Datasheet

ManualsBrandsARM ManualsComputer HardwareVERSION 1.2

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Floating-point Support

Then, if the

frob()

function raises Underflow, it is cleared again by

feclearexcept

, and

so no trap occurs when

feupdateenv

is called. However, if

frob()

raises Invalid

Operation, the sticky flag is set when

feupdateenv

is called, and so the trap handler is

invoked.

This mechanism is provided by C9X because C9X specifies no way to change exception

trapping for individual exceptions. A better method is to use

__ieee_status

to disable

the Underflow trap while leaving the Invalid Operation trap enabled. This has the

advantage that the Invalid Operation trap handler is provided with all the information

about the invalid operation (which operation was being performed on what data), and

can invent a result for the operation. Using the C9X method, the Invalid Operation trap

handler is called after the fact, receives no information about the cause of the exception,

and is called too late to provide a substitute result.

5.3.5 ARM extensions to the C9X interface

ARM provides some extensions to the C9X interface, to enable it to do everything that

the ARM floating-point environment is capable of. This includes trapping and

untrapping individual exception types, and also installing custom trap handlers.

The types

fenv_t

and

fexcept_t

are not defined by C9X to be anything in particular.

ARM defines them both to be the same structure type:

typedef struct {

unsigned statusword;

__ieee_handler_t invalid_handler;

__ieee_handler_t divbyzero_handler;

__ieee_handler_t overflow_handler;

__ieee_handler_t underflow_handler;

__ieee_handler_t inexact_handler;

} fenv_t, fexcept_t;

The members of the above structure are:

•

statusword

is the same status variable that the function

__ieee_status

sees, laid

out in the same format (see The __ieee_status function on page 5-8).

• five function pointers giving the address of the trap handler for each exception.

By default each is NULL. This means that if the exception is trapped then the

default exception trap action happens. The default is to cause a

SIGFPE

signal.

Writing custom exception trap handlers

If you want to install a custom exception trap handler, declare it as a function like this: