Safe function to add fixed point numbers clamping overflow.
Although fixed point numbers can be added with the normal '+' integer
operator, that doesn't provide any protection against overflow. If overflow
is a problem, you should use this function instead. It is slower than using
integer operators, but if an overflow occurs it will set `errno' and clamp
the result, rather than just letting it wrap. Example:
/* This will put 5035 into `result'. */
result = fixadd(itofix(5000), itofix(35));
/* Sets `errno' and puts -32768 into `result'. */
result = fixadd(itofix(-31000), itofix(-3000));
ASSERT(!errno); /* This will fail. */
Returns the clamped result of adding `x' to `y', setting `errno' to ERANGE
if there was an overflow.
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