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MATHERR(3)			    Linux Programmer's Manual			       MATHERR(3)

NAME
       matherr - SVID math library exception handling

SYNOPSIS
       #define _SVID_SOURCE		/* See feature_test_macros(7) */
       #include <math.h>

       int matherr(struct exception *exc);

       extern _LIB_VERSION_TYPE _LIB_VERSION;

       Link with -lm.

DESCRIPTION
       The  System  V  Interface  Definition  (SVID) specifies that various math functions should
       invoke a function called matherr() if a math exception  is  detected.   This  function  is
       called  before the math function returns; after matherr() returns, the system then returns
       to the math function, which in turn returns to the caller.

       The matherr() mechanism is supported by glibc,  but  is	now  obsolete:	new  applications
       should use the techniques described in math_error(7) and fenv(3).  This page documents the
       glibc matherr() mechanism as an aid for maintaining and porting older applications.

       To employ matherr(), the programmer  must  define  the  _SVID_SOURCE  feature  test  macro
       (before	including any header files), and assign the value _SVID_ to the external variable
       _LIB_VERSION.

       The system provides a default version  of  matherr().   This  version  does  nothing,  and
       returns zero (see below for the significance of this).  The default matherr() can be over-
       ridden by a programmer-defined version, which will be invoked when  an  exception  occurs.
       The function is invoked with one argument, a pointer to an exception structure, defined as
       follows:

	   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 */
	   }

       The type field has one of the following values:

       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; errno is
		   set to EDOM.

       SING	   A pole error occurred (the function result is an infinity).	The return  value
		   in  most  cases  is HUGE (the largest single precision floating-point number),
		   appropriately signed.  In most cases, errno is set to EDOM.

       OVERFLOW    An overflow occurred.  In most cases, the value HUGE is returned, and errno is
		   set to ERANGE.

       UNDERFLOW   An underflow occurred.  0.0 is returned, and errno is set to ERANGE.

       TLOSS	   Total loss of significance.	0.0 is returned, and errno is set to ERANGE.

       PLOSS	   Partial  loss  of significance.  This value is unused on glibc (and many other
		   systems).

       The arg1 and arg2 fields are the arguments supplied to the function (arg2 is undefined for
       functions that take only one argument).

       The  retval  field  specifies  the  return value that the math function will return to its
       caller.	The programmer-defined matherr() can modify this field to change the return value
       of the math function.

       If the matherr() function returns zero, then the system sets errno as described above, and
       may print an error message on standard error (see below).

       If the matherr() function returns a nonzero value, then the system does not set errno, and
       doesn't print an error message.

   Math functions that employ matherr()
       The  table  below lists the functions and circumstances in which matherr() is called.  The
       "Type" column indicates the value assigned  to  exc->type  when	calling  matherr().   The
       "Result" column is the default return value assigned to exc->retval.

       The  "Msg?"  and  "errno" columns describe the default behavior if matherr() returns zero.
       If the "Msg?" columns contains "y", then the system prints an error  message  on  standard
       error.

       The table uses the following notations and abbreviations:

	      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 <math.h>

       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
       atanh(|x|>1)	    DOMAIN	NAN		y     EDOM
       atanh(|x|==1)	    SING	(x>0.0)?	y     EDOM
					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
       log2(x<0)	    DOMAIN	-HUGE		n     EDOM
       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
       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

EXAMPLE
       The  example  program  demonstrates the use of matherr() when calling log(3).  The program
       takes up to three command-line arguments.  The first argument is the floating-point number
       to  be given to log(3).	If the optional second argument is provided, then _LIB_VERSION is
       set to _SVID_ so that matherr() is called, and the integer supplied  in	the  command-line
       argument  is  used as the return value from matherr().  If the optional third command-line
       argument is supplied, then it specifies an alternative return value that matherr()  should
       assign as the return value of the math function.

       The  following  example	run, where log(3) is given an argument of 0.0, does not use math-
       err():

	   $ ./a.out 0.0
	   errno: Numerical result out of range
	   x=-inf

       In the following run, matherr() is called, and returns 0:

	   $ ./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

       The message "log: SING error" was printed by the C library.

       In the following run, matherr() is called, and returns a nonzero value:

	   $ ./a.out 0.0 1
	   matherr SING exception in log() function
		   args:   0.000000, 0.000000
		   retval: -340282346638528859811704183484516925440.000000
	   x=-340282346638528859811704183484516925440.000000

       In this case, the C library did not print a message, and errno was not set.

       In the following run, matherr() is called, changes the return value of the math	function,
       and returns a nonzero value:

	   $ ./a.out 0.0 1 12345.0
	   matherr SING exception in log() function
		   args:   0.000000, 0.000000
		   retval: -340282346638528859811704183484516925440.000000
	   x=12345.000000

   Program source

       #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's return value? */
       static double new_retval;       /* New function return value */

       int
       matherr(struct exception *exc)
       {
	   fprintf(stderr, "matherr %s exception in %s() function\n",
		  (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\n",
		   exc->arg1, exc->arg2);
	   fprintf(stderr, "	    retval: %f\n", 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>]]\n", 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\n", x);
	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       fenv(3), math_error(7), standards(7)

COLOPHON
       This  page  is  part of release 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2010-09-10				       MATHERR(3)
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