NAME

matherr − error-handling function

SYNOPSIS

#include <math.h>

int matherr (x)
struct exception ∗x;

DESCRIPTION

Matherr is invoked by functions in the Math Library when errors are detected.  Users may define their own procedures for handling errors by including a function named matherr in their programs.  Matherr must be of the form described above.  A pointer to the exception structure x will be passed to the user-supplied matherr function when an error occurs.  This structure, which is defined in the <math.h> header file, is as follows:

struct exception {
int type;
char ∗name;
double arg1, arg2, retval;
};

The element type is an integer describing the type of error that has occurred, from the following list of constants (defined in the header file):

DOMAINdomain error
SINGsingularity
OVERFLOWoverflow
UNDERFLOW underflow
TLOSStotal loss of significance
PLOSSpartial loss of significance

The element name points to a string containing the name of the function that had the error.  The variables arg1 and arg2 are the arguments to the function that had the error.  Retval is a double that is returned by the function having the error.  If it supplies a return value, the user’s matherr must return non-zero.  If the default error value is to be returned, the user’s matherr must return 0. 

If matherr is not supplied by the user, the default error-handling procedures, described with the math functions involved, will be invoked upon error.  These procedures are also summarized in the table below.  In every case, errno is set to non-zero and the program continues. 

EXAMPLE

matherr(x)
register struct exception ∗x;
{
switch (x−>type) {
case DOMAIN:
case SING: /∗ print message and abort ∗/
fprintf(stderr, "domain error in %s\n", x−>name);
abort( );
case OVERFLOW:
if (!strcmp("exp", x−>name)) {
/∗ if exp, print message, return the argument ∗/
fprintf(stderr, "exp of %f\n", x−>arg1);
x−>retval = x−>arg1;
} else if (!strcmp("sinh", x−>name)) {
/∗ if sinh, set errno, return 0 ∗/
errno = ERANGE;
x−>retval = 0;
} else
/∗ otherwise, return HUGE ∗/
x−>retval = HUGE;
break;
case UNDERFLOW:
return (0); /∗ execute default procedure ∗/
case TLOSS:
case PLOSS:
/∗ print message and return 0 ∗/
fprintf(stderr, "loss of significance in %s\n", x−>name);
x−>retval = 0;
break;
}
return (1);
}

Version 2.5  —  April 22, 1987