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Andreas,

The latest version of the page is below.

===

Hi Andreas,

(Andries suggested that you probably have the background knowledge to

help here.)

The math man pages in man-pages are in a somewhat sorry state, with

respect to the following:

* Few of the pages properly describe the special cases for Inf, -Inf,

NaN arguments (e.g., compare "man 3 log" with the POSIX.1 page "man 3p

log").

* There isn't a clear discussion of error cases, and how to determine

if an error occurrred using errno and/or fetestexcept(3).

I'm planning to fix each of the math man pages to address these

issues, and use a new page, math_error.7, as an anchor page referenced

by all of the math pages for discussion of how to handle errors.

Would you be willing to review this new page (below) to see whether it

correctly describes the glibc details? Might you also be willing to

look at a sampling of the changed math page pages that I'll make later

this week/early next week in order to let me know I'm on the right

track in terms of the changes I'm making?

Cheers,

Michael

.\" Copyright (c) 2008, Linux Foundation, written by Michael Kerrisk

.\" <mtk.manpages@xxxxxxxxx>

.\"

.\" Permission is granted to make and distribute verbatim copies of this

.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

.\"

.\" Permission is granted to copy and distribute modified versions of this

.\" manual under the conditions for verbatim copying, provided that the

.\" entire resulting derived work is distributed under the terms of a

.\" permission notice identical to this one.

.\"

.\" Since the Linux kernel and libraries are constantly changing, this

.\" manual page may be incorrect or out-of-date. The author(s) assume no

.\" responsibility for errors or omissions, or for damages resulting from

.\" the use of the information contained herein. The author(s) may not

.\" have taken the same level of care in the production of this manual,

.\" which is licensed free of charge, as they might when working

.\" professionally.

.\"

.\" Formatted or processed versions of this manual, if unaccompanied by

.\" the source, must acknowledge the copyright and authors of this work.

.\"

.TH MATH_ERROR 7 2008-07-21 "Linux" "Linux Programmer's Manual"

.SH SYNOPSIS

.nf

.B #include <math.h>

.B #include <errno.h>

.B #include <fenv.h>

.fi

.SH NAME

math_error \- detecting errors from mathematical functions

.SH DESCRIPTION

On error, many of the mathematical functions declared in

.IR <math.h>

return a NaN (not a number).

However, rather than looking at the return value

(which is not always possible)

one can also check whether an error was signaled.

There are two signaling mechanisms:

the older one sets

.IR errno ;

the newer one uses the floating-point exception mechanism (the use of

.BR feclearexcept (3)

and

.BR fetestexcept (3),

as outlined below)

described in

.BR fenv (3).

C99 and POSIX.1-2001 specify a

.I math_errhandling

identifier,

which is supposed to indicate which of these two mechanisms is in use;

the standards require that at least one be in use,

but permit both to be available.

Although glibc does not support this identifier,

in practice it supports both mechanisms.

A portable program that needs to check for an error from a mathematical

function should set

.I errno

to zero, and make the following call

.in +4n

.nf

feclearexcept(FE_ALL_EXCEPT);

.fi

.in

before calling a mathematical function.

Upon return from the mathematical function, if

.I errno

is non-zero, or the following call (see

.BR fenv (3))

returns non-zero

.in +4n

.nf

fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW |

FE_UNDERFLOW);

.fi

.in

.\" enum

.\" {

.\" FE_INVALID = 0x01,

.\" __FE_DENORM = 0x02,

.\" FE_DIVBYZERO = 0x04,

.\" FE_OVERFLOW = 0x08,

.\" FE_UNDERFLOW = 0x10,

.\" FE_INEXACT = 0x20

.\" };

then an error occurred in the mathematical function.

The error conditions that can occur for mathematical functions

are described below.

.SS Domain Error

A

.I domain error

occurs when a mathematical function is supplied with an argument whose

value falls outside the domain for which the function

is defined (e.g., giving a negative argument to

.BR log (3)).

When a domain error occurs,

.I errno

is set to

.BR EDOM ,

and an "invalid"

.RB ( FE_INVALID )

floating-point exception is raised.

.SS Pole Error

A

.I pole error

occurs when the mathematical result of a function is an exact infinity

(e.g., the logarithm of 0 is negative infinity).

When a pole error occurs,

the function returns the (signed) value

.BR HUGE_VAL ,

.BR HUGE_VALF ,

or

.BR HUGE_VALL ,

depending on whether the function result type is

.IR double ,

.IR float ,

or

.IR "long double" .

The sign of the result is that which is mathematically correct for

the function.

.I errno

is set to

.BR ERANGE ,

and a "divide-by-zero"

.RB ( FE_DIVBYZERO )

floating-point exception is raised.

.SS Range Error

A

.I range error

occurs when the magnitude of the function result means that it

cannot be represented in the result type of the function.

The return value of the function depends on whether the range error

was an overflow or an underflow.

A floating result

.I overflows

if the result is finite,

but is too large to represented in the result type.

When an overflow occurs,

the function returns the value

.BR HUGE_VAL ,

.BR HUGE_VALF ,

or

.BR HUGE_VALL ,

depending on whether the function result type is

.IR double ,

.IR float ,

or

.IR "long double" .

.I errno

is set to

.BR ERANGE ,

and an "overflow"

.RB ( FE_OVERFLOW )

floating-point exception is raised.

A floating result

.I underflows

if the result is too small to be represented in the result type.

If an underflow occurs,

a mathematical function typically returns 0.0

(C99 says a function shall return "an implementation-defined value

whose magnitude is no greater than the smallest normalized

positive number in the specified type").

.\" FIXME(mtk) POSIX.1 says "may" for the following two cases; need to

.\" investigate this further for specific functions.

.I errno

may be set to

.BR ERANGE ,

and an "overflow"

.RB ( FE_UNDERFLOW )

floating-point exception may be raised.

Some functions deliver a range error if the supplied argument value,

or the correct function result, would be

.IR subnormal .

A subnormal value is one that is non-zero,

but with a magnitude that is so small that

it can't be presented in normalized form

(i.e., with a 1 in the most significant bit of the significand).

The representation of a subnormal number will contain one

or more leading zeros in the significand.

.SH NOTES

The

.I math_errhandling

identifier specified by C99 and POSIX.1-2001 is not supported.

.\" See CONFORMANCE in the glibc 2.8 (and earlier) source.

To avoid the complexities of using

.I errno

and

.BR fetestexcept (3)

for error checking,

it is often advised that one should instead check for bad argument

values before each call.

.\" http://www.securecoding.cert.org/confluence/display/seccode/FLP32-C.+Prevent+or+detect+domain+and+range+errors+in+math+functions

For example, the following code ensures that

.BR log (3)'s

argument is not a NaN and is not zero (a pole error) or

less than zero (a domain error):

.in +4n

.nf

double x, r;

if (isnan(x) || islessequal(x, 0)) {

/* Deal with NaN / pole error / domain error */

}

r = log(x);

.fi

.in

The discussion on this page does not apply to the complex

mathematical functions (i.e., those declared by

.IR <complex.h> ),

which in general are not required to return errors by C99

and POSIX.1-2001.

The

.BR gcc (1)

.I "-fno-math-errno"

option causes the executable to employ implementations of some

mathematical functions that are faster than the standard

implementations, but do not set

.I errno

on error.

(The

.BR gcc (1)

.I "-ffast-math"

option also enables

.IR "-fno-math-errno" .)

An error can still be tested for using

.BR fetestexcept (3).

.fi

.in

.SH SEE ALSO

.BR gcc (1),

.BR errno (3),

.BR fenv (3),

.BR fpclassify (3),

.BR INFINITY (3),

.BR isgreater (3),

.BR matherr (3),

.BR nan (3)

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**Next message:**Cyrill Gorcunov: "Re: [PATCH] vfs: use kstrdup() and check failing allocation"**Previous message:**Li Zefan: "Re: [PATCH] vfs: use kstrdup()"**Next in thread:**Michael Kerrisk: "Re: math_error.7 draft 3, for review"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]