Re: kvm programming tutorial

[Artur Grabowski <art@blahonga.org>]

Andreas Schuldei <andreas@schuldei.org> writes:

> I am looking for a straight forward example/tutorial of kvm
> programming.
> 
> I looked at ps and top src and do NOT consider them easy to
> understand.
> 
> I want to understand the organisation and structure of the
> returned data and how to use it.

You really, really don't want to use kvm.

But if you really need it here is a hack i sometimes use to extract stuff
from the kernel if it's not available through sysctl.

The argument is the symbol in kernel memory you want to look at. I assume that
it's size is sizeof(long).

//art

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <err.h>
#include <nlist.h>
#include <fcntl.h>
#include <kvm.h>

int
main(int argc, char **argv)
{
        struct nlist nl[2];
        kvm_t *kd;
        long i;

        if (argc != 2)
                err(1, "two arguments required");

        nl[0].n_name = argv[1];
        nl[1].n_name = NULL;

        if ((kd = kvm_open(NULL, NULL, NULL, O_RDONLY, "foo")) == NULL)
                err(1, "kvm_open");

        if (kvm_nlist(kd, nl) != 0)
                errx(1, "kvm_nlist: %s", kvm_geterr(kd));

        if (kvm_read(kd, nl[0].n_value, &i, sizeof(i)) != sizeof(i))
                errx(1, "kvm_read: %s", kvm_geterr(kd));

        kvm_close(kd);

        printf("%s: %ld\n", nl[0].n_name, i);

        return 0;
}