[LAD] C - Change in memory causing seg.fault (but why?)

Hello all,

I'm working on a little app utilizing 'lasound' and I've got bassically
what I want. However, somewhere there is a fault. Editing a random
variable causes a segmentation fault. I've tried gdb but except it
telling me where the application crashed (which I already knew) I did
not get anything useful out of it. To minimize my own potential errors I
have here below the example script from where I started (though I have
removed some unnecessary stuff) and commented out the 'long *outvl' and
'*outvl = -1'. Uncommenting, compile, and run this and I get a
segmentation fault.

The original script is available at the following link:
http://stackoverflow.com/questions/7657624/get-master-sound-volume-in-c-in-linux

Can someone tell me where this is going wrong?

Thanks in advance, Maarten

#include <unistd.h>
#include <fcntl.h>

#include <alsa/asoundlib.h>

/*
  Drawbacks. Sets volume on both channels but gets volume on one. Can be
easily adapted.
 */
int audio_volume(long* outvol)
{
    snd_mixer_t* handle;
    snd_mixer_elem_t* elem;
    snd_mixer_selem_id_t* sid;

    static const char* mix_name = "Fake";
    static const char* card = "default";
    static int mix_index = 0;

// long *outvl;
 // *outvl = -1;
    int ret = 0;
    long pmin, pmax;

    snd_mixer_selem_id_alloca(&sid);

    //sets simple-mixer index and name
    snd_mixer_selem_id_set_index(sid, mix_index);
    snd_mixer_selem_id_set_name(sid, mix_name);

        if ((snd_mixer_open(&handle, 0)) < 0)
        return -1;
    if ((snd_mixer_attach(handle, card)) < 0) {
        snd_mixer_close(handle);
        return -2;
    }
    if ((snd_mixer_selem_register(handle, NULL, NULL)) < 0) {
        snd_mixer_close(handle);
        return -3;
    }
    ret = snd_mixer_load(handle);
    if (ret < 0) {
        snd_mixer_close(handle);
        return -4;
    }
    elem = snd_mixer_find_selem(handle, sid);
    if (!elem) {
        snd_mixer_close(handle);
        return -5;
    }

    long minv, maxv;

    snd_mixer_selem_get_playback_volume_range (elem, &minv, &maxv);
    fprintf(stderr, "Volume range <%i,%i>\n", minv, maxv);

        if(snd_mixer_selem_get_playback_volume(elem, 0, outvol) < 0) {
            snd_mixer_close(handle);
            return -6;
        }

        fprintf(stderr, "Get volume %i with status %i\n", *outvol, ret);
        /* make the value bound to 100 */
        *outvol -= minv;
        maxv -= minv;
        minv = 0;
        *outvol = 100 * (*outvol) / maxv; // make the value bound from 0
to 100

    snd_mixer_close(handle);
    return 0;
    }

int main(void)
{
    long vol = -1;
    printf("Ret %i\n", audio_volume(&vol));

    return 0;
}

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Received on Fri Nov 9 00:15:02 2012