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.TH MATHERR 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
matherr \- SVID math library exception handling
.SH SYNOPSIS
.nf
.B #include <math.h>
.PP
.BI "int matherr(struct exception *" exc );
.PP
.B extern _LIB_VERSION_TYPE _LIB_VERSION;
.fi
.PP
Link with \fI\-lm\fP.
.SH DESCRIPTION
.IR Note :
the mechanism described in this page is no longer supported by glibc.
Before glibc 2.27, it had been marked as obsolete.
Since glibc 2.27,
.\" glibc commit 813378e9fe17e029caf627cab76fe23eb46815fa
the mechanism has been removed altogether.
New applications should use the techniques described in
.BR math_error (7)
and
.BR fenv (3).
This page documents the
.BR matherr ()
mechanism as an aid for maintaining and porting older applications.
.PP
The System V Interface Definition (SVID) specifies that various
math functions should invoke a function called
.BR matherr ()
if a math exception is detected.
This function is called before the math function returns;
after
.BR matherr ()
returns, the system then returns to the math function,
which in turn returns to the caller.
.PP
To employ
.BR matherr (),
the programmer must define the
.B _SVID_SOURCE
feature test macro
(before including
.I any
header files),
and assign the value
.B _SVID_
to the external variable
.BR _LIB_VERSION .
.PP
The system provides a default version of
.BR matherr ().
This version does nothing, and returns zero
(see below for the significance of this).
The default
.BR matherr ()
can be overridden by a programmer-defined
version, which will be invoked when an exception occurs.
The function is invoked with one argument, a pointer to an
.I exception
structure, defined as follows:
.PP
.in +4n
.EX
struct exception {
    int    type;      /* Exception type */
    char  *name;      /* Name of function causing exception */
    double arg1;      /* 1st argument to function */
    double arg2;      /* 2nd argument to function */
    double retval;    /* Function return value */
}
.EE
.in
.PP
The
.I type
field has one of the following values:
.TP 12
.B DOMAIN
A domain error occurred (the function argument was outside the range
for which the function is defined).
The return value depends on the function;
.I errno
is set to
.BR EDOM .
.TP
.B SING
A pole error occurred (the function result is an infinity).
The return value in most cases is
.B HUGE
(the largest single precision floating-point number),
appropriately signed.
In most cases,
.I errno
is set to
.BR EDOM .
.TP
.B OVERFLOW
An overflow occurred.
In most cases, the value
.B HUGE
is returned, and
.I errno
is set to
.BR ERANGE .
.TP
.B UNDERFLOW
An underflow occurred.
0.0 is returned, and
.I errno
is set to
.BR ERANGE .
.TP
.B TLOSS
Total loss of significance.
0.0 is returned, and
.I errno
is set to
.BR ERANGE .
.TP
.B PLOSS
Partial loss of significance.
This value is unused on glibc
(and many other systems).
.PP
The
.I arg1
and
.I arg2
fields are the arguments supplied to the function
.RI ( arg2
is undefined for functions that take only one argument).
.PP
The
.I retval
field specifies the return value that the math
function will return to its caller.
The programmer-defined
.BR matherr ()
can modify this field to change the return value of the math function.
.PP
If the
.BR matherr ()
function returns zero, then the system sets
.I errno
as described above, and may print an error message on standard error
(see below).
.PP
If the
.BR matherr ()
function returns a nonzero value, then the system does not set
.IR errno ,
and doesn't print an error message.
.SS Math functions that employ matherr()
The table below lists the functions and circumstances in which
.BR matherr ()
is called.
The "Type" column indicates the value assigned to
.I exc\->type
when calling
.BR matherr ().
The "Result" column is the default return value assigned to
.IR exc\->retval .
.PP
The "Msg?" and "errno" columns describe the default behavior if
.BR matherr ()
returns zero.
If the "Msg?" columns contains "y",
then the system prints an error message on standard error.
.PP
The table uses the following notations and abbreviations:
.PP
.RS
.TS
l l.
x        first argument to function
y        second argument to function
fin      finite value for argument
neg      negative value for argument
int      integral value for argument
o/f      result overflowed
u/f      result underflowed
|x|      absolute value of x
X_TLOSS  is a constant defined in \fI<math.h>\fP
.TE
.RE
.\" Details below from glibc 2.8's sysdeps/ieee754/k_standard.c
.\" A subset of cases were test by experimental programs.
.TS
lB lB lB cB lB
l l l c l.
Function	Type	Result	Msg?	errno
acos(|x|>1)	DOMAIN	HUGE	y	EDOM
asin(|x|>1)	DOMAIN	HUGE	y	EDOM
atan2(0,0)	DOMAIN	HUGE	y	EDOM
acosh(x<1)	DOMAIN	NAN	y	EDOM	\" retval is 0.0/0.0
atanh(|x|>1)	DOMAIN	NAN	y	EDOM	\" retval is 0.0/0.0
atanh(|x|==1)	SING	(x>0.0)?	y	EDOM	\" retval is x/0.0
\ 	\ 	HUGE_VAL :
\ 	\ 	\-HUGE_VAL
cosh(fin) o/f	OVERFLOW	HUGE	n	ERANGE
sinh(fin) o/f	OVERFLOW	(x>0.0) ?	n	ERANGE
\ 	\ 	HUGE : \-HUGE
sqrt(x<0)	DOMAIN	0.0	y	EDOM
hypot(fin,fin) o/f	OVERFLOW	HUGE	n	ERANGE
exp(fin) o/f	OVERFLOW	HUGE	n	ERANGE
exp(fin) u/f	UNDERFLOW	0.0	n	ERANGE
exp2(fin) o/f	OVERFLOW	HUGE	n	ERANGE
exp2(fin) u/f	UNDERFLOW	0.0	n	ERANGE
exp10(fin) o/f	OVERFLOW	HUGE	n	ERANGE
exp10(fin) u/f	UNDERFLOW	0.0	n	ERANGE
j0(|x|>X_TLOSS)	TLOSS	0.0	y	ERANGE
j1(|x|>X_TLOSS)	TLOSS	0.0	y	ERANGE
jn(|x|>X_TLOSS)	TLOSS	0.0	y	ERANGE
y0(x>X_TLOSS)	TLOSS	0.0	y	ERANGE
y1(x>X_TLOSS)	TLOSS	0.0	y	ERANGE
yn(x>X_TLOSS)	TLOSS	0.0	y	ERANGE
y0(0)	DOMAIN	\-HUGE	y	EDOM
y0(x<0)	DOMAIN	\-HUGE	y	EDOM
y1(0)	DOMAIN	\-HUGE	y	EDOM
y1(x<0)	DOMAIN	\-HUGE	y	EDOM
yn(n,0)	DOMAIN	\-HUGE	y	EDOM
yn(x<0)	DOMAIN	\-HUGE	y	EDOM
lgamma(fin) o/f	OVERFLOW	HUGE	n	ERANGE
lgamma(\-int) or	SING	HUGE	y	EDOM
\ \ lgamma(0)
tgamma(fin) o/f	OVERFLOW	HUGE_VAL	n	ERANGE
tgamma(\-int)	SING	NAN	y	EDOM
tgamma(0)	SING	copysign(	y	ERANGE
\ 	\ 	HUGE_VAL,x)
log(0)	SING	\-HUGE	y	EDOM
log(x<0)	DOMAIN	\-HUGE	y	EDOM
log2(0)	SING	\-HUGE	n	EDOM	\" different from log()
log2(x<0)	DOMAIN	\-HUGE	n	EDOM	\" different from log()
log10(0)	SING	\-HUGE	y	EDOM
log10(x<0)	DOMAIN	\-HUGE	y	EDOM
pow(0.0,0.0)	DOMAIN	0.0	y	EDOM
pow(x,y) o/f	OVERFLOW	HUGE	n	ERANGE
pow(x,y) u/f	UNDERFLOW	0.0	n	ERANGE
pow(NaN,0.0)	DOMAIN	x	n	EDOM
0**neg	DOMAIN	0.0	y	EDOM	\" +0 and -0
neg**non-int	DOMAIN	0.0	y	EDOM
scalb() o/f	OVERFLOW	(x>0.0) ?	n	ERANGE
\ 	\ 	HUGE_VAL :
\ 	\ 	\-HUGE_VAL
scalb() u/f	UNDERFLOW	copysign(	n	ERANGE
\ 	\ 	\ \ 0.0,x)
fmod(x,0)	DOMAIN	x	y	EDOM
remainder(x,0)	DOMAIN	NAN	y	EDOM	\" retval is 0.0/0.0
.TE
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR matherr ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH EXAMPLES
The example program demonstrates the use of
.BR matherr ()
when calling
.BR log (3).
The program takes up to three command-line arguments.
The first argument is the floating-point number to be given to
.BR log (3).
If the optional second argument is provided, then
.B _LIB_VERSION
is set to
.B _SVID_
so that
.BR matherr ()
is called, and the integer supplied in the
command-line argument is used as the return value from
.BR matherr ().
If the optional third command-line argument is supplied,
then it specifies an alternative return value that
.BR matherr ()
should assign as the return value of the math function.
.PP
The following example run, where
.BR log (3)
is given an argument of 0.0, does not use
.BR matherr ():
.PP
.in +4n
.EX
.RB "$" " ./a.out 0.0"
errno: Numerical result out of range
x=\-inf
.EE
.in
.PP
In the following run,
.BR matherr ()
is called, and returns 0:
.PP
.in +4n
.EX
.RB "$" " ./a.out 0.0 0"
matherr SING exception in log() function
        args:   0.000000, 0.000000
        retval: \-340282346638528859811704183484516925440.000000
log: SING error
errno: Numerical argument out of domain
x=\-340282346638528859811704183484516925440.000000
.EE
.in
.PP
The message "log: SING error" was printed by the C library.
.PP
In the following run,
.BR matherr ()
is called, and returns a nonzero value:
.PP
.in +4n
.EX
.RB "$" " ./a.out 0.0 1"
matherr SING exception in log() function
        args:   0.000000, 0.000000
        retval: \-340282346638528859811704183484516925440.000000
x=\-340282346638528859811704183484516925440.000000
.EE
.in
.PP
In this case, the C library did not print a message, and
.I errno
was not set.
.PP
In the following run,
.BR matherr ()
is called, changes the return value of the math function,
and returns a nonzero value:
.PP
.in +4n
.EX
.RB "$" " ./a.out 0.0 1 12345.0"
matherr SING exception in log() function
        args:   0.000000, 0.000000
        retval: \-340282346638528859811704183484516925440.000000
x=12345.000000
.EE
.in
.SS Program source
\&
.EX
#define _SVID_SOURCE
#include <errno.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

static int matherr_ret = 0;     /* Value that matherr()
                                   should return */
static int change_retval = 0;   /* Should matherr() change
                                   function\(aqs return value? */
static double new_retval;       /* New function return value */

int
matherr(struct exception *exc)
{
    fprintf(stderr, "matherr %s exception in %s() function\en",
           (exc\->type == DOMAIN) ?    "DOMAIN" :
           (exc\->type == OVERFLOW) ?  "OVERFLOW" :
           (exc\->type == UNDERFLOW) ? "UNDERFLOW" :
           (exc\->type == SING) ?      "SING" :
           (exc\->type == TLOSS) ?     "TLOSS" :
           (exc\->type == PLOSS) ?     "PLOSS" : "???",
            exc\->name);
    fprintf(stderr, "        args:   %f, %f\en",
            exc\->arg1, exc\->arg2);
    fprintf(stderr, "        retval: %f\en", exc\->retval);

    if (change_retval)
        exc\->retval = new_retval;

    return matherr_ret;
}

int
main(int argc, char *argv[])
{
    double x;

    if (argc < 2) {
        fprintf(stderr, "Usage: %s <argval>"
                " [<matherr\-ret> [<new\-func\-retval>]]\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    if (argc > 2) {
        _LIB_VERSION = _SVID_;
        matherr_ret = atoi(argv[2]);
    }

    if (argc > 3) {
        change_retval = 1;
        new_retval = atof(argv[3]);
    }

    x = log(atof(argv[1]));
    if (errno != 0)
        perror("errno");

    printf("x=%f\en", x);
    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR fenv (3),
.BR math_error (7),
.BR standards (7)