How to call exported kernel module functions from another module?

Question

I\'m writing an API as a kernel module that provides device drivers with various functions. I wrote three functions in mycode.c. I then built and loaded the

Answer 1

Minimal QEMU + Buildroot example

I have tested the following in a fully reproducible QEMU + Buildroot environment, so maybe having this working version version will help you find out what is wong with your code.

GitHub upstream is centered on the files:

• dep.c
• dep2.c
• Makefile

dep.c

#include  /* usleep_range */
#include 
#include 
#include 

MODULE_LICENSE("GPL");

int lkmc_dep = 0;
EXPORT_SYMBOL(lkmc_dep);
static struct task_struct *kthread;

static int work_func(void *data)
{
    while (!kthread_should_stop()) {
        printk(KERN_INFO "%d\n", lkmc_dep);
        usleep_range(1000000, 1000001);
    }
    return 0;
}

static int myinit(void)
{
    kthread = kthread_create(work_func, NULL, "mykthread");
    wake_up_process(kthread);
    return 0;
}

static void myexit(void)
{
    kthread_stop(kthread);
}

module_init(myinit)
module_exit(myexit)

dep2.c

#include  /* usleep_range */
#include 
#include 
#include 

MODULE_LICENSE("GPL");

extern int lkmc_dep;
static struct task_struct *kthread;

static int work_func(void *data)
{
    while (!kthread_should_stop()) {
        usleep_range(1000000, 1000001);
        lkmc_dep++;
    }
    return 0;
}

static int myinit(void)
{
    kthread = kthread_create(work_func, NULL, "mykthread");
    wake_up_process(kthread);
    return 0;
}

static void myexit(void)
{
    kthread_stop(kthread);
}

module_init(myinit)
module_exit(myexit)

And now you can do:

insmod dep.ko
insmod dep2.ko

With that Buildroot setup, things are already configuring depmod /lib/module/*/depmod with the dependency, so just this is enough to load both:

modprobe dep

Also, if you built your kernel with CONFIG_KALLSYMS_ALL=y, then the exported symbol can be seen with:

grep lkmc_dep /proc/kallsyms

see also: Does kallsyms have all the symbol of kernel functions?