gnu-c 对浮点型数据除0.0的处理

程序中有分母为 0 的情况发生，确切的说是 (double) 强制转换后的 浮点 0.0，结果发现输出的结果是 inf，即对于 如 1/(double)0的情形是 inf，而 如 0/(double)0 的情形是 -nan。inf 在比较时作为 正无穷 ∞ 处理，而 -nan 不可比较。

gcc c lib 手册 如下：

20.5.2 Infinity and NaN

IEEE 754 floating point numbers can represent positive or negative infinity, and NaN (not a number). These three values arise from calculations whose result is undefined or cannot be represented accurately. You can also deliberately set a floating-point variable to any of them, which is sometimes useful. Some examples of calculations that produce infinity or NaN:

1/0 = ∞
log (0) = -∞
sqrt (-1) = NaN
When a calculation produces any of these values, an exception also occurs; see FP Exceptions.

The basic operations and math functions all accept infinity and NaN and produce sensible output. Infinities propagate through calculations as one would expect: for example, 2 + ∞ = ∞, 4/∞ = 0, atan (∞) = π/2. NaN, on the other hand, infects any calculation that involves it. Unless the calculation would produce the same result no matter what real value replaced NaN, the result is NaN.

In comparison operations, positive infinity is larger than all values except itself and NaN, and negative infinity is smaller than all values except itself and NaN. NaN is unordered: it is not equal to, greater than, or less than anything, including itself. x == x is false if the value of x is NaN. You can use this to test whether a value is NaN or not, but the recommended way to test for NaN is with the isnan function (see Floating Point Classes). In addition, <, >, <=, and >= will raise an exception when applied to NaNs.

math.h defines macros that allow you to explicitly set a variable to infinity or NaN.

Macro: float INFINITY
An expression representing positive infinity. It is equal to the value produced by mathematical operations like 1.0 / 0.0. -INFINITY represents negative infinity.

You can test whether a floating-point value is infinite by comparing it to this macro. However, this is not recommended; you should use the isfinite macro instead. See Floating Point Classes.

This macro was introduced in the ISO C99 standard.

[1] https://www.gnu.org/software/libc/manual/html_node/Infinity-and-NaN.html