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# feholdexcept

From cppreference.com
< c‎ | numeric‎ | fenv

Defined in header `<fenv.h>` |  |
int feholdexcept( fenv_t* envp ); |  | (since C99)

First, saves the current floating-point environment to the object pointed to by `envp` (similar to fegetenv), then clears all floating-point status flags, and then installs the non-stop mode: future floating-point exceptions will not interrupt execution (will not trap), until the floating-point environment is restored by feupdateenv or fesetenv.

This function may be used in the beginning of a subroutine that must hide the floating-point exceptions it may raise from the caller. If only some exceptions must be suppressed, while others must be reported, the non-stop mode is usually ended with a call to feupdateenv after clearing the unwanted exceptions.

### [edit] Parameters

envp | - | pointer to the object of type fenv_t where the floating-point environment will be stored

### [edit] Return value

​0​ on success, non-zero otherwise.

### [edit] Example

Run this code

    #include <stdio.h>
    #include <fenv.h>
    #include <float.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");
    }

    double x2 (double x)   /* times two */
    {
        fenv_t curr_excepts;

        /* Save and clear current f-p environment. */
        feholdexcept(&curr_excepts);

        /* Raise inexact and overflow exceptions. */
        printf("In x2():  x = %f\n", x=x*2.0);
        show_fe_exceptions();
        feclearexcept(FE_INEXACT);   /* hide inexact exception from caller */

        /* Merge caller's exceptions (FE_INVALID)        */
        /* with remaining x2's exceptions (FE_OVERFLOW). */
        feupdateenv(&curr_excepts);
        return x;
    }

    int main(void)
    {
        feclearexcept(FE_ALL_EXCEPT);
        feraiseexcept(FE_INVALID);   /* some computation with invalid argument */
        show_fe_exceptions();
        printf("x2(DBL_MAX) = %f\n", x2(DBL_MAX));
        show_fe_exceptions();

        return 0;
    }

Output:

    current exceptions raised:  FE_INVALID
    In x2():  x = inf
    current exceptions raised:  FE_INEXACT FE_OVERFLOW
    x2(DBL_MAX) = inf
    current exceptions raised:  FE_INVALID FE_OVERFLOW

### [edit] References

- C11 standard (ISO/IEC 9899:2011):

  - 7.6.4.2 The feholdexcept function (p: 213-214)

- C99 standard (ISO/IEC 9899:1999):

  - 7.6.4.2 The feholdexcept function (p: 194-195)

### [edit] See also

feupdateenv (C99) | restores the floating-point environment and raises the previously raise exceptions (function) [edit]
fegetenv fesetenv (C99) | saves or restores the current floating point environment (function) [edit]
FE_DFL_ENV (C99) | default floating-point environment (macro constant) [edit]
C++ documentation for feholdexcept

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