0x09-static_libraries

0x09. C - Static libraries

A static library is a collection of reusable code that can be shared and reused. A library is usually in form of an archive (ends with the .a extension).

How to create libraries from C source files

1. convert C source files to object files

Remember the stages of compilation

1. Preprocessing

2. Compilation

3. Assembly

4. Linking

An object file is a file that has not gone through the last stage of compilation - linking.

To create object files we need to invoke gcc with the -c option.

$ ls
add.c
subtract.c
$ gcc -c add.c subtract.c
$ ls
add.c
add.o
subtract.c
subtract.o

object files usually end with .o

2. Use ar to create a library

ar is the command we use to create libraries. We need to tell ar the name of the library we want to create.

Let's call our library "calculate" 😎.

$ ar -rc libcalculate.a add.o subtract.o
$ ls
add.c
add.o
libcalculate.a
subtract.c
subtract.o

libraries usually start with lib end with .a, hence libcalculate.a

What does the -rc mean??

Well, it tells ar to create a new libcalculate.h library if it doesn't exist, but if it does, it should recreate the library.

3. Index the library

This step is optional but pretty useful. When you index a library, you generate an index containing all the cool stuffs in the library. Which makes linking very fast!

How do you index a library??

by using a command called ranlib like this

$ ranlib libcalculate.h

4. Use your library 🕺

But first you need to do on little thing.

*Create an header file containing the prototypes of all the functions in your library, easy!.

Inside our calcute.h header file...

int add(int a, int b);

int subtract(int a, int b);

Pheew!, finally.

#include "calculate.h"
#include <stdio.h>

int main(void)
{
    int a = 5;
    int b = 3;
    int result;

    result = add(a, b);
    printf("%d + %d = %d\n", a, b, result);
    result = subtract(a, 3);
    printf("%d - %d = %d\n", a, b, result);

    return (0);
}

How do you compile??

Easy! You just need to tell gcc where to find your library and the name of you library.

$ gcc main.c -L . -l calculate

The -L option tells gcc where to find your library, and we supplied . which means your current working directory

The -l option tells gcc the name of the library without the lib prefix and the .a suffix

$ ./a.out
5 + 3 = 8
5 - 3 = 2

---

Tasks

0. A library is not a luxury but one of the necessities of life

Create the static library libmy.a containing all the functions listed below:

int _putchar(char c);
int _islower(int c);
int _isalpha(int c);
int _abs(int n);
int _isupper(int c);
int _isdigit(int c);
int _strlen(char *s);
void _puts(char *s);
char *_strcpy(char *dest, char *src);
int _atoi(char *s);
char *_strcat(char *dest, char *src);
char *_strncat(char *dest, char *src, int n);
char *_strncpy(char *dest, char *src, int n);
int _strcmp(char *s1, char *s2);
char *_memset(char *s, char b, unsigned int n);
char *_memcpy(char *dest, char *src, unsigned int n);
char *_strchr(char *s, char c);
unsigned int _strspn(char *s, char *accept);
char *_strpbrk(char *s, char *accept);
char *_strstr(char *haystack, char *needle);

library

1. Without libraries what have we? We have no past and no future

Create a script called create_static_lib.sh that creates a static library called liball.a from all the .c files that are in the current directory.

solution