NAME

ieee_flags − mode and status function for IEEE standard arithmetic

SYNOPSIS

#include <sys/ieeefp.h>

int ieee_flags(action, mode, in, out)
char ∗action, ∗mode, ∗in, ∗∗out;

DESCRIPTION

This function provides easy access to the modes and status required to fully exploit ANSI/IEEE Std 754-1985 arithmetic in a C program.  All arguments are pointers to strings.  Results arising from invalid arguments and invalid combinations are undefined for efficiency. 

There are four types of action: get, set, clear and clearall.  There are three valid settings for mode, two corresponding to modes of IEEE arithmetic:

direction current rounding direction mode

precision current rounding precision mode

and one corresponding to status of IEEE arithmetic:

exception accrued exception-occurred status

There are fourteen types of in and out:

nearest round toward nearest

tozero round toward zero

negative round toward negative infinity

positive round toward positive infinity

extended

double

single

inexact

division division by zero exception

underflow

overflow

invalid

all all five exceptions above

common invalid, overflow, and division exceptions

Note: all and common only make sense with set or clear. 

For clearall, ieee_flags() returns 0 and restores all default modes and status.  Nothing will be assigned to out. Thus

char ∗mode, ∗out, ∗in;
ieee_flags("clearall", mode, in, &out);

set rounding direction to nearest, rounding precision to extended, and all accrued exception-occurred status to zero. 

For clear, ieee_flags() returns 0 and restores the default mode or status.  Nothing will be assigned to out. Thus

char ∗out, ∗in;
ieee_flags("clear", "direction", in, &out);       ... set rounding direction to round to nearest.

For set, ieee_flags() returns 0 if the action is successful and 1 if the corresponding required status or mode is not available (for instance, not supported in hardware). Nothing will be assigned to out. Thus

char ∗out, ∗in;
ieee_flags ("set", "direction", "tozero", &out);set rounding direction to round toward zero;

For get, we have the following cases:

Case 1: mode is direction.  In that case, out returns one of the four strings nearest, tozero, positive, negative, and ieee_flags() returns a value corresponding to out according to the enum fp_direction_type defined in <sys/ieeefp.h>. 

Case 2: mode is precision.  In that case, out returns one of the three strings extended, double and single, and ieee_flags() returns a value corresponding to out according to the enum fp_precision_type defined in <sys/ieeefp.h>. 

Case 3: mode is exception.  In that case, out returns

not available if information on exception is not available. 

no exception if no accrued exception. 

the accrued exception that has the highest priority according to the following list:

the exception named by in
invalid
overflow
division
underflow
inexact

In this case ieee_flags() returns a five or six bit value where each bit (see enum fp_exception_type in <sys/ieeefp.h>) corresponds to an exception-occurred accrued status flag: 0 = off, 1 = on.  The bit corresponding to a particular exception varies among architectures (see <sys/ieeefp.h>). 

Example:

char ∗out; int k, ieee_flags();
ieee_flags("clear", "exception", "all", &out);/∗ clear all accrued exceptions ∗/
...
code that generates three exceptions: overflow, invalid, inexact
...
k = ieee_flags("get", "exception", "overflow", &out);

then out is overflow, and on a Sun-3, k is 25. 

Sun Release 4.1  —  Last change: 16 May 1989