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man pages section 3: Extended Library Functions, Volume 2

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Updated: Thursday, June 13, 2019
 
 

feupdateenv(3M)

Name

feupdateenv - update floating-point environment

Synopsis

c99 [ flag... ] file... –lm [ library... ]
#include <fenv.h>

int feupdateenv(const fenv_t *envp);

Description

The feupdateenv() function attempts to save the currently raised floating-point exceptions in its automatic storage, attempts to install the floating-point environment represented by the object pointed to by envp, and then attempts to raise the saved floating-point exceptions. The envp argument points to an object set by a call to fegetenv(3M) or feholdexcept(3M), or equals a floating-point environment macro.

Return Values

The feupdateenv() function returns 0 if and only if all the required actions were successfully carried out.

Errors

No errors are defined.

Examples

The following example demonstrates sample code to hide spurious underflow floating-point exceptions:

Example 1 Hide spurious underflow floating-point exceptions.
#include <fenv.h>
double f(double x)
{
#   pragma STDC FENV_ACCESS ON
    double result;
    fenv_t save_env;
    feholdexcept(&save_env);
    // compute result
    if (/* test spurious underflow */)
    feclearexcept(FE_UNDERFLOW);
    feupdateenv(&save_env);
    return result;
}

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
Availability
system/library/math
MT-Level
MT-Safe
Standard

See Also

fenv.h(3HEAD), fegetenv(3M), feholdexcept(3M), attributes(7), standards(7)

Notes

In a multithreaded program, the feupdateenv() function affects the floating point environment only for the calling thread.

When the FEX_CUSTOM handling mode is in effect for an exception, raising that exception using feupdateenv() causes the handling function to be invoked. The handling function can then modify the exception flags to be set as described in fex_set_handling(3M). Any result value the handler supplies will be ignored.

The feupdateenv() function automatically installs and deinstalls SIGFPE handlers and sets and clears the trap enable mode bits in the floating point status register as needed. If a program uses these functions and attempts to install a SIGFPE handler or control the trap enable mode bits independently, the resulting behavior is not defined.

As described in fex_set_handling(3M), when a handling function installed in FEX_CUSTOM mode is invoked, all exception traps are disabled (and will not be reenabled while SIGFPE is blocked). Thus, attempting to change the environment from within a handler by calling fesetenv(3M) or feupdateenv() might not produce the expected results.