Hello,
I have tried to understand how to open an L2CAP socket, and have used the BlueZ API. This is what I have got so far:
/*
* l2cap_listen(l2cap_channel)
* Use this channel as a listening post
*/
static int l2cap_listen(const bdaddr_t *bdaddr, unsigned short psm, int lm, int backlog)
{
    struct sockaddr_l2 addr;
    struct l2cap_options opts;
    int sk;

    /* allocate a free socket */
    if ((sk = socket(PF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP)) < 0)
        return -1;

    /* open connection to remote device */
    memset(&addr, 0, sizeof(addr));
    addr.l2_family = AF_BLUETOOTH;
    bacpy(&addr.l2_bdaddr, bdaddr); /* update source address of socket */
    addr.l2_psm = htobs(psm);

    /* bind socket to newly formed socket */
    if (bind(sk, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
        perror("ERROR: Could not bind socket");
        close(sk);
        return -1;
    }

    setsockopt(sk, SOL_L2CAP, L2CAP_LM, &lm, sizeof(lm));

    memset(&opts, 0, sizeof(opts));
    opts.imtu = HIDP_DEFAULT_MTU;
    opts.omtu = HIDP_DEFAULT_MTU;
    opts.flush_to = 0xffff;

    setsockopt(sk, SOL_L2CAP, L2CAP_OPTIONS, &opts, sizeof(opts));

    /* backlog: specifies the number of unaccepted connection
    * that the system will allow before refusing new connections
    */
    if (listen(sk, backlog) < 0) {
        close(sk);
        return -1;
    }

    return sk;
}

/*
* l2cap_connect_req()
* Establishes connection with specific socket
* Request to establish connection from remote
* communication partner.
* Such a socket has to have been previously created
* by an application with the command listen.
*/
static int l2cap_connect(bdaddr_t *src, bdaddr_t *dst, unsigned short psm)
{
    struct sockaddr_l2 addr;
    struct l2cap_options opts;
    int sk;

    /* socket allocation */
    if ((sk = socket(PF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP)) < 0)
        perror("ERROR: Could not create socket");
        return -1;

    memset(&addr, 0, sizeof(addr));
    addr.l2_family = AF_BLUETOOTH;
    bacpy(&addr.l2_bdaddr, src); /*update source address of socket */

    /* binds socket to psm */
    if (bind(sk, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
        close(sk);
        return -1;
    }

    memset(&opts, 0, sizeof(opts));
    opts.imtu = HIDP_DEFAULT_MTU;
    opts.omtu = HIDP_DEFAULT_MTU;
    opts.flush_to = 0xffff;

    setsockopt(sk, SOL_L2CAP, L2CAP_OPTIONS, &opts, sizeof(opts));

    memset(&addr, 0, sizeof(addr));
    addr.l2_family = AF_BLUETOOTH;
    bacpy(&addr.l2_bdaddr, dst);
    addr.l2_psm = htobs(psm);

    if (connect(sk, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
        close(sk);
        return -1;
    }

    return sk;
}

/*
* l2cap_accept()
* (Permits incoming connection)
* Blocks and accepts incoming connection
* Function switches from BT_LISTEN to BT_CONNECTED
*/
static int l2cap_accept(int sk, bdaddr_t *bdaddr)
{
    struct sockaddr_l2 addr;
    socklen_t addrlen;
    int nsk;

    memset(&addr, 0, sizeof(addr));
    addrlen = sizeof(addr);

    // accept one connection
    if ((nsk = accept(sk, (struct sockaddr *) &addr, &addrlen)) < 0)
        return -1;

    if (bdaddr)
        bacpy(bdaddr, &addr.l2_bdaddr);

    return nsk;
}
--------------------
I am eventually aiming to register the mouse HID events, and send it via the L2CAP socket to "control" another bluetooth-enabled PC.
How do I use an SDP service in conjunction with the existing L2CAP code? How do I use the control channel (L2CAP_PSM_HIDP_CTRL 0x11) and the interrupt channel (L2CAP_PSM_HIDP_INTR 0x13)
I think the control channel is used by SDP for signalling, (device discovery); the interrupt channel is used for data transfer (service discovery)
Thanks in advance for any help!