我可以让 gcc 在运行时告诉我计算结果是 NaN 还是 inf 吗?

问题描述

有没有办法告诉 gcc 抛出 SIGFPE 或类似的东西来响应在运行时导致 NaN 或 (-)inf 的计算，就像它会被零除吗?

Is there a way to tell gcc to throw a SIGFPE or something similar in response to a calculation that results in NaN or (-)inf at runtime, like it would for a divide-by-zero?

我尝试了 -fsignaling-nans 标志，但似乎没有帮助.

I've tried the -fsignaling-nans flag, which doesn't seem to help.

推荐答案

几乎任何从非 NaN 输入产生 NaN 的浮点运算或数学库函数也应该发出无效运算"浮点异常的信号；同样，从有限输入产生无穷大的计算通常会发出被零除"或溢出"浮点异常的信号.因此，您需要某种方式将这些异常转换为 SIGFPE.

Almost any floating-point operation or math library function that produces a NaN from non-NaN inputs should also signal the 'invalid operation' floating-point exception; similarly, a calculation that produces an infinity from finite inputs will typically signal either the 'divide-by-zero' or 'overflow' floating-point exception. So you want some way of turning these exceptions into a SIGFPE.

我怀疑答案将高度依赖于系统，因为浮点陷阱和标志的控制可能由平台 C 库而不是 gcc 本身提供.但这是一个适用于我的例子，在 Linux 上.它使用 fenv.h 中的 feenableexcept 函数._GNU_SOURCE 定义是声明此函数所必需的.

I suspect the answer will be highly system-dependent, since control of floating-point traps and flags is likely to be supplied by the platform C library rather than by gcc itself. But here's an example that works for me, on Linux. It uses the feenableexcept function from fenv.h. The _GNU_SOURCE define is necessary for this function to be declared.

#define _GNU_SOURCE
#include <fenv.h>

int main(void) {
    double x, y, z;
    feenableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);

    x = 1e300;
    y = 1e300;
    z = x * y; /* should cause an FPE */

    return 0;
}

警告:我认为在某些设置下，异常可能直到 next 浮点操作之后才真正生成(理论上)应该导致它，所以你有时需要无操作浮点运算(例如乘以 1.0)来触发异常.

A caveat: I think it's possible with some setups that the exception isn't actually generated until the next floating-point operation after the one that (in theory) should have caused it, so you sometimes need a no-op floating-point operation (e.g. multiplying by 1.0) to trigger the exception.

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