IOCTL

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What is it?

       The ioctl() system call manipulates the underlying device parameters
       of special files.  In particular, many operating characteristics of
       character special files (e.g., terminals) may be controlled with
       ioctl() requests.  The argument fd must be an open file descriptor.

       The second argument is a device-dependent request code.  The third
       argument is an untyped pointer to memory.  It's traditionally char
       *argp (from the days before void * was valid C), and will be so named
       for this discussion.

       An ioctl() request has encoded in it whether the argument is an in
       parameter or out parameter, and the size of the argument argp in
       bytes.  Macros and defines used in specifying an ioctl() request are
       located in the file <sys/ioctl.h>.