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Manual page for IEEE_FLAGS(3M)

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.