HP-UX Reference (11i v2 03/08) - 3 Library Functions A-M (vol 6)

f
fesetflushtozero(3M) fesetflushtozero(3M)
NAME
fesetflushtozero( ) - set floating-point underflow mode
SYNOPSIS
#include <fenv.h>
void fesetflushtozero(int);
DESCRIPTION
The fesetflushtozero()
function sets the current underflow mode. If the argument is 1, the
underflow mode is set to flush-to-zero mode. If the argument is zero, the underflow mode is set to IEEE-
754-compliant (gradual) underflow mode. For arguments other than 1 or zero, the effect is undefined.
The default underflow mode is IEEE-754-compliant.
Flush-to-zero mode, also known as fast underflow mode, is supported on most PA1.1 systems and on all
PA2.0 and Itanium(R)-based systems. In IEEE-754-compliant mode, cases that might underflow may be
handled by trapping into the kernel, where the IEEE-mandated conversion of the result into a denormal-
ized value or zero is accomplished by software emulation.
On PA-RISC systems, flush-to-zero mode allows the substitution of a zero for denormal operands and
operation results, without trapping into the kernel.
On Itanium-based systems, flush-to-zero mode causes a zero to be substituted for denormal results (but
not operands), without trapping into the kernel.
Flush-to-zero mode may offer a significant performance improvement for some applications.
USAGE
To use this function, compile either with the default
-Ae option or with the -Aa
and -D_HPUX_SOURCE
options. Make sure your program includes <fenv.h>.
For Itanium-based systems, specify
+Ofenvaccess on the compiler command line or place the call to
this function under the effect of an affirmative
FENV_ACCESS pragma:
#pragma STDC FENV_ACCESS ON
If the FENV_ACCESS pragma is placed outside of any top-level declarations in a file, the pragma will
apply to all declarations in the compilation following the pragma until another FENV_ACCESS pragma
is encountered or until the end of the file is reached.
If the
FENV_ACCESS pragma is placed at the beginning of a block (compound statement), the pragma
will apply until another FENV_ACCESS pragma is encountered or until the end of the block is reached.
For PA-RISC, you might need to use the
+Onomoveflops compiler option in order to prevent optimiza-
tions that can undermine the specified behavior of this function.
Link in the math library by specifying
-lm on the compiler or linker command line.
For more information, see the HP-UX Floating-Point Guide.
RETURN VALUE
None.
ERRORS
No errors are defined.
EXAMPLES
Save the current underflow mode, set flush-to-zero mode, and restore the previous mode.
#include <fenv.h>
/*...*/
int fm_saved;
fm_saved = fegetflushtozero();
fesetflushtozero(1);
/*...*/
fesetflushtozero(fm_saved);
Section 3−−292 Hewlett-Packard Company − 1 − HP-UX 11i Version 2: August 2003