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在头文件<fenv.h>中定义 | ||
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#define FE_DFL_ENV / *实现定义* / | (自C99以来) |
宏常量FE_DFL_ENV扩展为类型const fenv_t *的表达式,该表达式指向默认浮点环境的完整副本,即在程序启动时加载的环境。
其他以FE_开头,后跟大写字母并且类型为const fenv_t *的宏可以由实现支持。
#include <stdio.h>#include <fenv.h> #pragma STDC FENV_ACCESS ON void show_fe_exceptions(void){ printf("current exceptions raised: "); if(fetestexcept(FE_DIVBYZERO)) printf(" FE_DIVBYZERO"); if(fetestexcept(FE_INEXACT)) printf(" FE_INEXACT"); if(fetestexcept(FE_INVALID)) printf(" FE_INVALID"); if(fetestexcept(FE_OVERFLOW)) printf(" FE_OVERFLOW"); if(fetestexcept(FE_UNDERFLOW)) printf(" FE_UNDERFLOW"); if(fetestexcept(FE_ALL_EXCEPT)==0) printf(" none"); printf("\n");} void show_fe_rounding_method(void){ printf("current rounding method: "); switch (fegetround()) { case FE_TONEAREST: printf ("FE_TONEAREST"); break; case FE_DOWNWARD: printf ("FE_DOWNWARD"); break; case FE_UPWARD: printf ("FE_UPWARD"); break; case FE_TOWARDZERO: printf ("FE_TOWARDZERO"); break; default: printf ("unknown"); }; printf("\n");} void show_fe_environment(void){ show_fe_exceptions(); show_fe_rounding_method();} int main(){ printf("On startup:\n"); show_fe_environment(); // Change environment fesetround(FE_DOWNWARD); // change rounding mode feraiseexcept(FE_INVALID); // raise exception printf("\nBefore restoration:\n"); show_fe_environment(); fesetenv(FE_DFL_ENV); // restore printf("\nAfter restoring default environment:\n"); show_fe_environment();}
输出:
On startup:current exceptions raised: none current rounding method: FE_TONEAREST Before restoration:current exceptions raised: FE_INVALID current rounding method: FE_DOWNWARD After restoring default environment:current exceptions raised: none current rounding method: FE_TONEAREST
C11标准(ISO / IEC 9899:2011):
7.6 / 9浮点环境<fenv.h>(p:208)
C99标准(ISO / IEC 9899:1999):
7.6 / 8浮点环境<fenv.h>(p:188-189)
fegetenvfesetenv(C99) | 保存或恢复当前的浮点环境(功能) |
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feupdateenv(C99) | 恢复浮点环境并引发以前引发的异常(函数) |
| FE_DFL_ENV的C ++文档 |