How to cast or convert an unsigned int to int in C?

c casting types int unsigned
Eric Brotto picture Eric Brotto · Feb 26, 2011 · Viewed 124.7k times · Source

My apologies if the question seems weird. I'm debugging my code and this seems to be the problem, but I'm not sure.

Thanks!

Answer

Chris Cooper picture Chris Cooper · Feb 26, 2011

It depends on what you want the behaviour to be. An int cannot hold many of the values that an unsigned int can.

You can cast as usual:

int signedInt = (int) myUnsigned;

but this will cause problems if the unsigned value is past the max int can hold. This means half of the possible unsigned values will result in erroneous behaviour unless you specifically watch out for it.

You should probably reexamine how you store values in the first place if you're having to convert for no good reason.

EDIT: As mentioned by ProdigySim in the comments, the maximum value is platform dependent. But you can access it with INT_MAX and UINT_MAX.

For the usual 4-byte types:

4 bytes = (4*8) bits = 32 bits

If all 32 bits are used, as in unsigned, the maximum value will be 2^32 - 1, or 4,294,967,295.

A signed int effectively sacrifices one bit for the sign, so the maximum value will be 2^31 - 1, or 2,147,483,647. Note that this is half of the other value.

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