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HP-UX 11i Version 3: February 2007

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fesetflushtozero() — set floating-point underflow mode


#include <fenv.h>

void fesetflushtozero(int);


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 HP Integrity servers. 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 denormalized 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 Integrity servers, 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.


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 Integrity servers, specify +Ofenvaccess on the compiler command line or place the call to this function under the effect of an affirmative FENV_ACCESS pragma:


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 optimizations 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 for HP Integrity servers at the following site: http://www.hp.com/go/fp.




No errors are defined.


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);


fesetflushtozero() was developed by HP and is not required by any current standard.

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