fegetflushtozero(3M)fegetflushtozero(3M)NAMEfegetflushtozero() - get floating-point underflow mode
The function retrieves the value representing the current underflow
mode, which is either IEEE-754-compliant (gradual) underflow mode or
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 Itnaium(R)-based systems. In
IEEE-754-compliant mode, underflow cases 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 HP Integrity servers, flush-to-zero mode causes a zero to
be substituted for denormal results (but not operands), without trap‐
ping into the kernel. Flush-to-zero mode may offer a significant per‐
formance improvement for some applications.
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:
The function returns zero if the current underflow mode is
IEEE-754-compliant. The function returns 1 if the current underflow
mode is flush-to-zero.
On systems that do not support flush-to-zero mode, this function
returns an undefined value.
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 ALSOfesetflushtozero(3M), fenv(5).