Adding Shared Library

In this section, you are going to learn

What is shared library ?

How to create shared library ?

How to link shared library ?

Topics in this section,

• Adding Shared Library : FAQs

• Adding Shared Library : Shared Library without Makefile

• Adding Shared Library : Shared Library with Makefile

Adding Shared Library : FAQs

Let us answer few basic questions about shared library

What is the use of library ?

See Answer

• It is a collection of pre-compiled functions that can be used by a program.

• Used to reduce the redundancy of code.

What are the types of library ?

See Answer

• Two types of library:

  • Static Library

  • Shared Library

What is Shared Library ?

See Answer

• Collection of compiled object code that is linked to the source code only at the runtime.

How to link the object files to generate the shared library ?

See Answer

• object files should be generated with -fPIC option in gcc command

• -shared option in gcc is used to generate the static library.

gcc -fPIC file.c file.o
gcc -shared -o mysharedlib.so file.o

How to link the shared library with the source code ?

See Answer

• set LD_LIBRARY_PATH to your shared library path and run the executable binary.

export LD_LIBRARY_PATH=<YOUR_LIBRARY_PATH>:$LD_LIBRARY_PATH
./a.out

What “nm” command does ?

See Answer

• nm command is used to display information about symbol in objects file or binaries.

$ nm -D libmathfunc.so
w __cxa_finalize
w __gmon_start__
w _ITM_deregisterTMCloneTable
w _ITM_registerTMCloneTable
0000000000001127 T mul
0000000000001111 T sub
00000000000010f9 T sum

• -D option is used to display dynamic symbols.

What objdump command does ?

See Answer

• objdump is used to provide detailed information about object files and libraries.

$ objdump -T libmathfunc.so

libmathfunc.so:     file format elf64-x86-64

DYNAMIC SYMBOL TABLE:
0000000000000000  w   D  *UND*  0000000000000000 __cxa_finalize
0000000000000000  w   D  *UND*  0000000000000000 _ITM_registerTMCloneTable
0000000000000000  w   D  *UND*  0000000000000000 _ITM_deregisterTMCloneTable
0000000000000000  w   D  *UND*  0000000000000000 __gmon_start__
0000000000001127 g    DF .text  0000000000000017 mul
00000000000010f9 g    DF .text  0000000000000018 sum
0000000000001111 g    DF .text  0000000000000016 sub

• -T option is used to display dynamic symbol table.

What readelf command does ?

See Answer

• readelf is used for displaying information about ELF (Executable and Linkable Format) format binaries.

What file command does ?

See Answer

• file is used for providing basic information about the file including whether it is a shared library.

$ file libmathfunc.so
libmathfunc.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=39a0f0d0a3ff4477eb4fc706a0b85ad38f4501f8, not stripped

Let us now explore it in depth !

Adding Shared Library Without Makefile

Creating Function Prototype

• Step 1: Create an header file with all the necessary function prototypes.

int sum(int a,int b);
int sub(int a,int b);
int mul(int a,int b);

Creating Function Definition

• Step 2: Create source code file with function definitions for all the functions defined in header file.

add.c

int sum (int a,int b)
{
	return a+b;
}

sub.c

int sub (int a, int b)
{
	return a-b;
}

mul.c

int mul (int a, int b)
{
	return a*b;
}

Generating Shared Library

• Step 3: Generate Shared Library.

  $ gcc -c -fPIC add.c
  $ gcc -c -fPIC sub.c
  $ gcc -c -fPIC mul.c
  
  $ gcc -shared -o libmathfunc.so add.o sub.o mul.o
  

Creating an Application

• Step 4: Create an application and use the functions being defined in the static library file created.

  #include <stdio.h>
  #include "math.h"
  
  int main(void)
  {
      int x = 2;
      int y = 4;
      int result;
  
      result = sum(x, y);
      printf("Sum = %d\n", result);
  
      result = sub(x, y);
      printf("Sub = %d\n", result);
  
      result = mul(x, y);
      printf("Product = %d\n", result);
  
      return 0;
  }
  

Compiling the Application

• Step 5: Compile the application by linking the static library with gcc command.

  export LD_LIBRARY_PATH=$(PWD):$LD_LIBRARY_PATH
  gcc -o output app.c -L. -lmathfunc
  

Executing the Application

• Step 6: Executing the application by running the binary file.

  $ ./output
  Sum = 6
  Sub = -2
  Product = 8
  

Adding Shared Library : Shared Library With Makefile

Creating Function Prototype

• Step 1: Declare all the function which needs to be linked with the static library in the header file.

int sum(int a,int b);
int sub(int a,int b);
int mul(int a,int b);

Creating Function Definition

• Step 2: Create source code file with function definitions for all the functions defined in header file.

add.c

int sum (int a,int b)
{
	return a+b;
}

sub.c

int sub (int a, int b)
{
	return a-b;
}

mul.c

int mul (int a, int b)
{
	return a*b;
}

Creating Makefile

• Step 3: Creating Makefile with all the instructions required to generate the static library.

CC = gcc  
CFLAGS = -fPIC -Wall -Wextra -O2 -g  
LDFLAGS = -shared   
RM = rm -f 
TARGET_LIB = libmathfunc.so  
APP = app 

SRCS = src/add.c src/sub.c src/mul.c  
OBJS = $(SRCS:.c=.o)

.PHONY: all 
all: ${TARGET_LIB} ${APP}

${APP}:
	gcc -L$(PWD)/obj -Wall -o app src/app.c -lmathfunc -I./hdr
	mv app obj 

$(TARGET_LIB): $(OBJS)
	$(CC) ${LDFLAGS} -o $'@' $^
	mv src/*.o obj/
	mv src/*.d obj/
	mv $(TARGET_LIB) obj/

$(SRCS:.c=.d):%.d:%.c
	$(CC) $(CFLAGS) -MM $< >$@

include $(SRCS:.c=.d)

.PHONY: clean
clean:
	rm -f obj/*
	rm -f src/*.d

Compiling the Application

$ make
gcc   -Wall -Wextra -O2 -g   -MM src/mul.c >src/mul.d
gcc   -Wall -Wextra -O2 -g   -MM src/sub.c >src/sub.d
gcc   -Wall -Wextra -O2 -g   -MM src/add.c >src/add.d
gcc   -Wall -Wextra -O2 -g     -c -o src/add.o src/add.c
gcc   -Wall -Wextra -O2 -g     -c -o src/sub.o src/sub.c
gcc   -Wall -Wextra -O2 -g     -c -o src/mul.o src/mul.c
gcc   -c src/add.c src/sub.c src/mul.c
ar -rc libmathfunc.a   src/add.o src/sub.o src/mul.o
mv *.o obj/
mv src/*.o obj/
mv src/*.d obj/
mv libmathfunc.so   obj/
gcc -Wall -o app src/app.c -L./obj -lmathfunc -I./hdr
mv app obj

Executing the Application

$ ./app
Sum = 6
Sub = -2
Product = 8

See Also

• Other topics of linux x86

  • Adding Hello World Application

  • Adding Static Library

  • Adding Hello World Kernel Module

  • Adding Patch to an Application

  • Adding Patch to a Static Library

  • Adding Patch to a Shared Library

  • Adding Patch to Kernel Module

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