C Function: return value incorrect?

Question

I have written a C library function, but the return value seems to be incorrect, even though it is correct in the function.

Here is relevant code:

(In dcml_private.c) The offending function:

dcml_status _dcml_get_status(struct dcml_device *dev)
{
    uint64_t data;
    dcml_status ret;
    int len;

    libusb_bulk_transfer(dev->handle,
                     DCML_ENDPOINT | LIBUSB_ENDPOINT_IN,
                     (unsigned char *) &data,
                     DCML_REPORT_SZ, &len, RX_TIMEOUT);

    printf("data = %ld\n", data);

    if (len != DCML_REPORT_SZ)
        printf("DCML: LIBUSB ERROR (%s)\n", libusb_error_name(len));
        return STATUS_UNKNOWN;

    ret = data & ~(1>>17);
    return (ret);
}

The calling function:

void _dcml_cmd(dcml_context *ctx, dcml_cmd cmd,
      dcml_status quit_cond, int dur)
{
    struct timeval start;
    struct timeval cur;
    uint32_t stat;

    (void)gettimeofday(&start, NULL);
    (void)gettimeofday(&cur, NULL);
    _dcml_send_cmd(ctx->active, cmd);

    while(difftimeval(cur, start) < dur) {
            sleep(POLL_PERIOD);
            stat = _dcml_get_status(ctx->active);

            printf("status (%d), quit_cond (%d)", stat, quit_cond);
            if (stat == quit_cond)
                  break;

            (void)gettimeofday(&cur, NULL);
    }

    _dcml_send_cmd(ctx->active, CMD_NONE);
}

As you see, I have print statements in my functions. In _dcml_cmd, typical output of that print statement would be

status (65535), quit_cond (2048)

Where _dcml_get_status prints:

data = 128

What this means is that the return value is correct IMMEDIATELY before exiting _dcml_get_status, but incorrect immediately after it is return to the calling function (and always has a value of 65535 here...)

It's probably helpful to know that "dmcl_status" is an enum. Switching the return type to uint16_t does not fix the issue. I thought it might be an overflow issue or something, but changing types, explicit casts and adding a mask line doesn't fix it.

Any thoughts?

Answer 1

It's because you have a bad habit of not putting { } after your if statements ALWAYS

   if (len != DCML_REPORT_SZ) {
        printf("DCML: LIBUSB ERROR (%s)\n", libusb_error_name(len));
        return STATUS_UNKNOWN;
   }

also use fprintf(stderr,... to dump out errors. Sending errors to printf is bad practice:

   if (len != DCML_REPORT_SZ) {
        fprintf(stderr, "DCML: LIBUSB ERROR (%s)\n", libusb_error_name(len));
        return STATUS_UNKNOWN;
   }

The calling function has the same issues around:

       printf("status (%d), quit_cond (%d)", stat, quit_cond);
        if (stat == quit_cond)
              break;

Use:

       fprintf(stderr, "status (%d), quit_cond (%d)", stat, quit_cond);
        if (stat == quit_cond) {
              break;
        }

Yes it uses up an extra line but darn it at 3:00AM when you are debugging this by adding fprintfs all over the place it won't break your logic. :^)