381 lines
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381 lines
8.5 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of MATH_ERROR</TITLE>
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</HEAD><BODY>
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<H1>MATH_ERROR</H1>
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Section: Linux Programmer's Manual (7)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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math_error - detecting errors from mathematical functions
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/math.h">math.h</A>></B>
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<B>#include <<A HREF="file:///usr/include/errno.h">errno.h</A>></B>
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<B>#include <<A HREF="file:///usr/include/fenv.h">fenv.h</A>></B>
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</PRE>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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When an error occurs,
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most library functions indicate this fact by returning a special value
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(e.g., -1 or NULL).
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Because they typically return a floating-point number,
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the mathematical functions declared in
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<I><<A HREF="file:///usr/include/math.h">math.h</A>></I>
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indicate an error using other mechanisms.
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There are two error-reporting mechanisms:
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the older one sets
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<I>errno</I>;
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the newer one uses the floating-point exception mechanism (the use of
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<B><A HREF="/cgi-bin/man/man2html?3+feclearexcept">feclearexcept</A></B>(3)
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and
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<B><A HREF="/cgi-bin/man/man2html?3+fetestexcept">fetestexcept</A></B>(3),
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as outlined below)
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described in
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<B><A HREF="/cgi-bin/man/man2html?3+fenv">fenv</A></B>(3).
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<P>
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A portable program that needs to check for an error from a mathematical
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function should set
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<I>errno</I>
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to zero, and make the following call
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<P>
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feclearexcept(FE_ALL_EXCEPT);
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<P>
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before calling a mathematical function.
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<P>
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Upon return from the mathematical function, if
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<I>errno</I>
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is nonzero, or the following call (see
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<B><A HREF="/cgi-bin/man/man2html?3+fenv">fenv</A></B>(3))
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returns nonzero
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<P>
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fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW |
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<BR> FE_UNDERFLOW);
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<P>
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then an error occurred in the mathematical function.
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<P>
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The error conditions that can occur for mathematical functions
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are described below.
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<A NAME="lbAE"> </A>
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<H3>Domain error</H3>
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A
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<I>domain error</I>
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occurs when a mathematical function is supplied with an argument whose
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value falls outside the domain for which the function
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is defined (e.g., giving a negative argument to
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<B><A HREF="/cgi-bin/man/man2html?3+log">log</A></B>(3)).
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When a domain error occurs,
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math functions commonly return a NaN
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(though some functions return a different value in this case);
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<I>errno</I>
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is set to
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<B>EDOM</B>,
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and an "invalid"
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(<B>FE_INVALID</B>)
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floating-point exception is raised.
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<A NAME="lbAF"> </A>
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<H3>Pole error</H3>
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A
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<I>pole error</I>
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occurs when the mathematical result of a function is an exact infinity
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(e.g., the logarithm of 0 is negative infinity).
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When a pole error occurs,
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the function returns the (signed) value
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<B>HUGE_VAL</B>,
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<B>HUGE_VALF</B>,
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or
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<B>HUGE_VALL</B>,
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depending on whether the function result type is
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<I>double</I>,
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<I>float</I>,
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or
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<I>long double</I>.
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The sign of the result is that which is mathematically correct for
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the function.
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<I>errno</I>
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is set to
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<B>ERANGE</B>,
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and a "divide-by-zero"
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(<B>FE_DIVBYZERO</B>)
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floating-point exception is raised.
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<A NAME="lbAG"> </A>
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<H3>Range error</H3>
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A
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<I>range error</I>
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occurs when the magnitude of the function result means that it
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cannot be represented in the result type of the function.
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The return value of the function depends on whether the range error
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was an overflow or an underflow.
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<P>
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A floating result
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<I>overflows</I>
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if the result is finite,
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but is too large to represented in the result type.
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When an overflow occurs,
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the function returns the value
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<B>HUGE_VAL</B>,
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<B>HUGE_VALF</B>,
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or
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<B>HUGE_VALL</B>,
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depending on whether the function result type is
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<I>double</I>,
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<I>float</I>,
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or
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<I>long double</I>.
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<I>errno</I>
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is set to
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<B>ERANGE</B>,
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and an "overflow"
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(<B>FE_OVERFLOW</B>)
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floating-point exception is raised.
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<P>
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A floating result
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<I>underflows</I>
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if the result is too small to be represented in the result type.
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If an underflow occurs,
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a mathematical function typically returns 0.0
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(C99 says a function shall return "an implementation-defined value
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whose magnitude is no greater than the smallest normalized
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positive number in the specified type").
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<I>errno</I>
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may be set to
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<B>ERANGE</B>,
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and an "overflow"
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(<B>FE_UNDERFLOW</B>)
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floating-point exception may be raised.
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<P>
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Some functions deliver a range error if the supplied argument value,
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or the correct function result, would be
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<I>subnormal</I>.
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A subnormal value is one that is nonzero,
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but with a magnitude that is so small that
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it can't be presented in normalized form
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(i.e., with a 1 in the most significant bit of the significand).
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The representation of a subnormal number will contain one
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or more leading zeros in the significand.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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The
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<I>math_errhandling</I>
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identifier specified by C99 and POSIX.1 is not supported by glibc.
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This identifier is supposed to indicate which of the two
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error-notification mechanisms
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(<I>errno</I>,
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exceptions retrievable via
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<B><A HREF="/cgi-bin/man/man2html?3+fettestexcept">fettestexcept</A></B>(3))
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is in use.
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The standards require that at least one be in use,
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but permit both to be available.
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The current (version 2.8) situation under glibc is messy.
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Most (but not all) functions raise exceptions on errors.
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Some also set
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<I>errno</I>.
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A few functions set
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<I>errno</I>,
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but don't raise an exception.
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A very few functions do neither.
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See the individual manual pages for details.
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<P>
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To avoid the complexities of using
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<I>errno</I>
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and
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<B><A HREF="/cgi-bin/man/man2html?3+fetestexcept">fetestexcept</A></B>(3)
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for error checking,
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it is often advised that one should instead check for bad argument
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values before each call.
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For example, the following code ensures that
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<B><A HREF="/cgi-bin/man/man2html?3+log">log</A></B>(3)'s
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argument is not a NaN and is not zero (a pole error) or
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less than zero (a domain error):
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<P>
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double x, r;
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<P>
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if (isnan(x) || islessequal(x, 0)) {
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<BR> /* Deal with NaN / pole error / domain error */
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}
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<P>
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r = log(x);
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<P>
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The discussion on this page does not apply to the complex
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mathematical functions (i.e., those declared by
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<I><<A HREF="file:///usr/include/complex.h">complex.h</A>></I>),
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which in general are not required to return errors by C99
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and POSIX.1.
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<P>
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The
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<B><A HREF="/cgi-bin/man/man2html?1+gcc">gcc</A></B>(1)
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<I>-fno-math-errno</I>
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option causes the executable to employ implementations of some
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mathematical functions that are faster than the standard
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implementations, but do not set
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<I>errno</I>
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on error.
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(The
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<B><A HREF="/cgi-bin/man/man2html?1+gcc">gcc</A></B>(1)
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<I>-ffast-math</I>
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option also enables
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<I>-fno-math-errno</I>.)
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An error can still be tested for using
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<B><A HREF="/cgi-bin/man/man2html?3+fetestexcept">fetestexcept</A></B>(3).
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<A NAME="lbAI"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?1+gcc">gcc</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?3+errno">errno</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+fenv">fenv</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+fpclassify">fpclassify</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+INFINITY">INFINITY</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+isgreater">isgreater</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+matherr">matherr</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+nan">nan</A></B>(3)
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<P>
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<I>info libc</I>
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<A NAME="lbAJ"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="1"><A HREF="#lbAB">NAME</A><DD>
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<DT id="2"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="3"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DL>
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<DT id="4"><A HREF="#lbAE">Domain error</A><DD>
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<DT id="5"><A HREF="#lbAF">Pole error</A><DD>
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<DT id="6"><A HREF="#lbAG">Range error</A><DD>
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</DL>
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<DT id="7"><A HREF="#lbAH">NOTES</A><DD>
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<DT id="8"><A HREF="#lbAI">SEE ALSO</A><DD>
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<DT id="9"><A HREF="#lbAJ">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:06:09 GMT, March 31, 2021
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