date and time created 85/08/21 15:33:50 by miriam

SCCS-vsn: lib/libm/common_source/infnan.3 1.1

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+.TH INFNAN 3M  "20 August 1985"
+.UC 4
+.ds nn \fINaN\fR
+.SH NAME
+.nf
+.ta \w'infnan \- 'u+1n
+infnan \-      signals invalid floating\-point operations on a VAX
+       TEMPORARY! ... Subject to Change.
+.ta
+.fi
+.SH SYNOPSIS
+.nf
+.B #include <math.h>
+.PP
+.B double infnan(iarg)
+.B int iarg;
+.SH DESCRIPTION
+At some time in the future, some of the useful properties of
+the Infinities and \*(nns in the IEEE standard 754 for Binary
+Floating\-Point Arithmetic will be simulated in UNIX on the
+DEC VAX by using its Reserved Operands.  Meanwhile, the
+Invalid, Overflow and Divide\-by\-Zero exceptions of the
+IEEE standard are being approximated on a VAX by calls to a
+procedure \fIinfnan\fR in appropriate places in \fIlibm\fR.  When
+better exception\-handling is implemented in UNIX, only
+\fIinfnan\fR among the codes in \fIlibm\fR will have to be changed.
+And users of \fIlibm\fR can design their own \fIinfnan\fR now to
+insulate themselves from future changes.
+.PP
+Whenever an elementary function code in \fIlibm\fR has to
+simulate one of the aforementioned IEEE exceptions, it calls
+infnan(iarg) with an appropriate value of \fIiarg\fR.  Then a
+reserved operand fault stops computation.  But \fIinfnan\fR could
+be replaced by a function with the same name that returns
+some plausible value, assigns an apt value to the global
+variable \fIerrno\fR, and allows computation to resume.
+Alternatively, the Reserved Operand Fault Handler could be
+changed to respond by returning that plausible value etc.
+instead of aborting.
+.PP
+In the table below, the first two columns show various
+exceptions signaled by the IEEE standard, and the default
+result it prescribes.  The third column shows what value is
+given to \fIiarg\fR by functions in \fIlibm\fR when they
+invoke infnan(iarg) under analogous circumstances on a VAX.
+Currently \fIinfnan\fR stops computation under all those
+circumstances.  The last two columns offer an alternative;
+they suggest a setting for \fIerrno\fR and a value for a
+revised \fIinfnan\fR to return.  And a C program to
+implement that suggestion follows. 
+.sp 0.5
+.RS
+.nf
+.ta \w'Div\-by\-0'u+2n +\w'+Infinity'u+1n +\w'+ERANGE'u+1n +\w'ERANGE or EDOM'u+4n +\w'+HUGE'u+1n
+IEEE   IEEE
+Signal Default \fIiarg\fR      \fIerrno\fR     \fIinfnan\fR
+.if t \
+\l'4i'
+.if n \
+\l'5i'
+Invalid        \*(nn   EDOM    EDOM    0
+.if n \{\
+Overflow       \(+-Infinity    ERANGE  ERANGE  HUGE
+Div\-by\-0     \(+-Infinity    \(+-ERANGE      ERANGE or EDOM  \(+-HUGE \}
+.if t \{\
+Overflow       \(+-\(if        ERANGE  ERANGE  HUGE
+Div\-by\-0     \(+-\(if        \(+-ERANGE      ERANGE or EDOM  \(+-HUGE \}
+.sp 0.5
+       (HUGE = 1.7e38 ... nearly  2.0**127)
+.ta
+.fi
+.RE
+.sp 0.5
+.PP
+.nf
+.ta \w'\fBextern int\fR'u+1n +\w'\fBdefault:\fR'u+1n +\w'\fB\-ERANGE:\fR'u+1n +\w'\fBerrno = ERANGE;\fR'u+1n
+ALTERNATIVE  \fIinfnan\fR:\fB
+.sp 0.5
+#include       <math.h>
+#include       <errno.h>
+extern int     errno ;
+double infnan(iarg)
+int    iarg ;
+{
+       switch(iarg) {
+       case    \0ERANGE:       errno = ERANGE; return(HUGE);
+       case    \-ERANGE:       errno = EDOM;   return(\-HUGE);
+       default:                errno = EDOM;   return(0);
+       }
+}\fR
+.ta
+.fi
+.SH SEE ALSO
+intro(3M), intro(2), signal(3).
+.PP
+ERANGE and EDOM are defined in <errno.h>.  See intro(2)
+for interpretations of EDOM and ERANGE from the operating
+system's point of view.