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fex_set_handling(3m)
Mathematical Library Functions fex_set_handling(3M)
NAME
fex_set_handling, fex_get_handling, fex_getexcepthandler, fex_setex‐
cepthandler - control floating point exception handling modes
SYNOPSIS
c99 [ flag... ] file... -lm [ library... ]
#include <fenv.h>
int fex_set_handling(int ex, int mode, void(*handler);
int fex_get_handling(int ex);
void fex_getexcepthandler(fex_handler_t *buf, int ex);
void fex_setexcepthandler(const fex_handler_t *buf, int ex);
DESCRIPTION
These functions provide control of floating point exception handling
modes. For each function, the ex argument specifies one or more excep‐
tions indicated by a bitwise-OR of any of the following values defined
in <fenv.h>:
FEX_INEXACT
FEX_UNDERFLOW
FEX_OVERFLOW
FEX_DIVBYZERO division by zero
FEX_INV_ZDZ 0/0 invalid operation
FEX_INV_IDI infinity/infinity invalid operation
FEX_INV_ISI infinity-infinity invalid operation
FEX_INV_ZMI 0*infinity invalid operation
FEX_INV_SQRT square root of negative operand
FEX_INV_SNAN signaling NaN
FEX_INV_INT invalid integer conversion
FEX_INV_CMP invalid comparison
For convenience, the following combinations of values are also defined:
FEX_NONE no exceptions
FEX_INVALID all invalid operation exceptions
FEX_COMMON overflow, division by zero, and invalid operation
FEX_ALL all exceptions
The fex_set_handling() function establishes the specified mode for han‐
dling the floating point exceptions identified by ex. The selected mode
determines the action to be taken when one of the indicated exceptions
occurs. It must be one of the following values:
FEX_NOHANDLER Trap but do not otherwise handle the exception, evok‐
ing instead whatever ambient behavior would normally
be in effect. This is the default behavior when the
exception's trap is enabled. The handler parameter is
ignored.
FEX_NONSTOP Provide the IEEE 754 default result for the operation
that caused the exception, set the exception's flag,
and continue execution. This is the default behavior
when the exception's trap is disabled. The handler
parameter is ignored.
FEX_ABORT Call abort(3C). The handler parameter is ignored.
FEX_SIGNAL Invoke the function *handler with the parameters nor‐
mally supplied to a signal handler installed with
sigfpe(3C).
FEX_CUSTOM Invoke the function *handler as described in the next
paragraph.
In FEX_CUSTOM mode, when a floating point exception occurs, the handler
function is invoked as though its prototype were:
#include <fenv.h>
void handler(int ex, fex_info_t *info);
On entry, ex is the value (of the first twelve listed above) corre‐
sponding to the exception that occurred, info->op indicates the opera‐
tion that caused the exception, info->op1 and info->op2 contain the
values of the operands, info->res contains the default untrapped result
value, and info->flags reflects the exception flags that the operation
would have set had it not been trapped. If the handler returns, the
value contained in info->res on exit is substituted for the result of
the operation, the flags indicated by info->flags are set, and execu‐
tion resumes at the point where the exception occurred. The handler
might modify info->res and info->flags to supply any desired result
value and flags. Alternatively, if the exception is underflow or over‐
flow, the hander might set
info->res.type = fex_nodata;
which causes the exponent-adjusted result specified by IEEE 754 to be
substituted. If the handler does not modify info->res or info->flags,
the effect is the same as if the exception had not been trapped.
Although the default untrapped result of an exceptional operation is
always available to a FEX_CUSTOM handler, in some cases, one or both
operands may not be. In these cases, the handler may be invoked with
info->op1.type == fex_nodata or info->op2.type == fex_nodata to indi‐
cate that the respective data structures do not contain valid data.
(For example, info->op2.type == fex_nodata if the exceptional operation
is a unary operation.) Before accessing the operand values, a custom
handler should always examine the type field of the operand data struc‐
tures to ensure that they contain valid data in the appropriate format.
The fex_get_handling() function returns the current handling mode for
the exception specified by ex, which must be one of the first twelve
exceptions listed above.
The fex_getexcepthandler() function saves the current handling modes
and associated data for the exceptions specified by ex in the data
structure pointed to by buf. The type fex_handler_t is defined in
<fenv.h>.
The fex_setexcepthandler() function restores the handling modes and
associated data for the exceptions specified by ex from the data struc‐
ture pointed to by buf. This data structure must have been set by a
previous call to fex_getexcepthandler(). Otherwise the effect on the
indicated modes is undefined.
RETURN VALUES
The fex_set_handling() function returns a non-zero value if the
requested exception handling mode is established. Otherwise, it returns
0.
EXAMPLES
The following example demonstrates how to substitute a predetermined
value for the result of a 0/0 invalid operation.
#include <math.h>
#include <fenv.h>
double k;
void presub(int ex, fex_info_t *info) {
info->res.type = fex_double;
info->res.val.d = k;
}
int main() {
double x, w;
int i;
fex_handler_t buf;
/*
* save current 0/0 handler
*/
(void) fex_getexcepthandler(&buf, FEX_INV_ZDZ);
/*
* set up presubstitution handler for 0/0
*/
(void) fex_set_handling(FEX_INV_ZDZ, FEX_CUSTOM, presub);
/*
* compute (k*x)/sin(x) for k=2.0, x=0.5, 0.4, ..., 0.1, 0.0
*/
k = 2.0;
(void) printf("Evaluating f(x) = (k*x)/sin(x)\n\n");
for (i = 5; i >= 0; i--) {
x = (double) i * 0.1;
w = (k * x) / sin(x);
(void) printf("\tx=%3.3f\t f(x) = % 1.20e\n", x, w);
}
/*
* restore old 0/0 handler
*/
(void) fex_setexcepthandler(&buf, FEX_INV_ZDZ);
return 0;
}
The output from the preceding program reads:
Evaluating f(x) = (k*x)/sin(x)
x=0.500 f(x) = 2.08582964293348816000e+00
x=0.400 f(x) = 2.05434596443822626000e+00
x=0.300 f(x) = 2.03031801709447368000e+00
x=0.200 f(x) = 2.01339581906893761000e+00
x=0.100 f(x) = 2.00333722632695554000e+00
x=0.000 f(x) = 2.00000000000000000000e+00
When x = 0, f(x) is computed as 0/0 and an invalid operation exception
occurs. In this example, the value 2.0 is substituted for the result.
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) cw(2.75i) lw(2.75i) lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE Availabilitysystem/library/math, SUNWlmxs Interface Sta‐
bilityCommitted MT-LevelMT-Safe (see Notes)
SEE ALSO
sigfpe(3C), feclearexcept(3M), fegetenv(3M), fex_set_log(3M),
attributes(7)
Numerical Computation Guide
NOTES
In a multithreaded application, the preceding functions affect excep‐
tion handling modes only for the calling thread.
The functions described on this page automatically install and dein‐
stall SIGFPE handlers and set and clear 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.
All traps are disabled before a handler installed in FEX_CUSTOM mode is
invoked. When the SIGFPE signal is blocked, as it is when such a han‐
dler is invoked, the floating point environment, exception flags, and
retrospective diagnostic functions described in feclearexcept(3M),
fegetenv(3M), and fex_set_log(3M) do not re-enable traps. Thus, the
handler itself always runs in FEX_NONSTOP mode with logging of retro‐
spective diagnostics disabled. Attempting to change these modes within
the handler may not produce the expected results.
Solaris 11.4 12 Jul 2006 fex_set_handling(3M)