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_control87, _controlfp

Get and set the floating-point control word.

unsigned int _control87( 
   unsigned int new,
   unsigned int mask 
unsigned int _controlfp( 
   unsigned int new,
   unsigned int mask 


New control-word bit values.
Mask for new control-word bits to set.

Return Value

The bits in the value returned indicate the floating-point control state. See FLOAT.H for a complete definition of the bits returned by _control87.


The _control87 function gets and sets the floating-point control word. The floating-point control word allows the program to change the precision, rounding, and infinity modes in the floating-point math package. You can also mask or unmask floating-point exceptions using _control87. If the value for mask is equal to 0, _control87 gets the floating-point control word. If mask is nonzero, a new value for the control word is set: For any bit that is on (equal to 1) in mask, the corresponding bit in new is used to update the control word. In other words, fpcntrl = ((fpcntrl & ~mask) | (new & mask)) where fpcntrl is the floating-point control word.

Note   The run-time libraries mask all floating-point exceptions by default.

_controlfp is a platform-independent, portable version of _control87. It is nearly identical to the _control87 function on Intel (x86) platforms and is also supported by the MIPS and ALPHA platforms. To ensure that your floating-point code is portable to MIPS or ALPHA, use _controlfp. If you are targeting x86 platforms, use either _control87 or _controlfp.

The difference between _control87 and _controlfp is the way these two functions treat DENORMAL values. For Intel (x86) platforms, _control87 can set and clear the DENORMAL OPERAND exception mask. ALPHA platforms do not support this exception, and _controlfp does not modify the DENORMAL OPERAND exception mask. The following example demonstrates the difference:

_control87( _EM_INVALID, _MCW_EM ); // DENORMAL is unmasked by this call
_controlfp( _EM_INVALID, _MCW_EM ); // DENORMAL exception mask remains unchanged

The possible values for the mask constant (mask) and new control values (new) are shown in the following Hexadecimal Values table. Use the portable constants listed below (_MCW_EM, _EM_INVALID, and so forth) as arguments to these functions, rather than supplying the hexadecimal values explicitly.

ALPHA platforms support the DENORMAL input and output values in software. The default behavior of Windows NT on these platforms is to flush the DENORMAL input and output values to zero. _controlfp provides a new mask to preserve and flush the input and output DENORMAL values.

Intel (x86) platforms support the DENORMAL input and output values in hardware. The behavior is to preserve DENORMAL values. _control87 does not provide a mask to change this behavior. The following example demonstrates this difference:

controlfp( _DN_SAVE, _MCW_DN);   // Denormal values preserved by software on ALPHA. NOP on x86
controlfp( _DN_FLUSH, _MCW_DN);   // Denormal values flushed to zero by hardware on Alpha. Ignored on x86

Hexadecimal Values

Regarding the _MCW_EM mask, clearing the mask sets the exception, which allows the hardware exception; setting the mask hides the exception. Note that if a _EM_UNDERFLOW or_EM_OVERFLOW occurs, no hardware exception will be thrown until the next floating point instruction is executed. To generate a hardware exception immediately after _EM_UNDERFLOW or_EM_OVERFLOW, call the FWAIT MASM instruction.

Mask Hex value Constant Hex value
_MCW_DN (Denormal control) 0x03000000 _DN_SAVE




_MCW_EM (Interrupt exception mask) 0x0008001F _EM_INVALID












_MCW_IC (Infinity control) 0x00040000 _IC_AFFINE




_MCW_RC (Rounding control) 0x00000300 _RC_CHOP








_MCW_PC (Precision control) 0x00030000 _PC_24 (24 bits)

_PC_53 (53 bits)

_PC_64 (64 bits)





Routine Required header Compatibility
_control87 <float.h> Win 98, Win Me, Win NT, Win 2000, Win XP
_controlfp <float.h> Win 98, Win Me, Win NT, Win 2000, Win XP

For additional compatibility information, see Compatibility in the Introduction.


All versions of the C run-time libraries.


// crt_cntrl87.c
/* This program uses _control87 to output the control 
 * word, set the precision to 24 bits, and reset the status to 
 * the default.

#include <stdio.h>
#include <float.h>

int main( void )
   double a = 0.1;

   /* Show original control word and do calculation. */
   printf( "Original: 0x%.4x\n", _control87( 0, 0 ) );
   printf( "%1.1f * %1.1f = %.15e\n", a, a, a * a );

   /* Set precision to 24 bits and recalculate. */
   printf( "24-bit:   0x%.4x\n", _control87( _PC_24, MCW_PC ) );
   printf( "%1.1f * %1.1f = %.15e\n", a, a, a * a );

   /* Restore to default and recalculate. */
   printf( "Default:  0x%.4x\n", 
          _control87( _CW_DEFAULT, 0xfffff ) );
   printf( "%1.1f * %1.1f = %.15e\n", a, a, a * a );


Original: 0x9001f
0.1 * 0.1 = 1.000000000000000e-002
24-bit:   0xa001f
0.1 * 0.1 = 9.999999776482582e-003
Default:  0x9001f
0.1 * 0.1 = 1.000000000000000e-002

See Also

Floating-Point Support Routines | _clear87 | _status87 | Run-Time Routines and .NET Framework Equivalents