C header files

#ifndef MYLIBRARY_H
#define MYLIBRARY_H

...

#endif

ifndef stands for if not defined.

This setup (header guard) stops the same thing being defined multiple times, and avoids circular dependancies etc

#pragma once does the exact same thing

---

Imagine some file a.c

#include <stdio.h>

int main() {
	int x = my_add(1, 2);
	printf("%d\n", x);
}

And you have my_add in b.c

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

Now you must make a header file, b.h, and then you can compile them like this:

[~/Desktop] 
$ ls
a.c  b.c  b.h 

[~/Desktop] 
$ cat b.h 
#pragma once

// (add documentation here)
int my_add(int a, int b);

[~/Desktop] 
$ gcc a.c b.c

[~/Desktop] 
$ ./a.out 
3

Success :)

---

Now how about if b.c was closed source, but you had an .so file and header file?

I’ll move b.c and b.h to a new folder, my_lib

and in a.c, rename #include "b.h" to #include "my_lib/b.h"

So now, gcc a.c my_lib/b.c would succeed

But, now let’s pretend b.c is closed source and we can’t use it

[~/Desktop] 
$ gcc a.c -L $(pwd)/my_lib -l b
/usr/bin/ld: cannot find -lb: No such file or directory
collect2: error: ld returned 1 exit status

ld is giving us an error.

you can debug with ld --verbose

$ ld -L $(pwd)/my_lib -l b --verbose
<...SNIP...>

==================================================
ld: mode elf_x86_64
attempt to open /home/connor/Desktop/my_lib/libb.so failed
<...SNIP...>

Of course, it does not find libb.so because we haven’t created it yet.

[~/Desktop/my_lib] 
$ ls
b.c  b.h

[~/Desktop/my_lib] 
$ gcc -fPIC -shared b.c -o libb.so

[~/Desktop/my_lib] 
$ ls
b.c  b.h  libb.so

[~/Desktop] 
$ gcc a.c -L $(pwd)/my_lib -l b

[~/Desktop] 
$ ./a.out 
./a.out: error while loading shared libraries: libb.so: cannot open shared object file: No such file or directory

Now compilation with linking succeeds but we need one final step to run it

[~/Desktop] 
$ ldd a.out 
        linux-vdso.so.1 (0x00007f3c2bf0a000)
        libb.so => not found
        libc.so.6 => /usr/lib/libc.so.6 (0x00007f3c2bce7000)
        /lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007f3c2bf0c000)

[~/Desktop] 
$ LD_LIBRARY_PATH=$(pwd)/my_lib ./a.out 
3