Adding Static Library

In this section, you are going to learn

What is static library ?

How to create static library ?

How to link static library with object files ?

Topics in this section,

Let us answer few basic questions about static 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 Static Library ?

See Answer

  • Collection of compiled object code that can be linked directly to the program at the compile time.

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

See Answer

  • ar command is used to generate the static library.

ar rcs <YOUR_LIBRARY_NAME> <OBJECT_FILE_NAMES>

What command is used to link the generated static library in compile time ?

See Answer

  • To use the static library we must use this gcc command

gcc sample.c -o sample -L<YOUR_LIBRARY_FILE_PATH> -l<YOUR_LIBRARY_NAME>

  • -L specifies the directory path where the library is located.

  • -l links with the specified library

In ar command what option “t” does ?

See Answer

  • option t is used to display the table listing of the object files linked with the static library.

$ ar t libmathfunc.a
add.o
mul.o
sub.o

What “nm” command does ?

See Answer

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

$ nm libmathfunc.a

add.o:
0000000000000000 T sum

mul.o:
0000000000000000 T mul

sub.o:
0000000000000000 T sub

Why the size of executable file linked with static library file is greater when compared to shared library file ?

See Answer

  • The linking of static library file happens at compile time.

  • This results in copying the whole static library file with all its function to the executable bin.

Let us now explore it in depth !

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

1int sum(int a,int b);
2int sub(int a,int b);
3int mul(int a,int b);

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

1int sum (int a,int b)
2{
3	return a+b;
4}

1int sub (int a, int b)
2{
3	return a-b;
4}

1int mul (int a, int b)
2{
3	return a*b;
4}

  • Step 3: Generate Static Library using ar command.

    $ gcc -c add.c
$ gcc -c sub.c
$ gcc -c mul.c

$ ar rcs libmathfunc.a add.o sub.o mul.o

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

     1#include <stdio.h>
 2#include "math.h"
 3
 4int main(void)
 5{
 6    int x = 2;
 7    int y = 4;
 8    int result;
 9
10    result = sum(x, y);
11    printf("Sum = %d\n", result);
12
13    result = sub(x, y);
14    printf("Sub = %d\n", result);
15
16    result = mul(x, y);
17    printf("Product = %d\n", result);
18
19    return 0;
20}

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

    gcc -o output app.c -L. -lmathfunc

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

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

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

1int sum(int a,int b);
2int sub(int a,int b);
3int mul(int a,int b);

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

1int sum (int a,int b)
2{
3	return a+b;
4}

1int sub (int a, int b)
2{
3	return a-b;
4}

1int mul (int a, int b)
2{
3	return a*b;
4}

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

 1CC = gcc  
 2CFLAGS = -Wall -Wextra -O2 -g  
 3RM = rm -f 
 4TARGET_LIB = libmathfunc.a  
 5APP = app
 6
 7SRCS = src/add.c src/sub.c src/mul.c  
 8OBJS = $(SRCS:.c=.o)
 9
10.PHONY: all
11all: ${TARGET_LIB} ${APP}
12
13${APP}:
14	gcc -Wall -o app src/app.c -L./obj -lmathfunc -I./hdr
15	mv app bin
16
17$(TARGET_LIB): $(OBJS)
18	$(CC) -c src/add.c src/sub.c src/mul.c
19	ar -rc $(TARGET_LIB) src/add.o src/sub.o src/mul.o
20	mv *.o obj/
21	mv src/*.o obj/
22	mv src/*.d obj/
23	mv $(TARGET_LIB) obj/
24
25$(SRCS:.c=.d):%.d:%.c
26	$(CC) $(CFLAGS) -MM $< >$@
27
28include $(SRCS:.c=.d)
29
30.PHONY: clean
31clean:
32	rm -f obj/*
33	rm -f src/*.d
34

$ 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.a   obj/
gcc -Wall -o app src/app.c -L./obj -lmathfunc -I./hdr
mv app bin

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