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characters

在头文件<fenv.h>中定义



int fegetenv(fenv_t * envp);

(1)

(自C99以来)

int fesetenv(const fenv_t * envp);

(2)

(自C99以来)

1)试图将浮点环境的状态存储在envp指向的对象中。

2)尝试从envp指向的对象中建立浮点环境。 该对象的值必须事先通过调用feholdexcept或fegetenv获得,或者是浮点宏常量。 如果在envp中设置了任何浮点状态标志,它们将在环境中设置(然后可用fetestexcept进行测试),但不会引发相应的浮点异常(执行继续不间断)。

参数

envp

-

pointer to the object of type fenv_t which holds the status of the floating-point environment

返回值

成功时为0,否则为非零。

#include <stdio.h>#include <math.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){
    fenv_t curr_env;
    int rtn; 
    /* Show default environment. */    show_fe_environment();    printf("\n"); 
    /* Perform some computation under default environment. */    printf("+11.5 -> %+4.1f\n", rint(+11.5)); /* midway between two integers */    printf("+12.5 -> %+4.1f\n", rint(+12.5)); /* midway between two integers */    show_fe_environment();    printf("\n"); 
    /* Save current environment. */
    rtn = fegetenv(&curr_env); 
    /* Perform some computation with new rounding method. */    feclearexcept(FE_ALL_EXCEPT);    fesetround(FE_DOWNWARD);    printf("1.0/0.0 = %f\n", 1.0/0.0);    printf("+11.5 -> %+4.1f\n", rint(+11.5));    printf("+12.5 -> %+4.1f\n", rint(+12.5));    show_fe_environment();    printf("\n"); 
    /* Restore previous environment. */
    rtn = fesetenv(&curr_env);    show_fe_environment(); 
    return 0;}

输出:

current exceptions raised: none
current rounding method:   FE_TONEAREST 
+11.5 -> +12.0+12.5 -> +12.0current exceptions raised: FE_INEXACT
current rounding method:   FE_TONEAREST 
1.0/0.0 = inf+11.5 -> +11.0+12.5 -> +12.0current exceptions raised: FE_DIVBYZERO FE_INEXACT
current rounding method:   FE_DOWNWARD
 
current exceptions raised: FE_INEXACT
current rounding method:   FE_TONEAREST

参考

  • C11标准(ISO / IEC 9899:2011):

    • 7.6.4.1 fegetenv函数(p:213)

    • 7.6.4.3 fesetenv函数(p:214)

  • C99标准(ISO / IEC 9899:1999):

    • 7.6.4.1 fegetenv函数(p:194)

    • 7.6.4.3 fesetenv函数(p:195)

扩展内容

feholdexcept(C99)

保存环境,清除所有状态标志并忽略所有将来的错误(功能)

feupdateenv(C99)

恢复浮点环境并引发以前引发的异常(函数)

FE_DFL_ENV(C99)

默认浮点环境(宏常量)

| fegetenv,fesetenv的C ++文档 |

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