I saw a line of code like this:
xxxxx = (uint16_t) -1;
Why cast -1 into a unsigned int? What is this code to get?
Thanks!
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I saw a line of code like this:
xxxxx = (uint16_t) -1;
Why cast -1 into a unsigned int? What is this code to get?
Thanks!
Obviously the answer is within reach of your fingertips. Just plug that code in and step through and you will see the value that you get when you cast -1 to a 16 bit unsigned integer.
The value turns out to be two’s complement of -1, which is: 0xFFFF hex, or 65535 decimal.
As to the actual reason for using the code like that, it’s simply a short-cut. Or maybe it’s just to satisfy a type compatibility requirement.
If you’re wondering how come -1 gets cast to 0xFFFF (65535) and not maybe 0 or 1, as one might expect, you have to understand that the reason for that is that the C language, although statically typed, is quite liberal when it comes to enforcing type restrictions. That means that it will happily cast – or interpret if you will – any memory location as whatever arbitrary type of data you tell it. This of course can have quite devastating consequences if used improperly but the trade-off is flexibility and a speed improvement due to the lack of strict sanity checks. This was very important a few decades ago when C was designed and it still is if you’re writing code for very tiny processors.
That being said, if you think about a cast as simply saying: “disregard what you think you know about the data at this memory location, just tell me what it would mean if you read it as a
<insert_your_type_here>“ and if you know that computers usually represent negative numbers as two’s complement (see above) then the answer should by now by pretty obvious: C is taking the value in memory and reading it back as an unsigned integer.As an ending note. I should point out that C is not the only language that will cast -1 to 0xFFFF but even more modern languages that are capable of stronger type checks will do the same, probably for compatibility and continuity reasons, as well as for the reason that it makes it possible to reverse the cast: 0xFFFF back to a signed 16 bit integer is -1.