mirror of https://github.com/mkerrisk/man-pages
391 lines
8.3 KiB
Groff
391 lines
8.3 KiB
Groff
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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.\" <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" References consulted:
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.\" Linux libc source code
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.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
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.\" 386BSD man pages
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.\" Modified 1993-07-24 by Rik Faith (faith@cs.unc.edu)
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.\" Modified 1995-08-14 by Arnt Gulbrandsen <agulbra@troll.no>
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.\" Modified 2002-07-27 by Walter Harms
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.\" (walter.harms@informatik.uni-oldenburg.de)
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.TH POW 3 2016-03-15 "" "Linux Programmer's Manual"
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.SH NAME
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pow, powf, powl \- power functions
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.SH SYNOPSIS
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.nf
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.B #include <math.h>
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.PP
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.BI "double pow(double " x ", double " y );
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.BI "float powf(float " x ", float " y );
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.BI "long double powl(long double " x ", long double " y );
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.fi
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.PP
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Link with \fI\-lm\fP.
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.PP
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.in -4n
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.in
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.PP
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.ad l
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.BR powf (),
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.BR powl ():
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.RS 4
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_ISOC99_SOURCE || _POSIX_C_SOURCE\ >=\ 200112L
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|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
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|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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.RE
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.ad
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.SH DESCRIPTION
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These functions return the value of
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.I x
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raised to the
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power of
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.IR y .
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.SH RETURN VALUE
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On success, these functions return the value of
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.I x
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to the power of
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.IR y .
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.PP
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If
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.I x
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is a finite value less than 0, and
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.I y
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is a finite noninteger, a domain error occurs,
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.\" The domain error is generated at least as far back as glibc 2.4
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and a NaN is returned.
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.PP
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If the result overflows,
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a range error occurs,
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.\" The range error is generated at least as far back as glibc 2.4
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and the functions return
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.BR HUGE_VAL ,
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.BR HUGE_VALF ,
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or
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.BR HUGE_VALL ,
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respectively, with the mathematically correct sign.
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.PP
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If result underflows, and is not representable,
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a range error occurs,
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and 0.0 is returned.
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.\" POSIX.1 does not specify the sign of the zero,
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.\" but http://sources.redhat.com/bugzilla/show_bug.cgi?id=2678
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.\" points out that the zero has the wrong sign in some cases.
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.PP
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Except as specified below, if
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.I x
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or
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.I y
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is a NaN, the result is a NaN.
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.PP
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If
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.I x
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is +1, the result is 1.0 (even if
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.I y
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is a NaN).
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.PP
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If
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.I y
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is 0, the result is 1.0 (even if
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.I x
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is a NaN).
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.PP
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If
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.I x
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is +0 (\-0),
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and
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.I y
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is an odd integer greater than 0,
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the result is +0 (\-0).
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.PP
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If
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.I x
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is 0,
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and
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.I y
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greater than 0 and not an odd integer,
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the result is +0.
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.PP
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If
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.I x
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is \-1,
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and
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.I y
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is positive infinity or negative infinity,
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the result is 1.0.
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.PP
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If the absolute value of
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.I x
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is less than 1,
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and
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.I y
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is negative infinity,
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the result is positive infinity.
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.PP
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If the absolute value of
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.I x
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is greater than 1,
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and
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.I y
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is negative infinity,
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the result is +0.
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.PP
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If the absolute value of
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.I x
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is less than 1,
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and
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.I y
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is positive infinity,
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the result is +0.
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.PP
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If the absolute value of
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.I x
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is greater than 1,
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and
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.I y
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is positive infinity,
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the result is positive infinity.
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.PP
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If
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.I x
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is negative infinity,
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and
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.I y
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is an odd integer less than 0,
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the result is \-0.
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.PP
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If
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.I x
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is negative infinity,
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and
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.I y
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less than 0 and not an odd integer,
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the result is +0.
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.PP
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If
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.I x
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is negative infinity,
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and
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.I y
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is an odd integer greater than 0,
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the result is negative infinity.
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.PP
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If
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.I x
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is negative infinity,
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and
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.I y
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greater than 0 and not an odd integer,
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the result is positive infinity.
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.PP
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If
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.I x
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is positive infinity,
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and
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.I y
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less than 0,
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the result is +0.
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.PP
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If
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.I x
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is positive infinity,
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and
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.I y
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greater than 0,
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the result is positive infinity.
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.PP
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If
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.I x
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is +0 or \-0,
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and
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.I y
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is an odd integer less than 0,
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a pole error occurs and
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.BR HUGE_VAL ,
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.BR HUGE_VALF ,
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or
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.BR HUGE_VALL ,
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is returned,
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with the same sign as
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.IR x .
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.PP
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If
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.I x
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is +0 or \-0,
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and
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.I y
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is less than 0 and not an odd integer,
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a pole error occurs and
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.\" The pole error is generated at least as far back as glibc 2.4
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.RB + HUGE_VAL ,
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.RB + HUGE_VALF ,
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or
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.RB + HUGE_VALL ,
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is returned.
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.SH ERRORS
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.\" FIXME . review status of this error
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.\" longstanding bug report for glibc:
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.\" http://sources.redhat.com/bugzilla/show_bug.cgi?id=369
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.\" For negative x, and -large and +large y, glibc 2.8 gives incorrect
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.\" results
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.\" pow(-0.5,-DBL_MAX)=nan
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.\" EDOM FE_INVALID nan; fail-errno fail-except fail-result;
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.\" FAIL (expected: range-error-overflow (ERANGE, FE_OVERFLOW); +INF)
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.\"
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.\" pow(-1.5,-DBL_MAX)=nan
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.\" EDOM FE_INVALID nan; fail-errno fail-except fail-result;
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.\" FAIL (expected: range-error-underflow (ERANGE, FE_UNDERFLOW); +0)
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.\"
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.\" pow(-0.5,DBL_MAX)=nan
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.\" EDOM FE_INVALID nan; fail-errno fail-except fail-result;
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.\" FAIL (expected: range-error-underflow (ERANGE, FE_UNDERFLOW); +0)
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.\"
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.\" pow(-1.5,DBL_MAX)=nan
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.\" EDOM FE_INVALID nan; fail-errno fail-except fail-result;
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.\" FAIL (expected: range-error-overflow (ERANGE, FE_OVERFLOW); +INF)
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See
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.BR math_error (7)
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for information on how to determine whether an error has occurred
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when calling these functions.
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.PP
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The following errors can occur:
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.TP
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Domain error: \fIx\fP is negative, and \fIy\fP is a finite noninteger
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.I errno
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is set to
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.BR EDOM .
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An invalid floating-point exception
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.RB ( FE_INVALID )
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is raised.
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.TP
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Pole error: \fIx\fP is zero, and \fIy\fP is negative
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.I errno
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is set to
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.BR ERANGE
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(but see BUGS).
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A divide-by-zero floating-point exception
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.RB ( FE_DIVBYZERO )
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is raised.
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.TP
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Range error: the result overflows
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.I errno
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is set to
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.BR ERANGE .
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An overflow floating-point exception
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.RB ( FE_OVERFLOW )
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is raised.
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.TP
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Range error: the result underflows
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.I errno
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is set to
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.BR ERANGE .
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An underflow floating-point exception
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.RB ( FE_UNDERFLOW )
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is raised.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lbw21 lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR pow (),
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.BR powf (),
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.BR powl ()
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T} Thread safety MT-Safe
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.TE
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.SH CONFORMING TO
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C99, POSIX.1-2001, POSIX.1-2008.
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.PP
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The variant returning
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.I double
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also conforms to
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SVr4, 4.3BSD, C89.
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.SH BUGS
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On 64-bits,
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.\"
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.\" https://sourceware.org/bugzilla/show_bug.cgi?id=13932
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.BR pow ()
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may be more than 10,000 times slower for some (rare) inputs
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than for other nearby inputs.
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This affects only
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.BR pow (),
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and not
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.BR powf ()
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nor
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.BR powl ().
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.PP
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In glibc 2.9 and earlier,
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.\"
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.\" http://sources.redhat.com/bugzilla/show_bug.cgi?id=6776
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when a pole error occurs,
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.I errno
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is set to
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.BR EDOM
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instead of the POSIX-mandated
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.BR ERANGE .
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Since version 2.10,
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.\" or possibly 2.9, I haven't found the source code change
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.\" and I don't have a 2.9 system to test
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glibc does the right thing.
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.PP
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If
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.I x
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is negative,
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then large negative or positive
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.I y
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values yield a NaN as the function result, with
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.I errno
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set to
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.BR EDOM ,
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and an invalid
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.RB ( FE_INVALID )
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floating-point exception.
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For example, with
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.BR pow (),
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one sees this behavior when the absolute value of
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.I y
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is greater than about 9.223373e18.
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.\" see bug http://sources.redhat.com/bugzilla/show_bug.cgi?id=3866
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.\" and http://sources.redhat.com/bugzilla/show_bug.cgi?id=369
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.PP
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In version 2.3.2 and earlier,
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.\" FIXME . Actually, 2.3.2 is the earliest test result I have; so yet
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.\" to confirm if this error occurs only in 2.3.2.
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when an overflow or underflow error occurs, glibc's
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.BR pow ()
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generates a bogus invalid floating-point exception
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.RB ( FE_INVALID )
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in addition to the overflow or underflow exception.
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.SH SEE ALSO
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.BR cbrt (3),
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.BR cpow (3),
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.BR sqrt (3)
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