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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Math library compatibility modes</TITLE
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>Prev</A
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>Chapter 13. C and math library overview</TD
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="MATH-LIBRARY-COMPATIBILITY-MODES">Math library compatibility modes</H1
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><P
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>This math library is capable of being operated in several
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different compatibility modes. These options deal solely with how
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errors are handled. </P
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><P
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>There are 4 compatibility modes: ANSI/POSIX 1003.1;
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IEEE-754; X/Open Portability Guide issue 3 (XPG3); and
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System V Interface Definition Edition 3. </P
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><P
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>In IEEE mode, the <TT
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CLASS="FUNCTION"
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>matherr()</TT
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> function
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(see below) is never called, no warning messages are printed on
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the stderr output stream, and errno is never set. </P
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><P
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>In ANSI/POSIX mode, errno is set correctly,
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but <TT
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CLASS="FUNCTION"
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>matherr()</TT
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> is never called and no warning messages
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are printed on the stderr output stream. </P
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><P
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>In X/Open mode, errno is set correctly,
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<TT
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CLASS="FUNCTION"
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>matherr()</TT
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> is called, but no warning messages are printed
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on the stderr output stream. </P
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><P
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>In SVID mode, functions which overflow return
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a value HUGE (defined in <TT
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CLASS="FILENAME"
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>math.h</TT
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>), which is the maximum
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single precision floating point value (as opposed to
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HUGE_VAL which is meant to stand for infinity). errno is
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set correctly and <TT
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CLASS="FUNCTION"
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>matherr()</TT
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> is called. If
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<TT
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CLASS="FUNCTION"
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>matherr()</TT
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> returns 0, warning messages are printed on
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the stderr output stream for some errors. </P
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><P
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>The mode can be compiled-in as IEEE-only, or any one of the
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above methods settable at run-time. </P
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><DIV
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CLASS="NOTE"
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><BLOCKQUOTE
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CLASS="NOTE"
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><P
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><B
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>Note: </B
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>This math library assumes that the hardware (or software floating
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point emulation) supports IEEE-754 style arithmetic, 32-bit 2's
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complement integer arithmetic, doubles are in 64-bit IEEE-754 format.</P
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></BLOCKQUOTE
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN10153">matherr()</H2
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><P
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>As mentioned above, in X/Open or SVID modes, the user
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            can supply a function <TT
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CLASS="FUNCTION"
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>matherr()</TT
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> of
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            the form:</P
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><TABLE
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>int <TT
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CLASS="FUNCTION"
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>matherr</TT
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>( struct exception *e )</PRE
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></TD
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></TR
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></TABLE
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><P
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>where struct exception is defined as:</P
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>struct exception {
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 int type;
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 char *name;
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 double arg1, arg2, retval;
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}; </PRE
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></TD
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></TABLE
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><P
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>type is the exception type and is one of:</P
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><P
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></P
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CLASS="VARIABLELIST"
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><DL
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><DT
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>DOMAIN</DT
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><DD
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><P
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>argument domain exception</P
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></DD
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><DT
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>SING</DT
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><DD
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><P
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>argument singularity</P
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></DD
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><DT
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>OVERFLOW</DT
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><DD
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><P
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>overflow range exception</P
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></DD
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><DT
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>UNDERFLOW</DT
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><DD
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><P
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>underflow range exception</P
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></DD
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><DT
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>TLOSS</DT
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><DD
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><P
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>total loss of significance</P
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></DD
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><DT
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>PLOSS</DT
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><DD
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><P
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>partial loss of significance</P
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></DD
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></DL
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></DIV
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><P
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><TT
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CLASS="PARAMETER"
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><I
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>name</I
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></TT
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> is a string containing the name of the
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function</P
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><P
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><TT
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CLASS="PARAMETER"
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><I
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>arg1</I
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></TT
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> and <TT
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CLASS="PARAMETER"
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><I
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>arg2</I
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></TT
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> are the
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arguments passed to the function</P
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><P
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><TT
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CLASS="PARAMETER"
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><I
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>retval</I
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></TT
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> is the default value that will be returned
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by the function, and can be changed by <TT
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CLASS="FUNCTION"
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>matherr()</TT
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></P
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><DIV
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CLASS="NOTE"
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><BLOCKQUOTE
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CLASS="NOTE"
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><P
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><B
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>Note: </B
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>matherr must have &#8220;C&#8221; linkage, not &#8220;C&#0043;&#0043;&#8221; linkage.</P
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></BLOCKQUOTE
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></DIV
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><P
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>If matherr returns zero, or the user doesn't supply
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their own matherr, then the following <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>usually</I
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></SPAN
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> happens
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in SVID mode:</P
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><DIV
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CLASS="TABLE"
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><A
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NAME="AEN10199"><P
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><B
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>Table 13-1. Behavior of math exception handling</B
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></P
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><TABLE
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BORDER="1"
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CLASS="CALSTABLE"
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><THEAD
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><TR
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><TH
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ALIGN="LEFT"
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VALIGN="TOP"
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>Type</TH
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><TH
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ALIGN="LEFT"
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VALIGN="TOP"
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>Behavior</TH
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></TR
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></THEAD
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><TBODY
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>DOMAIN</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>0.0 returned,
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errno=EDOM, and a message printed on stderr</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>SING</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>HUGE of appropriate
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sign is returned, errno=EDOM, and a message is printed
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on stderr</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>OVERFLOW</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>HUGE of
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appropriate sign is returned, and errno=ERANGE</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>UNDERFLOW</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>0.0 is
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returned and errno=ERANGE</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>TLOSS</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>0.0 is returned,
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errno=ERANGE, and a message is printed on stderr</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>PLOSS</TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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>The current
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implementation doesn't return this type</TD
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></TR
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></TBODY
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></TABLE
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></DIV
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><P
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>X/Open mode is similar except that the message is
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not printed on stderr and HUGE_VAL is used in place of
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HUGE</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="AEN10226">Thread-safety and re-entrancy</H2
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><P
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>With the appropriate configuration options set below, the
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math library is fully thread-safe if:</P
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><P
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></P
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><UL
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><LI
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><P
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>Depending on the compatibility mode, the
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                setting of the errno variable from the C library is
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                thread-safe</P
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></LI
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><LI
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><P
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>Depending on the compatibility mode, sending error messages
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to the stderr output stream using the C library
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<TT
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CLASS="FUNCTION"
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>fputs()</TT
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>
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 function is thread-safe </P
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></LI
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><LI
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><P
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>Depending on the compatibility mode, the user-supplied
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<TT
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CLASS="FUNCTION"
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>matherr()</TT
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>
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 function and anything it depends on are thread-safe </P
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></LI
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></UL
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><P
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>In addition, with the exception of the <TT
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CLASS="FUNCTION"
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>gamma*()</TT
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> and <TT
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CLASS="FUNCTION"
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>lgamma*()</TT
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> functions,
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the math library is reentrant (and thus safe to use from interrupt handlers)
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if the Math library is always in IEEE mode.</P
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