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 14 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”, “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

(a) “not available” if information on exception is not available,
(b) “no exception” if no accrued exception,
(c) the accrued exception that has the highest priority according to the list below

(1) the exception named by in,
(2) “invalid”,
(3) “overflow”,
(4) “division”,
(5) “underflow”,
(6) “inexact”.

In this case ieee_flags() returns a five bit value where each bit (cf. 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. 

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 = “overflow”, and on a Sun-3, k=25. 

FILES

/usr/include/sys/ieeefp.h

/usr/lib/libm.a

Sun Release 4.0  —  Last change: 21 October 1987