FE_DFL_ENV
定义于头文件 <fenv.h>
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#define FE_DFL_ENV /*implementation defined*/ |
(C99 起) | |
宏常量 FE_DFL_ENV 展开成 const fenv_t* 类型表达式,指向默认浮点环境,即在程序启动时加载的浮点环境的完整副本。
实现可以支持以 FE_
后随大写字母开始,且拥有 const fenv_t* 类型的附加宏。
示例
运行此代码
#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(void) { printf("On startup:\n"); show_fe_environment(); // 更改环境 fesetround(FE_DOWNWARD); // 更改舍入模式 feraiseexcept(FE_INVALID); // 引发异常 printf("\nBefore restoration:\n"); show_fe_environment(); fesetenv(FE_DFL_ENV); // 恢复 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
引用
- C11 standard (ISO/IEC 9899:2011):
- 7.6/9 Floating-point environment <fenv.h> (p: 208)
- C99 standard (ISO/IEC 9899:1999):
- 7.6/8 Floating-point environment <fenv.h> (p: 188-189)
参阅
(C99)(C99) |
保存或恢复当前浮点环境,包括异常的标志和数字的舍弃模式 (函数) |
(C99) |
恢复之前保存的浮点环境,并引发之前已经引发过的异常,使其存在于当前内存环境中 (函数) |