Module structure
- C course
- Expert
- Lesson 8/47
- 5 min
Learn the structure of a kernel module.
Module structure and macros
A kernel module needs two special macros: module_init and module_exit. They name the functions to call on load and remove.
Meta macros like MODULE_LICENSE, MODULE_AUTHOR, MODULE_DESCRIPTION are required. Without MODULE_LICENSE("GPL"), the kernel treats the module as non-GPL and restricts access to some symbols. Module source includes <linux/module.h> and <linux/init.h>. This structure keeps the module compatible with the kernel.
Init and exit functions
The function declared with module_init is called when the module loads. The __init marker tells the kernel this code can be discarded after loading.
Similarly, the module_exit function is marked __exit and called only on removal. The init function returns 0 on success, a negative code on error. On error the kernel rolls back the module and refuses the load, so init functions check each resource and release earlier ones on the error path.
Practice Print module metadata
Define struct module_info { const char *key; const char *value; }. In main build a four-element array: {"module_init", "start"}, {"module_exit", "cleanup"}, {"MODULE_LICENSE", "GPL"}, {"MODULE_AUTHOR", "Ayşe"}. Loop with for and print each with printf("%s=%s\n", ...).
Starter code
#include <stdio.h>
// Define module_info struct
// Build a four-element array and print it with a for loop
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
struct module_info { const char *key; const char *value; };
int main(void) {
struct module_info info[] = {
{"module_init", "start"},
{"module_exit", "cleanup"},
{"MODULE_LICENSE", "GPL"},
{"MODULE_AUTHOR", "Ayşe"}
};
int n = sizeof(info) / sizeof(info[0]);
for (int i = 0; i < n; i++) {
printf("%s=%s\n", info[i].key, info[i].value);
}
return 0;
}Expected output
module_init=start module_exit=cleanup MODULE_LICENSE=GPL MODULE_AUTHOR=Ayşe
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match module macros
| Term | Meaning |
|---|---|
module_init |
Initialization |
module_exit |
Cleanup |
MODULE_LICENSE |
License |
MODULE_AUTHOR |
Author |
2 Which macro declares the module's init function?
- MODULE_AUTHOR
- printk
- MODULE_LICENSE
- module_init Correct answer
Explanation: module_init declares the load function.
3 Complete the MODULE_LICENSE call
MODULE_LICENSE("____");MODULE_LICENSE("GPL");Explanation: GPL gives full access to kernel symbols.