Static Library - An Introduction

Static library is a group of object files bundled together in an archive by the archiver program. The static libraries have a .a extension.

Advantages
1. The executable is not dependent on any library as the library code is included in the binary
2. In some cases performance improvement

Command for creating a static library

ar rcs -o libmylibrary.a file1.o file2.o file3.o

Steps for generating the static library libemployee.a for the Employee Program written in the previous post

1. Compiling source files to object files

g++ -Wall -c -fPIC -O -I. -o Employee.o Employee.cpp

2. Command for building the archive file libemployee.a

ar -rcs -o libemployee.a Employee.o

The Client Program

 1 #include "Employee.h"
 2 #include <iostream>
 3 
 4 int main()
 5 {
 6     Employee *emp = new Employee("Pankaj", 26);
 7     std::cout << "Employee Name: " << emp->getName() << std::endl;
 8     std::cout << "Employee Age : " << emp->getAge() << std::endl;
 9     delete emp;
10     return 0;
11 }

Compiling the Client Program

g++ -Wall -c -fPIC -O -I. -o rClient.o Client.cpp

Linking the client executable with the static library

gcc -o client Client.o -fPIC -Wl,-rpath,. -L. -lstdc++ -lemployee

Running ldd against the client executable gives us the following result:

$ ldd ./client 
 linux-vdso.so.1 =>  (0x00007fff7cde3000)
 libstdc++.so.6 => /usr/lib64/libstdc++.so.6 (0x00007fb85d978000)
 libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fb85d762000)
 libc.so.6 => /lib64/libc.so.6 (0x00007fb85d3f5000)
 libm.so.6 => /lib64/libm.so.6 (0x00007fb85d19e000)
 /lib64/ld-linux-x86-64.so.2 (0x00007fb85dc81000)

The client executable is not dependent on the employee library