memory if multiple programs use the same library.

Key Commands:

• gcc -c to compile source files into object files.
• ar rcs to create the static library.
• gcc -L. -l to link the static library to your program.