First of all go to /usr/src/include and create a header file with this structure:

NAME.h

_PROTOTYPE(int FUNCTION_NAME, (int PARAM));

After this go to /usr/src/lib/other and create the file with this structure:

NAME.c

#include<lib.h>
#include<unistd.h>
#include<stdio.h>

PUBLIC int FUNCTION_NAME(int PARAM) {
// WHAT TO DO?!
}

Edit the file Makefile.in in the same directory (/usr/src/lib/other). Add the name of your file in the list using alphabetical order.

What to do now?

You need to regenerate the Makefile.

$ cd /usr/src/lib/other/
$ make Makefile

Generate the libraries: (slow process)

$ cd /usr/src/tools
$ make libraries

And when it ends without erros "compile" the system:

$ cd /usr/src/tools
$ make hdboot

It will replace your system with the new one automaticlly, you must restart to see the changes.

Now how to test? Create a file in /root dir.

testTest.c

#include <stdio.h>
#include <unistd.h>
#include <lib.h>
#include <fcntl.h>
#include <NAME.h>

int main(int argc, char *argv[]) {
FUNCTION_NAME(PARAM);
}

Now you can create your own libraries to Minix system.

Example of library created by user to Minix

Best Regards,
Matheus