module doesn't work when it's app uses STDOUT

Hello,

sorry, this might be a beginner question, but I couldn't find any infos in the FAQ.

I'm writing a driver (module) for a PCI card with a neural processor (NP-processor) on it for kernel 2.4.1. The registers of this processor are mapped to a memory area. I access the different registers of the NP-processor with ioctl's (there are 8bit and 16bit registers):

    switch (cmd) {
      case NP_IOCSGCR:
          get_user (b, (byte *) arg);
          writeb (b, np.mem + Z_GCR);
          break;
      case NP_IOCGGCR:
          b = readb (np.mem + Z_GCR);
          put_user (b, (byte *) arg);
          break;
      case NP_IOCSMIF:
          get_user (w, (word *) arg);
          writew (w, np.mem + Z_MIF);
          break;
      case NP_IOCGMIF:
          w = readw (np.mem + Z_MIF);
          put_user (w, (word *) arg);
          break;

When I load the module and init the device, I can read the registers. Some of them have default values after a reset of the chip and they are ok. But when I load data to the registers and let the NP-processor categorize the data, strange things happen:

I wrote a program to test the driver which loads some data to the NP-processor and tries to categorize it. The program prints its result to STDOUT.
- everything works ok, results are correct, if I redirect STDOUT to a file
- if I don't redirect, I get reproducibly wrong results when I print to STDOUT

My first idea was to disable/enable interrupts when entering/leaving the ioctl-function (using save_flags(); cli(); and restore_flags();), but it
didn't help.

Armin.