fesetflushtozero(3M)fesetflushtozero(3M)NAMEfesetflushtozero() - set floating-point underflow mode
The 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) under‐
flow mode. For arguments other than 1 or zero, the effect is unde‐
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
On PA-RISC systems, flush-to-zero mode allows the substitution of a
zero for denormal operands and operation results, without trapping into
On Integrity servers, flush-to-zero mode causes a zero to be substi‐
tuted for denormal results (but not operands), without trapping into
Flush-to-zero mode may offer a significant performance improvement for
To use this function, compile either with the default option or with
the and options. Make sure your program includes
For Integrity servers, specify on the compiler command line or place
the call to this function under the effect of an affirmative pragma:
If the pragma is placed outside of any top-level declarations in a
file, the pragma will apply to all declarations in the compilation fol‐
lowing the pragma until another pragma is encountered or until the end
of the file is reached.
If the pragma is placed at the beginning of a block (compound state‐
ment), the pragma will apply until another pragma is encountered or
until the end of the block is reached.
For PA-RISC, you might need to use the compiler option in order to pre‐
vent optimizations that can undermine the specified behavior of this
Link in the math library by specifying on the compiler or linker com‐
For more information, see the at the following site:
No errors are defined.
Save the current underflow mode, set flush-to-zero mode, and restore
the previous mode.
was developed by HP and is not required by any current standard.
SEE ALSOfegetflushtozero(3M), fenv(5).