Call functions from outside a static library in C

Question

I need to generate a static library MyLib.lib that contains unimplemented functions.

Inside the project, I called the unimplemented function as shown below:

/* Inside MyLib.c */

#include "MyLib.h"

void foo(void)
{
   func(); // To be implemented by the user.
}

And In the header file MyLib.h, I included a header file

#include "user.h" // contains user_imlplementation_of_func()

...


#define func()         user_imlplementation_of_func()

To make things simple, let's just give an example of the user.c:

void user_imlplementation_of_func(void)
{
   printf("OK");
}

I would like to know is it possible to do this call? Otherwise, Is there any other solution to use unimplemented functions inside a static library and let the user define them after compressing the project from a source code to a .lib file

A static library is really nothing more than a collection of object files. Therefore calling "functions from outside a static library" is just the same as calling function from other object files. — Some programmer dude, Mar 05 '19 at 09:00
You may want to pass your user defined function as a callback to your library, take a look at https://stackoverflow.com/questions/142789/what-is-a-callback-in-c-and-how-are-they-implemented — kalehmann, Mar 05 '19 at 09:02
@Someprogrammerdude Correct me if I am wrong: If the user includes the library in his project and forget to add the `#define func() user_imlplementation_of_func()`, he will get a compilation problem. If he adds the define, everything is going to be fine? — Pryda, Mar 05 '19 at 09:03
Depending on your compiler you could implement a default 'weak' function and if the user wants his own, he could simply "overwrite" it. — theSealion, Mar 05 '19 at 09:20

kalehmann · Accepted Answer · 2019-03-05T09:53:22.627

You need to declare the unimplemented function as extern in the header of your library. This tells the compiler, that the function will be defined somewhere else.

Example:

MyLib.h

void foo();

extern void func();

MyLib.c

#include "MyLib.h"

void foo(void)
{
    func();
}

main.c

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

void func()
{
    printf("Hello, world!\n");
}

int main()
{
    foo();
}

Example build:

cc -o MyLib.o -c MyLib.c
cc -o out main.c MyLib.o

Output:

$ ./out
Hello, world!

However, for more readability I suggest you to pass your project functions as pointers to your library functions. This is commonly known as callback.

Example:

/* MyLib.c */

#include "MyLib.h"

void foo(void (*func)(void))
{
   func();
}

Now you can call the foo function in your project with:

foo(&user_imlplementation_of_func);

Edit:

As stated in the comments by the user theSealion a third solution is the usage of weak symbols. The wikipedia articel "Weak symbol" provides a good explanation with examples.

score 0 · Answer 2 · answered Mar 05 '19 at 09:37

You can do some else - function pointer:

//youLib.c

extern void (*func)(void) = NULL;

extern void foo()
{
    if (func != NULL) {
        func();
    }
}

//main.c

static void user_imlplementation_of_func(void)
{
    printf("Hello\n");
}

func = &user_imlplementation_of_func;
foo(); //Will printf "Hello"

Call functions from outside a static library in C

2 Answers2