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─────Run-Time Library───────────────────────────────────────────────────────
 
/* MATHERR.C illustrates writing an error routine for math functions.
 * The error function must be:
 *      _matherr
 *
 * To use _matherr, you must turn on the No Extended Dictionary flag
 * within the PWB environment (choose the LINK Options command from
 * the Options menu and then select Additional Global Options) or
 * use the /NOE linker option outside the environment. For example:
 *      CL matherr.c /link /NOE
 */
 
#include <math.h>
#include <string.h>
#include <stdio.h>
 
void main()
{
    /* Do several math operations that cause errors. The _matherr
     * routine handles _DOMAIN errors, but lets the system handle
     * other errors normally.
     */
    printf( "log( -2.0 ) = %e\n", log( -2.0 ) );
    printf( "log10( -5.0 ) = %e\n", log10( -5.0 ) );
    printf( "log( 0.0 ) = %e\n", log( 0.0 ) );
}
 
/* Handle several math errors caused by passing a negative argument
 * to log or log10 (_DOMAIN errors). When this happens, _matherr
 * returns the natural or base-10 logarithm of the absolute value
 * of the argument and suppresses the usual error message.
 */
int _matherr( struct _exception *except )
{
    /* Handle _DOMAIN errors for log or log10. */
    if( except->type == _DOMAIN )
    {
        if( strcmp( except->name, "log" ) == 0 )
        {
            except->retval = log( -(except->arg1) );
            printf( "Special: using absolute value: %s: _DOMAIN error\n",
                    except->name );
            return 1;
        }
        else if( strcmp( except->name, "log10" ) == 0 )
        {
            except->retval = log10( -(except->arg1) );
            printf( "Special: using absolute value: %s: _DOMAIN error\n",
                    except->name );
            return 1;
        }
    }
    else
    {
        printf( "Normal: " );
        return 0;    /* Else use the default actions */
    }
}
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