use and create library in c

#ifndef lib_h_
#def lib_h_
#define MAX_FOO  20

// a type definition exported by library:
struct foo_struct {
    int x;
    float y;
};
typedef struct foo_struct foo_struct;

// a global variable exported by library
// "extern" means that this is not a variable declaration, it
// just defines that a variable named total_foo of type int
// exits and you can use it (its declaration is in some library source file)
extern int total_foo;

// a function prototype for a function exported by library:
extern int foo(float y, float z);   // a very bad function name

#endif

• constants, struct and typedef (that are used in the program and you’ll need to use to pass argument to the library functions)
• typically, variables used by library functions, like “global offset”.
• how to call a library function
• all variables and functions here (without any implementation) need the keyword extern. This keyword sort of registered the variables and functions here so that the compiler would know how to find them. both lib.c and your application app.c would include this lib.h, meaning identical extern int total_foo; would appear in both.

you application would be a c source file like app.c

#include "lib.h"
int main (){
    total_foo = 2;
    foo(1.1, 2.2);
}

• use the variables directly.(don’t do int total_foo = 2;, would report error)

largely refered to this tutorial