#include <QApplication>

#include "/usr/include/jack/jack.h"

#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

jack_port_t *input_port;
jack_port_t *output_port;

int
process (jack_nframes_t nframes, void *arg)
{
        jack_default_audio_sample_t *out =
                (jack_default_audio_sample_t *) jack_port_get_buffer (output_port, nframes);
        jack_default_audio_sample_t *in =
                (jack_default_audio_sample_t *) jack_port_get_buffer (input_port, nframes);

        memcpy (out, in, sizeof (jack_default_audio_sample_t) * nframes);

        return 0;
 }

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    jack_client_t *client = 0;
    const char **ports;

    client = jack_client_open("my_client", JackNoStartServer, NULL);

    jack_set_process_callback (client, process, 0);

    input_port = jack_port_register (client, "input",
                                     JACK_DEFAULT_AUDIO_TYPE, JackPortIsInput, 0);
    output_port = jack_port_register (client, "output",
                                      JACK_DEFAULT_AUDIO_TYPE, JackPortIsOutput, 0);

    if (jack_activate (client)) {
        fprintf (stderr, "cannot activate client");
        return 1;
    }
   
    if ((ports = jack_get_ports (client, NULL, NULL, JackPortIsPhysical|JackPortIsOutput)) == NULL) {
        fprintf(stderr, "Cannot find any physical capture ports\n");
        exit(1);
    }

    if (jack_connect (client, ports[0], jack_port_name (input_port))) {
        fprintf (stderr, "cannot connect input ports\n");
    }

    if ((ports = jack_get_ports (client, NULL, NULL, JackPortIsPhysical|JackPortIsInput)) == NULL) {
    fprintf(stderr, "Cannot find any physical playback ports\n");
    exit(1);
    }

    if (jack_connect (client, jack_port_name (output_port), ports[0])) {
    fprintf (stderr, "cannot connect output ports\n");
    }

    return a.exec();
}

Cannot open my_client client
Cannot read socket fd = 19 err = Success
CheckRes error
JackSocketClientChannel read fail

jack_set_process_callback called with a NULL client