C: Casting minimum 32-bit integer (-2147483648) to float gives positive number (2147483648.0)

问题

I was working on an embedded project when I ran into something which I thought was strange behaviour. I managed to reproduce it on codepad (see below) to confirm, but don't have any other C compilers on my machine to try it on them.

Scenario: I have a #define for the most negative value a 32-bit integer can hold, and then I try to use this to compare with a floating point value as shown below:

#define INT32_MIN (-2147483648L)

void main()
{
    float myNumber = 0.0f;
    if(myNumber > INT32_MIN)
    {
        printf("Everything is OK");
    }
    else
    {
        printf("The universe is broken!!");
    }
}

Codepad link: http://codepad.org/cBneMZL5

To me it looks as though this this code should work fine, but to my surprise it prints out The universe is broken!!.

This code implicitly casts the INT32_MIN to a float, but it turns out that this results in a floating point value of 2147483648.0 (positive!), even though the floating point type is perfectly capable of representing -2147483648.0.

Does anyone have any insights into the cause of this behaviour?

CODE SOLUTION: As Steve Jessop mentioned in his answer, limits.h and stdint.h contain correct (working) int range defines already, so I'm now using these instead of my own #define

PROBLEM/SOLUTION EXPLANATION SUMMARY: Given the answers and discussions, I think this is a good summary of what's going on (note: still read the answers/comments because they provide a more detailed explanation):

I'm using a C89 compiler with 32-bit longs, so any values greater than LONG_MAX and less or equal to ULONG_MAX followed by the L postfix have a type of unsigned long
(-2147483648L) is actually a unary - on an unsigned long (see previous point) value: -(2147483648L). This negation operation 'wraps' the value around to be the unsigned long value of 2147483648 (because 32-bit unsigned longs have the range 0 - 4294967295).
This unsigned long number looks like the expected negative int value when it gets printed as an int or passed to a function because it is first getting cast to an int, which is wrapping this out-of-range 2147483648 around to -2147483648 (because 32-bit ints have the range -2147483648 to 2147483647)
The cast to float, however, is using the actual unsigned long value 2147483648 for conversion, resulting in the floating-point value of 2147483648.0.

回答1:

In C89 with a 32 bit long, 2147483648L has type unsigned long int (see 3.1.3.2 Integer constants). So once modulo arithmetic has been applied to the unary minus operation, INT32_MIN is the positive value 2147483648 with type unsigned long.

In C99, 2147483648L has type long if long is bigger than 32 bits, or long long otherwise (see 6.4.4.1 Integer constants). So there is no problem and INT32_MIN is the negative value -2147483648 with type long or long long.

Similarly in C89 with long larger than 32 bits, 2147483648L has type long and INT32_MIN is negative.

I guess you're using a C89 compiler with a 32 bit long.

One way to look at it is that C99 fixes a "mistake" in C89. In C99 a decimal literal with no U suffix always has signed type, whereas in C89 it may be signed or unsigned depending on its value.

What you should probably do, btw, is include limits.h and use INT_MIN for the minimum value of an int, and LONG_MIN for the minimum value of a long. They have the correct value and the expected type (INT_MIN is an int, LONG_MIN is a long). If you need an exact 32 bit type then (assuming your implementation is 2's complement):

for code that doesn't have to be portable, you could use whichever type you prefer that's the correct size, and assert it to be on the safe side.
for code that has to be portable, search for a version of the C99 header stdint.h that works on your C89 compiler, and use int32_t and INT32_MIN from that.
if all else fails, write stdint.h yourself, and use the expression in WiSaGaN's answer. It has type int if int is at least 32 bits, otherwise long.

回答2:

Replace

#define INT32_MIN (-2147483648L)

with

#define INT32_MIN (-2147483647 - 1)

-2147483648 is interpreted by the compiler to be the negation of 2147483648, which causes overflow on an int. So you should write (-2147483647 - 1) instead.
This is all C89 standard though. See Steve Jessop's answer for C99.
Also long is typically 32 bits on 32-bit machines, and 64 bits on 64-bit machines. int here gets the things done.

来源：https://stackoverflow.com/questions/11536389/c-casting-minimum-32-bit-integer-2147483648-to-float-gives-positive-number

标签

floating-point

int

long-integer

minimum