Defined in float.h, this function is
provided for compatibility with Microsoft products. It enables you
to control exception traps and rounding modes.
The function prototype for _controlfp() is:
unsigned int _controlfp(unsigned int new, unsigned int mask);
Note
This function requires you to select a floating-point model
that supports exceptions. For example, --fpmode=ieee_full or --fpmode=ieee_fixed.
_controlfp() also modifies a control
word using a mask to isolate the bits to modify. For every bit of mask that
is zero, the corresponding control word bit is unchanged. For every
bit of mask that is nonzero, the corresponding
control word bit is set to the value of the corresponding bit of new.
The return value is the previous state of the control word.
Note
This is different behavior to that of __ieee_status() or __fp_status(),
where you can toggle a bit by setting a zero in the mask word and
a one in the flags word.
Table 3 describes
the macros you can use to form the arguments to _controlfp().
Table 3. _controlfp argument macros
| Macro | Description |
|---|---|
_MCW_EM | Mask containing all exception bits |
_EM_INVALID | Bit describing the Invalid Operation exception |
_EM_ZERODIVIDE | Bit describing the Divide by Zero exception |
_EM_OVERFLOW | Bit describing the Overflow exception |
_EM_UNDERFLOW | Bit describing the Underflow exception |
_EM_INEXACT | Bit describing the Inexact Result exception |
_MCW_RC | Mask for the rounding mode field |
_RC_CHOP | Rounding mode value describing Round Toward Zero |
_RC_UP | Rounding mode value describing Round Up |
_RC_DOWN | Rounding mode value describing Round Down |
_RC_NEAR | Rounding mode value describing Round To Nearest |
Note
The values of these macros are not guaranteed to remain the same in future versions of ARM products. To ensure that your code continues to work if the value changes in future releases, use the macro rather than its value.
For example, to set the rounding mode to round down, call:
_controlfp(_RC_DOWN, _MCW_RC);
To trap the Invalid Operation exception and untrap all other exceptions:
_controlfp(_EM_INVALID, _MCW_EM);
To untrap the Inexact Result exception:
_controlfp(0, _EM_INEXACT);
See also- Tasks
Using ARM® C and C++ Libraries and Floating-Point Support:
- Reference
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