patty/src/ax25.c

#include <string.h>
#include <errno.h>

#include <patty/ax25.h>

struct _patty_ax25_address {
    char callsign[7];

    int ssid;
    int last;
    int repeated;
};

struct _patty_ax25_frame {
    struct _patty_ax25_address dest;
    struct _patty_ax25_address src;
    struct _patty_ax25_address repeaters[8];
    
    int hops;
    int repeated;

    enum patty_ax25_frame_type type;
    enum patty_ax25_proto      proto;

    uint8_t  control;
    uint8_t  pid;
    void *   info;
    uint16_t fcs;
};

static ssize_t frame_decode_address(patty_ax25_address *address, void *data, off_t offset) {
    int i, space = 0;
    uint8_t *info = (uint8_t *)data;

    /*
     * First, unpack each callsign octet, ensuring all are within the 7 bit
     * ASCII space and do not have the extended bit set to 1.
     */
    for (i=0; i<6; i++) {
        uint8_t c = (((uint8_t *)data) + offset)[i];

        if (!PATTY_AX25_ADDRESS_OCTET_VALID(c) || !PATTY_AX25_ADDRESS_OCTET_LAST(c)) {
            errno = EINVAL;

            goto error;
        }

        if (c == ' ') {
            /*
             * Take note if we have reached the end of the call sign.
             */
            if (space == 0) {
                space = 1;
            }

            address->callsign[i] = '\0';
        } else {
            /*
             * If we have already reached the end of the callsign, and a
             * non-space character is encountered, then the address is
             * erroneous.
             */
            if (space) {
                errno = EINVAL;

                goto error;
            }

            address->callsign[i] = c >> 1;
        }
    }

    /*
     * Now, unpack the data from the SSID field.
     */
    address->repeated = PATTY_AX25_ADDRESS_SSID_REPEATED(info[7]);
    address->ssid     = PATTY_AX25_ADDRESS_SSID_NUMBER(info[7]);
    address->last     = PATTY_AX25_ADDRESS_OCTET_LAST(info[7]);

    return 7;

error:
    return -1;
}

int patty_ax25_frame_decode(patty_ax25_frame *frame, void *data, size_t len) {  
    ssize_t decoded;
      off_t offset = 0;

    memset(frame, '\0', sizeof(*frame));

    if ((decoded = frame_decode_address(&frame->dest, data, offset)) < 0) {
        goto error_decode_dest;
    } else {
        offset += decoded;
    }

    /*
     * It would be considered erroneous if the destination address did have the
     * extension bit set to 1.
     */

    if ((decoded = frame_decode_address(&frame->src, data, offset)) < 0) {
        goto error_decode_src;
    } else {
        offset += decoded;
    }

    /*
     * Now, if the source address is not the final address in the frame, then
     * begin decoding repeater addresses.
     */
    if (!frame->src.last) {
        int i;

        for (i=0; i<PATTY_AX25_MAX_HOPS; i++) {
            if ((decoded = frame_decode_address(&frame->repeaters[i], data, offset)) < 0) {
                goto error_decode_repeater;
            }

            frame->hops++;

            if (frame->repeaters[i].last) {
                frame->repeated = frame->repeaters[i].repeated;

                break;
            }
        }

        /*
         * If the last hop does not have the address extension bit set, then
         * that's a big problem.
         */
        if (frame->hops && !frame->repeaters[i-1].last) {
            errno = EINVAL;

            goto error_decode_repeater;
        }
    }

    return 0;

error_decode_repeater:
error_decode_src:
error_decode_dest:
    return -1;
}