Once you are equipped with the four methods just described, the driver can be compiled and tested; it retains any data you write to it until you overwrite it with new data. The device acts like a data buffer whose length is limited only by the amount of real RAM available. You can try using cp, dd, and input/output redirection to test the driver.
The free command can be used to see how the amount of free memory shrinks and expands according to how much data is written into scull.
To get more confident with reading and writing one quantum at a time, you can add a printk at an appropriate point in the driver and watch what happens while an application reads or writes large chunks of data. Alternatively, use the strace utility to monitor the system calls issued by a program, together with their return values. Tracing a cp or an ls -l > /dev/scull0 will show quantized reads and writes. Monitoring (and debugging) techniques are presented in detail in the next chapter.
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