Question

I want to add a system call using KLD in FreeBSD 8.2 that has some arguments (1 argument here)
I’ve done the following (I’ve actually changed the the syscall.c in /usr/share/examples/kld/syscalls/module/syscall.c)

#include <sys/param.h>
#include <sys/proc.h>
#include <sys/module.h>
#include <sys/sysproto.h>
#include <sys/sysent.h>
#include <sys/kernel.h>
#include <sys/systm.h>

struct hellomet_args{
    int a;
};

static int
hellomet(struct thread *td, struct hellomet_args *arg)
{
    int a = arg->a;

    printf("hello secondish kernel %d  \n",a);
    return (0);
}

static struct sysent hellomet_sysent = {
    1,          
    hellomet            
};




static int offset = NO_SYSCALL;


static int
load(struct module *module, int cmd, void *arg)
{
    int error = 0;

    switch (cmd) {
    case MOD_LOAD :
        printf("syscall loaded at %d\n", offset);
        break;
    case MOD_UNLOAD :
        printf("syscall unloaded from %d\n", offset);
        break;
    default :
        error = EOPNOTSUPP;
        break;
    }
    return (error);
}

SYSCALL_MODULE(hellomet, &offset, &hellomet_sysent, load, NULL);

When I make this file using provided Makefile in module directory I get :

cc1: warnings being treated as errors syscall.c:56: warning: initialization from incompatible pointer type
*** Error code 1

Stop in /usr/share/examples/kld/syscall/module.
*** Error code 1

What’s the problem with this code?

Answer 1

You are initialising the sy_call member of hellomet_sysent with a function pointer that doesn’t match the typedef. sy_call is of type sy_call_t, which is defined as being a function that takes (struct thread* , void*) and returns int. Your call takes (struct thread*, struct hellomet_args *) instead.

Try something like this:

static struct sysent hellomet_sysent = {
    1,          
    (sy_call_t*) hellomet            
};