我遇到了奇怪的行为。在这个程序中，我尝试检查浮点值是否为零:
#include <cstdlib>
#include <cmath>
#include <iostream>
int main ()
{
float fa (0), fb (0);
double da (0), db (0);
long double la (0), lb (0);
cout << "Float: " << (FP_ZERO == fpclassify (fa - fb) ) << endl;
cout << "Double: " << (FP_ZERO == fpclassify (da - db) ) << endl;
cout << "Long double: " << (FP_ZERO == fpclassify (la - lb) ) << endl;
cout << "Float: " << (FP_ZERO == fpclassify (fa - 42) ) << endl;
cout << "Double: " << (FP_ZERO == fpclassify (da - 42) ) << endl;
cout << "Long double: " << (FP_ZERO == fpclassify (la - 42) ) << endl;
return EXIT_SUCCESS;
}
程序的结果是可预测的:
$ ./llvlg
Float: 1
Double: 1
Long double: 1
Float: 0
Double: 0
Long double: 0
但是，如果我通过 Valgrind 启动程序，长双零的结果将是错误的:
$ valgrind ./llvlg
==7521== Memcheck, a memory error detector
==7521== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==7521== Using Valgrind-3.12.0.SVN and LibVEX; rerun with -h for copyright info
==7521== Command: ./llvlg
==7521==
Float: 1
Double: 1
Long double: 0
Float: 0
Double: 0
Long double: 0
==7521==
==7521== HEAP SUMMARY:
==7521== in use at exit: 0 bytes in 0 blocks
==7521== total heap usage: 2 allocs, 2 frees, 73,728 bytes allocated
==7521==
==7521== All heap blocks were freed -- no leaks are possible
==7521==
==7521== For counts of detected and suppressed errors, rerun with: -v
==7521== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
这是错误还是预期的行为？
更新:
可用的程序源代码 here .
构建选项:g++ main.cpp -O0 -o llvlg。
我的环境:
$ uname -a Linux tiptop 4.8.0-53-generic#56-Ubuntu SMP Tue May 16 00:23:44 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux$ gcc --version gcc (Ubuntu 6.2.0-5ubuntu12) 6.2.0 20161005$ valgrind --version valgrind-3.12.0.SVN$ cat/proc/cpuinfo ... 型号名称:Intel(R) Core(TM) i7-4700MQ CPU @ 2.40GHz ...
最佳答案
这是 Valgrind 的预期行为 - 一个已知的限制。内部 long double 仅以 64 位(双)精度表示。
参见 here .
关于c++ - 使用 Valgrind 的 std::fpclassify for long double 的错误结果，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/44316523/