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       matherr - SVID math library exception handling


       #include <math.h>

       int matherr(struct exception *exc);


       Link with -lm.


       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, the mechanism  has  been  removed
       altogether.   New  applications  should  use the techniques described in math_error(7) and
       fenv(3).  This page documents the matherr()  mechanism  as  an  aid  for  maintaining  and
       porting older applications.

       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.

       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

       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
       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 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

       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

       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 :
       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
       tgamma(fin) o/f      OVERFLOW    HUGE_VAL        n     ERANGE
       tgamma(-int)         SING        NAN             y     EDOM
       tgamma(0)            SING        copysign(       y     ERANGE
       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 :
       scalb() u/f          UNDERFLOW   copysign(       n     ERANGE
       fmod(x,0)            DOMAIN      x               y     EDOM
       remainder(x,0)       DOMAIN      NAN             y     EDOM


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue   │
       │matherr() │ Thread safety │ MT-Safe │


       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

           $ ./a.out 0.0
           errno: Numerical result out of range

       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

       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

       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

   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 */

       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" : "???",
           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;

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

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

           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)

           printf("x=%f\n", x);


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


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