How does converting float to double work?

Question

double can represent every value a float can represent.

Does converting float to double simply extend the mantissa by adding 0 and extend the exponent part by filling sign bits?

I tested some data at http://www.binaryconvert.com/index.html . It works in this way. But I did not find any official definition of the conversion. Does the conversion have any corner case not working this way?

Answer 1

This has very little to do with C; its only guarantee is that converting from float to double must preserve the value.

So you're really asking about (presumably) IEEE-754 . There are at least a few ways in which your description doesn't hold:

1. The exponents for single-precision and double-precision have different biases. So conversion requires more than just sign extension.

2. Denormal numbers require special handling (they become normal numbers).

3. NaNs may also require special handling - distinguishing between signalling and quiet NaNs is dependent on the value of the mantissa.