Strange behavior when casting an int to float in C

Question

I have a doubt concerning the output of the following C program. I tried to compile it using both Visual C++ 6.0 and MinGW32 (gcc 3.4.2).

#include 

        

Answer 1

This is certainly a compiler bug. From the C11 standard we have the following guarantees (C99 was similar):

• Types have a set of representable values (implied)
• All values representable by float are also representable by double (6.2.5/10)
• Converting float to double does not change the value (6.3.1.5/1)
• Casting int to float, when the int value is in the set of representable values for float, gives that value.
• Casting int to float, when the magnitude of the int value is less than FLT_MAX and the int is not a representable value for float, causes either the next-highest or next-lowest float value to be selected, and which one is selected is implementation-defined. (6.3.1.4/2)

The third of these points guarantees that the float value supplied to printf is not modified by the default argument promotions.

If 2147483647 is representable in float, then (float)x and (float)2147483647 must give 2147483647.000000.

If 2147483647 is not representable in float, then (float)x and (float)2147483647 must either give the next-highest or next-lowest float. They don't both have to make the same selection. But this means that a printout of 2147483647.000000 is not permitted¹, each must either be the higher value or the lower value.

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¹ Well - it's theoretically possible that the next-lowest float was 2147483646.9999999... so when the value is displayed with 6-digit precision by printf then it is rounded to give what was seen. But this isn't true in IEEE754 and you could easily experiment to discount this possibility.