What is Static Library and Dynamic Library? How to generate the static and dynamic library?

How to Generate Static Library and Dynamic Library(Shared Libaries) Using Makefile:

First of all i would like give some introduction about static library and dynamic library.

The difference between static and dynamic libraries can be viewed as how you get to work. You can drive your car or you can take a train. Driving your own car is kinda like a static library. Taking the train is kinda like a shared library.

When you drive your car all you need is the car. When you take the train there is usually more than one train car by itself. Usually there's a locomotive, and then some passenger cars, and maybe a caboose to boot. As long as all the cars are working in harmony you continue to get to work. If any of the cars has a problem your chances of getting to work diminish.

When you compile a program with static libraries statically linked libraries are linked into the final executable by the linker. This increases the size of the executable. Likewise when a library needs to be updated you'll need to compile the new library and then recompile the application to take advantage of the new library. Ok, so why do we have static libraries then? Well if you're booting your system into maintenance mode static libraries can be beneficial.

What is Static Library?

Static libraries are simply a collection of ordinary object files; conventionally, static libraries end with the ``.a'' suffix. This collection is created using the ar (archiver) program.  

In static linking, the size of the executable becomes greater than in dynamic linking, as the library code is stored within the executable rather than in separate files

Static libraries permit users to link to programs without having to recompile its code, saving recompilation time. Note that recompilation time is less important given today's faster compilers, so this reason is not as strong as it once was. Static libraries are often useful for developers if they wish to permit programmers to link to their library, but don't want to give the library source code.

To create a static library, or to add additional object files to an existing static library, use a command like this:

                   ar rcs library.a File1.o File2.o

or 

                   ar -qcs library.a File1.o File2.o 

Makefile to Generate Static Library.

Let's take small example, suppose there are two files and generate the static library and link it to main file.

file1.c

int fun1(int a, int b)
{
  int c;

  /* add the Two Variables */
  c= a+b;
  return(c);
}

file2.c

int fun2(int a, int b)
{
  int c;

  /* add the Two Variables */
  c= a+b;
  return(c);
}
 Above there are two files, here you can find the Makefile to generate the library using file1.o and file2.o

Makefile to generate MyLibrary.a using file1.o and file2.o

CC = gcc

CFLAGS = -g -Wall

# .a extension for static library
# .so extension for dynamic library
# step 1:
# it goes to step2:

all: libMylibrary.a


#step 3:
# Return to step 2
# Declare this OBJS before it is used, Here OBJS is a variable.

OBJS = $(PWD)/file1.o \
       $(PWD)/file2.o

#step 2:

libMylibrary.a : $(OBJS)
        ar -qcs libMylibrary.a $(OBJS)


# step 4:
# Return to step 3

$(PWD)/%.o : %.c
        $(CC) $(CFLAGS) -c $< -o $@

clean:
        rm -f *.o
        rm -f *.a

How to Link this library to the Main function file to get .exe

mainFile.c

#include<stdio.h>
#include<stdlib.h>

extern int fun1(int a, int b);
extern int fun2(int a, int b);

int main()
{

  printf("The value of Function1: %d", fun1(4,5));
  printf("The value of Function2: %d", fun2(6,7));

  return 0;
}

Makefile links library and generate the test.exe

CC = gcc

CFLAGS = -g -Wall

# .a extension for static library
# .so extension for dynamic library
# step 1:
# it goes to step2:

all: test


#step 3:
# Return to step 2

OBJS = $(PWD)/mainFile.o \

#step 2:

test : $(OBJS)
        $(CC) $(CFLAGS) $(OBJS) $(LIBDIR) $(LIBLOG) -o test.exe


#step 4:
# Return to Step 3

$(PWD)/mainFile.o : mainFile.c
        $(CC) $(CFLAGS) -c mainFile.c

# Step 5:

# For static library, always include LIB DIR

# This is Mandatory
LIBDIR =  -L./
LIBLOG = -lMylibrary

clean:
        rm -f *.o
        rm -f *.a

 Important Point:
When ever you are linking static library to mainFile,  it is mandatory to mention the LIBDIR( where the library is present) using -L option. This $(LIBDIR) should be included when generating .exe file. 

Note: How to run your own makefile, suppose i have makefile as Makefile_org, then run using the below command.

make -f Makefile_org

clean :

 make -f Makefile_org clean

Real Time Example.

Suppose there are three modules BMODULE, CMODULE , DMODULE. BMODULE  is having dependency upon other two modules i.e. CMODULE , DMODULE . 

Hence, first compile CMODULE  and DMODULE and link their objects to BMODULE to generate the .exe.

Here  DMODULE generates the static library and CMODULE generates the object files(o's)

Step1:  Create Directories of BModule, DModule, CModule

-bash-3.2#  mkdir mreddya

-bash-3.2#  mkdir MAKEFILE_EXERCISE  

create directories under /root/mreddya/MAKEFILE_EXERCISE/as shown below.

-bash-3.2#  mkdir BMODULE

-bash-3.2#  mkdir CMODULE 

-bash-3.2#  mkdir DMODULE
 
Step2:  Create sub-directories of BModule, DModule, CModule
create sub directories under each module as shown below.

-bash-3.2# cd DMODULE/

-bash-3.2#  mkdir src

-bash-3.2#  mkdir inc

-bash-3.2#  mkdir obj 

-bash-3.2# cd CMODULE/

-bash-3.2#  mkdir src

-bash-3.2#  mkdir inc

-bash-3.2#  mkdir obj 

-bash-3.2# cd BMODULE/

-bash-3.2#  mkdir src

-bash-3.2#  mkdir inc

-bash-3.2#  mkdir obj 

Step 3:  Generate Library for DModule for the below files

-bash-3.2# cd DMODULE/

-bash-3.2# cd

Create four files under source directory, dModulefile1.c  dModulefile2.c  dModulefile3.c, Makefile

-bash-3.2# cd DMODULE/

-bash-3.2# cd inc

Create three files under inc,dModulefile1.h  dModulefile2.h  dModulefile3.h

dModulefile1.c

#include "dModulefile1.h"

int DModuleFunction1(int a)
{
   printf("I am in DModuleFunction1 and its Value:%d", a);
   a = a + FUNC_VALUE4;
   return(a);
}

dModulefile2.c

#include "dModulefile2.h"

int DModuleFunction2(int b)
{
   printf("I am in DModuleFunction2 and its Value:%d", b);
   b = b + FUNC_VALUE5;
   return(b);
}

dModulefile3.c

#include "dModulefile3.h"

int DModuleFunction3(int c)
{
   printf("I am in DModuleFunction3 and its Value:%d", c);
   c = c + FUNC_VALUE6;
   return(c);
}

dModulefile1.h

#include<stdio.h>

#define FUNC_VALUE4 14

dModulefile2.h

#include<stdio.h>

#define FUNC_VALUE5 11

dModulefile3.h
#include<stdio.h>
#define FUNC_VALUE6 13

Step 4:  Makefile for DMODULE (Makefile place in src directory)

# define the Compiler as gcc
CC = gcc

# define compiler flags "CFLAGS"  if you -g option it is for GDB, -wall to display all
# warnings

CFLAGS= -g -Wall

# TO include Directories, To include any directories give -I/Path Directory
# e.g: INCLUDEDIR = -I/VOBS/CMODULE/INC/, to include one more directory
# then Syntax will be INCLUDEDIR += -I/VOBS/CMODULE/INC/
#                     INCLUDEDIR  = -I/VOBS/BMODULE/INC/

INCLUDEDIR = -I/root/mreddya/MAKEFILE_EXERCISE/DMODULE/inc

# define MACRO for OBJECT DIRETORY
OBJ-DIR = /root/mreddya/MAKEFILE_EXERCISE/DMODULE/obj

# copy all .o( Objects) in to object directory
# All Objects are stored in OBJ-DIR
#
OBJS = $(OBJ-DIR)/dModulefile1.o \
        $(OBJ-DIR)/dModulefile2.o \
        $(OBJ-DIR)/dModulefile3.o \

# First whenever we give Make, it comes here, and Check OBJS
# Then It will check what and all .o are required from 

# OBJS directory
# then it will go Command to generate the .o's.

all:    libDmodule.a

# Generate the Library

libDmodule.a: $(OBJS)
        ar -qcs libDmodule.a $(OBJS)

#To Generate %.o means same directory.
# target ( It tells $(OBJ-DIR)/%.o : %.c, generate the %.c files # to .o files using command.)
# To get the Target the below command should be executed, 

# otherwise Linker error will be there.

$(OBJ-DIR)/%.o : %.c
        $(CC) $(CFLAGS) $(INCLUDEDIR) -c $< -o $@

clean:
        rm -f $(OBJ-DIR)/*.o
        rm -f *.a

Step 5:  Run the Makefile and check the results

-bash-3.2# pwd
/root/mreddya/MAKEFILE_EXERCISE/DMODULE/src

-bash-3.2# make
gcc -g -Wall -I/root/mreddya/MAKEFILE_EXERCISE/DMODULE/inc -c dModulefile1.c -o /root/mreddya/MAKEFILE_EXERCISE/DMODULE/obj/dModulefile1.o
gcc -g -Wall -I/root/mreddya/MAKEFILE_EXERCISE/DMODULE/inc -c dModulefile2.c -o /root/mreddya/MAKEFILE_EXERCISE/DMODULE/obj/dModulefile2.o
gcc -g -Wall -I/root/mreddya/MAKEFILE_EXERCISE/DMODULE/inc -c dModulefile3.c -o /root/mreddya/MAKEFILE_EXERCISE/DMODULE/obj/dModulefile3.o
ar -qcs libDmodule.a /root/mreddya/MAKEFILE_EXERCISE/DMODULE/obj/dModulefile1.o /root/mreddya/MAKEFILE_EXERCISE/DMODULE/obj/dModulefile2.o /root/mreddya/MAKEFILE_EXERCISE/DMODULE/obj/dModulefile3.o

-bash-3.2# ls
dModulefile1.c  dModulefile2.c  dModulefile3.c  libDmodule.a  Makefile

-bash-3.2# pwd
/root/mreddya/MAKEFILE_EXERCISE/DMODULE/obj

-bash-3.2# ls
dModulefile1.o  dModulefile2.o  dModulefile3.o

Step 6:  Generate objects for CMODULE

-bash-3.2# cd CMODULE/

-bash-3.2# cd

Create four files under source directory, cModulefile1.c  cModulefile2.c  cModulefile3.c, Makefile

-bash-3.2# cd CMODULE/

-bash-3.2# cd inc

Create three files under inc,cModulefile1.h  cModulefile2.h  cModulefile3.h

cModulefile1.c

#include "cModulefile1.h"

int CModuleFunction1(int a)
{
   printf("I am in CModuleFunction1 and its Value:%d", a);
   a = a + FUNC_VALUE1;
   return(a);
}

cModulefile2.c

#include "cModulefile2.h"

int CModuleFunction2(int b)
{
   printf("I am in CModuleFunction2 and its Value:%d", b);
   b = b + FUNC_VALUE2;
   return(b);
}

dModulefile3.c

#include "cModulefile3.h"

int CModuleFunction3(int c)
{
   printf("I am in CModuleFunction3 and its Value:%d", c);
   c = c + FUNC_VALUE3;
   return(c);
}

cModulefile1.h

#include<stdio.h>

#define FUNC_VALUE1 10

cModulefile2.h

#include<stdio.h>

#define FUNC_VALUE2 11

cModulefile2.h

#include<stdio.h>

#define FUNC_VALUE3 11

Step 7:  Makefile for CMODULE (Makefile place in src directory)

# define the Compiler as gcc
CC = gcc

# define compiler flags "CFLAGS"  if you -g option it is for GDB, -wall to display all
# warnings

CFLAGS= -g -Wall

# TO include Directories, To include any directories give -I/Path Directory
# e.g: INCLUDEDIR = -I/VOBS/CMODULE/INC/, to include one more directory
# then Syntax will be INCLUDEDIR += -I/VOBS/CMODULE/INC/
#                     INCLUDEDIR  = -I/VOBS/BMODULE/INC/

INCLUDEDIR = -I/root/mreddya/MAKEFILE_EXERCISE/CMODULE/inc

# define MACRO for OBJECT DIRETORY
OBJ-DIR = /root/mreddya/MAKEFILE_EXERCISE/CMODULE/obj

# copy all .o( Objects) in to object directory
# All Objects are stored in OBJ-DIR
#
OBJS = $(OBJ-DIR)/cModulefile1.o \
        $(OBJ-DIR)/cModulefile2.o \
        $(OBJ-DIR)/cModulefile3.o \

# First whenever we give Make, it comes here, and Check OBJS
# Then It will check what and all .o are required from 

# OBJS directory
# then it will go Command to generate the .o's.

all: $(OBJS)

#To Generate %.o means same directory.
# target ( It tells $(OBJ-DIR)/%.o : %.c, generate the %.c files to .o files using command.)
#    command.

#To Generate %.o means same directory.
# target ( It tells $(OBJ-DIR)/%.o : %.c, generate the %.c files to .o files using command.)
#    command.
# To get the Target the below command should be executed, otherwise
# Linker error will be there.

$(OBJ-DIR)/%.o : %.c
        $(CC) $(CFLAGS) $(INCLUDEDIR) -c $< -o $@

clean:
        rm -f $(OBJ-DIR)/*.o


Step 8:  Run the Makefile and check the results

-bash-3.2# pwd
/root/mreddya/MAKEFILE_EXERCISE/CMODULE/src

-bash-3.2# make clean
rm -f /root/mreddya/MAKEFILE_EXERCISE/CMODULE/obj/*.o

-bash-3.2# make
gcc -g -Wall -I/root/mreddya/MAKEFILE_EXERCISE/CMODULE/inc -c cModulefile1.c -o /root/mreddya/MAKEFILE_EXERCISE/CMODULE/obj/cModulefile1.o
gcc -g -Wall -I/root/mreddya/MAKEFILE_EXERCISE/CMODULE/inc -c cModulefile2.c -o /root/mreddya/MAKEFILE_EXERCISE/CMODULE/obj/cModulefile2.o
gcc -g -Wall -I/root/mreddya/MAKEFILE_EXERCISE/CMODULE/inc -c cModulefile3.c -o /root/mreddya/MAKEFILE_EXERCISE/CMODULE/obj/cModulefile3.o

-bash-3.2# pwd
/root/mreddya/MAKEFILE_EXERCISE/CMODULE/obj

-bash-3.2# ls
cModulefile1.o  cModulefile2.o  cModulefile3.o

Step 9: Get .exe by Linking the CMODULE and DMODULE to BMODULE 

-bash-3.2# cd BMODULE/

-bash-3.2# cd

Create two files under source directory, bModulefile.c,  Makefile

-bash-3.2# cd BMODULE/

-bash-3.2# cd inc

Create one file under inc,bModulefile.h

bModulefile.c

#include<stdio.h>

#include "bModulefile.h"

/* extern the Functions */
extern int CModuleFunction1(int );
extern int CModuleFunction2(int );
extern int CModuleFunction3(int );
extern int DModuleFunction1(int );
extern int DModuleFunction2(int );
extern int DModuleFunction3(int );

void getFunValue(void)
{
  int result = 0;

  result = CModuleFunction1(3)* FUNC_VALUE8;
  printf("The Result value with CModuleFunction1:%d\n", result);
  result = CModuleFunction2(4)* FUNC_VALUE8;
  printf("The Result value with CModuleFunction2:%d\n", result);
  result = CModuleFunction3(5)* FUNC_VALUE8;
  printf("The Result value with CModuleFunction3:%d\n", result);
  result = DModuleFunction1(3)* FUNC_VALUE8;
  printf("The Result value with DModuleFunction1:%d\n", result);
  result = DModuleFunction2(4)* FUNC_VALUE8;
  printf("The Result value with DModuleFunction2:%d\n", result);
  result = DModuleFunction3(5)* FUNC_VALUE8;
  printf("The Result value with DModuleFunction3:%d\n", result);
}
int main()
{
   /* call the getFunValue Function */
   getFunValue();
   return 0;
}

bModulefile.h

#define FUNC_VALUE8 1

Step 10:  Makefile for BMODULE (Makefile place in src directory)
This Makefile links both CMODULE and DMODULE and generates the test.exe

Some points to be Noted here:

Before compiling BMODULE, first it is mandate to compile CMODULE and DMODULE.

Once CMODULE and DMODULE compiled and run the Makefile of BMODULE.

Step 11:  Makefile compiles first CMODULE and DMODULE then BMODULE
Run this Makefile using make all command

# This is the Main Make file which incorporates all the files and
# generates the .exe file.

# define the Compiler Macro as gcc
CC = gcc

# define any Flags whether to compile as 64 bit(-m64) or with -g option
# to debug in gdb and also to show file name with warnings.
CFLAGS = -g -Wall

# Include the Directories which you want to link here.
# Include the .h directories.
# Whenever multiple directories are included then use +=  with -I option
INCL_DIR += -I/root/mreddya/MAKEFILE_EXERCISE/BMODULE/inc

# define MACRO for OBJECT DIRETORY
OBJ-DIR = /root/mreddya/MAKEFILE_EXERCISE/BMODULE/obj

EXE-DIR = /root/mreddya/MAKEFILE_EXERCISE/BMODULE/src

COBJ-DIR = /root/mreddya/MAKEFILE_EXERCISE/CMODULE/obj

DLIB_DIR = /root/mreddya/MAKEFILE_EXERCISE/DMODULE/src

OBJS = $(OBJ-DIR)/bModulefile.o

COBJS = $(COBJ-DIR)/cModulefile1.o \
        $(COBJ-DIR)/cModulefile2.o \
       $(COBJ-DIR)/cModulefile3.o \


EXTERNALLIBS = -L$(DLIB_DIR)
LIBLOG       = -lDmodule


CMODULE_COMPILE:
        cd /root/mreddya/MAKEFILE_EXERCISE/CMODULE/src && make
DMODULE_COMPILE:
        cd /root/mreddya/MAKEFILE_EXERCISE/DMODULE/src && make
BMODULE_COMPILE:
        $(CC) $(CFLAGS) $(INCL_DIR) -c *.c -o $(OBJS)

all:
        make all_modules

all_modules:
        make CMODULE_COMPILE
        make DMODULE_COMPILE
        make BMODULE_COMPILE
        $(CC) -o test.exe $(OBJS) $(EXTERNALLIBS) $(LIBLOG) $(COBJS)


clean:
        rm -f $(OBJ-DIR)/*.o
        rm -f $(EXE-DIR)/*.exe
        rm -f $(COBJ-DIR)/*.o
        rm -f $(DOBJ-DIR)/*.o

There are several ways to write make files, this is one way.
write small script and compile the Makefiles like show below.

build.sh script

cd /root/mreddya/MAKEFILE_EXERCISE/CMODULE/src
make
cd /root/mreddya/MAKEFILE_EXERCISE/DMODULE/src
make

cd /root/mreddya/MAKEFILE_EXERCISE/BMODULE/src
make

What is Dynamic Library or Shared Library (.so)?

Dynamic library is also called as Shared Library. 

Shared Library is a binary file(.so file) contains object (.o) files. All object files form as shared library  and this library can be loaded or unloaded dynamically.

By using shared library concept, one can make the code is position-independent, meaning that the code may be loaded anywhere in memory. Applications linked to shared objects at run time.
 

How to generate a shared library:

Step 1: Create object code, it means generate object files (.o files) 

 gcc -fPIC -c <filename.c>

The -fPIC option tells gcc to create position independent code which is necessary for shared libraries.

  e.g: gcc -fPIC -c myfilename.c

Step 2: Create shared library 

 gcc -g -Wall -shared -o  <library Name>  <objectfile1.o,  objectfile2.o>

 It will create the library with .so extension.

e.g: gcc -g -Wall -shared -o  libMyfuncs.so file1.o  file2.o

How to use the shared library to your application

Step 1:

Set the environment for Library using 

 export LD_LIBRARY_PATH=.

The above indicates the path directory where the library is present. ./ or  .  indicates the present directory

Step 3:

Compile your Main file and generate the object code.

e.g: gcc -c mainFile

And then Link the shared library  to the main file as mentioned below.

 e.g: gcc  -L./ -lMyfuncs mainFile.o -o test.exe

 Notes:

suppose if  "-L/opt/exp", means library is present in this directory and link the library using -llibName. The name of the library is Myfuncs.so.
The libraries will NOT be included in the executable but will be dynamically linked during run time execution.

That's reason size of .exe is small when we compare with static library.

How to check the list of shared libraries using by the application.

Use simple command.

ldd <.exe>

 -bash-3.2# ldd test.exe
        libMyFun.so => ./libMyFun.so (0x00002ab25d70f000)
        libc.so.6 => /lib64/libc.so.6 (0x0000003ef7c00000)
        /lib64/ld-linux-x86-64.so.2 (0x0000003ef6c00000)

Simple Makefile to generate shared library

file1.c

int func1(int a, int b)
{
    int c;
    /* Add */
    c= a+b;
    return(c);
}

file2.c

 int func1(int a, int b)
{
    int c;
    /* Add */
    c= a+b;
    return(c);
}

Makefile to generate shared library as lib

CC = gcc

CFLAGS = -g -Wall

OBJS = $(PWD)/file1.o \
        $(PWD)/file2.o

all: libMyFun.so

libMyFun.so : $(OBJS)
        $(CC) $(CFLAGS) -shared -o libMyFun.so $(OBJS)

$(PWD)/%.o : %.c
        $(CC) $(CFLAGS) -fPIC -c *.c
clean:
        rm -f *.o
        rm -f *.so

 How to Link this shared library to the mainFile.c

mainFile.c


#include<stdio.h>

extern int func1(int, int);
extern int func2(int, int);

int main()
{

  printf("The result of Function 1 is: %d\n", func1(4,5));
  printf("The result of Function 1 is: %d\n", func2(10,20));
  return 0;
}

Build.sh  <Generate the test.exe using small script>

#step 1: clean the Library
make clean

#step 2: Generated the Library
make

#step 3: Generates the mainFile.o

gcc -c mainFile.c

#step 4: clean the .exe
rm -f *.exe

#step 5: Link the Shared library to mainFile.o and
#generates the test.exe file.

gcc -L./ -lMyFun mainFile.o -o test.exe

Run the Build.sh

chmod +x Build.sh

./Build.sh