mirror of https://github.com/mkerrisk/man-pages
248 lines
8.3 KiB
Plaintext
248 lines
8.3 KiB
Plaintext
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
|
|
.TH "<fenv.h>" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
|
|
.\" <fenv.h>
|
|
.SH NAME
|
|
fenv.h \- floating-point environment
|
|
.SH SYNOPSIS
|
|
.LP
|
|
\fB#include <fenv.h>\fP
|
|
.SH DESCRIPTION
|
|
.LP
|
|
The \fI<fenv.h>\fP header shall define the following data types through
|
|
\fBtypedef\fP:
|
|
.TP 7
|
|
\fBfenv_t\fP
|
|
Represents the entire floating-point environment. The floating-point
|
|
environment refers collectively to any floating-point
|
|
status flags and control modes supported by the implementation.
|
|
.TP 7
|
|
\fBfexcept_t\fP
|
|
Represents the floating-point status flags collectively, including
|
|
any status the implementation associates with the flags. A
|
|
floating-point status flag is a system variable whose value is set
|
|
(but never cleared) when a floating-point exception is raised,
|
|
which occurs as a side effect of exceptional floating-point arithmetic
|
|
to provide auxiliary information. A floating-point control
|
|
mode is a system variable whose value may be set by the user to affect
|
|
the subsequent behavior of floating-point arithmetic.
|
|
.sp
|
|
.LP
|
|
The \fI<fenv.h>\fP header shall define the following constants if
|
|
and only if the implementation supports the
|
|
floating-point exception by means of the floating-point functions
|
|
\fIfeclearexcept\fP(), \fIfegetexceptflag\fP(), \fIferaiseexcept\fP(),
|
|
\fIfesetexceptflag\fP(), and \fIfetestexcept\fP(). Each expands to
|
|
an integer constant expression with values such that
|
|
bitwise-inclusive ORs of all combinations of the constants result
|
|
in distinct values.
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
FE_DIVBYZERO
|
|
FE_INEXACT
|
|
FE_INVALID
|
|
FE_OVERFLOW
|
|
FE_UNDERFLOW
|
|
.fi
|
|
.RE
|
|
.LP
|
|
The \fI<fenv.h>\fP header shall define the following constant, which
|
|
is simply the bitwise-inclusive OR of all
|
|
floating-point exception constants defined above:
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
FE_ALL_EXCEPT
|
|
.fi
|
|
.RE
|
|
.LP
|
|
The \fI<fenv.h>\fP header shall define the following constants if
|
|
and only if the implementation supports getting and
|
|
setting the represented rounding direction by means of the \fIfegetround\fP()
|
|
and \fIfesetround\fP() functions. Each expands to an integer constant
|
|
expression whose values
|
|
are distinct non-negative vales.
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
FE_DOWNWARD
|
|
FE_TONEAREST
|
|
FE_TOWARDZERO
|
|
FE_UPWARD
|
|
.fi
|
|
.RE
|
|
.LP
|
|
The \fI<fenv.h>\fP header shall define the following constant, which
|
|
represents the default floating-point environment
|
|
(that is, the one installed at program startup) and has type pointer
|
|
to const-qualified \fBfenv_t\fP. It can be used as an
|
|
argument to the functions within the \fI<fenv.h>\fP header that manage
|
|
the floating-point environment.
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
FE_DFL_ENV
|
|
.fi
|
|
.RE
|
|
.LP
|
|
The following shall be declared as functions and may also be defined
|
|
as macros. Function prototypes shall be provided.
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
\fBint feclearexcept(int);
|
|
int fegetexceptflag(fexcept_t *, int);
|
|
int feraiseexcept(int);
|
|
int fesetexceptflag(const fexcept_t *, int);
|
|
int fetestexcept(int);
|
|
int fegetround(void);
|
|
int fesetround(int);
|
|
int fegetenv(fenv_t *);
|
|
int feholdexcept(fenv_t *);
|
|
int fesetenv(const fenv_t *);
|
|
int feupdateenv(const fenv_t *);
|
|
\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
The FENV_ACCESS pragma provides a means to inform the implementation
|
|
when an application might access the floating-point
|
|
environment to test floating-point status flags or run under non-default
|
|
floating-point control modes. The pragma shall occur
|
|
either outside external declarations or preceding all explicit declarations
|
|
and statements inside a compound statement. When
|
|
outside external declarations, the pragma takes effect from its occurrence
|
|
until another FENV_ACCESS pragma is encountered, or
|
|
until the end of the translation unit. When inside a compound statement,
|
|
the pragma takes effect from its occurrence until another
|
|
FENV_ACCESS pragma is encountered (including within a nested compound
|
|
statement), or until the end of the compound statement; at
|
|
the end of a compound statement the state for the pragma is restored
|
|
to its condition just before the compound statement. If this
|
|
pragma is used in any other context, the behavior is undefined. If
|
|
part of an application tests floating-point status flags, sets
|
|
floating-point control modes, or runs under non-default mode settings,
|
|
but was translated with the state for the FENV_ACCESS pragma
|
|
off, the behavior is undefined. The default state (on or off) for
|
|
the pragma is implementation-defined. (When execution passes from
|
|
a part of the application translated with FENV_ACCESS off to a part
|
|
translated with FENV_ACCESS on, the state of the floating-point
|
|
status flags is unspecified and the floating-point control modes have
|
|
their default settings.)
|
|
.LP
|
|
\fIThe following sections are informative.\fP
|
|
.SH APPLICATION USAGE
|
|
.LP
|
|
This header is designed to support the floating-point exception status
|
|
flags and directed-rounding control modes required by the
|
|
IEC\ 60559:1989 standard, and other similar floating-point state information.
|
|
Also it is designed to facilitate code
|
|
portability among all systems.
|
|
.LP
|
|
Certain application programming conventions support the intended model
|
|
of use for the floating-point environment:
|
|
.IP " *" 3
|
|
A function call does not alter its caller's floating-point control
|
|
modes, clear its caller's floating-point status flags, nor
|
|
depend on the state of its caller's floating-point status flags unless
|
|
the function is so documented.
|
|
.LP
|
|
.IP " *" 3
|
|
A function call is assumed to require default floating-point control
|
|
modes, unless its documentation promises otherwise.
|
|
.LP
|
|
.IP " *" 3
|
|
A function call is assumed to have the potential for raising floating-point
|
|
exceptions, unless its documentation promises
|
|
otherwise.
|
|
.LP
|
|
.LP
|
|
With these conventions, an application can safely assume default floating-point
|
|
control modes (or be unaware of them). The
|
|
responsibilities associated with accessing the floating-point environment
|
|
fall on the application that does so explicitly.
|
|
.LP
|
|
Even though the rounding direction macros may expand to constants
|
|
corresponding to the values of FLT_ROUNDS, they are not
|
|
required to do so.
|
|
.LP
|
|
For example:
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
\fB#include <fenv.h>
|
|
void f(double x)
|
|
{
|
|
#pragma STDC FENV_ACCESS ON
|
|
void g(double);
|
|
void h(double);
|
|
/* ... */
|
|
g(x + 1);
|
|
h(x + 1);
|
|
/* ... */
|
|
}
|
|
\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
If the function \fIg\fP() might depend on status flags set as a side
|
|
effect of the first \fIx\fP+1, or if the second
|
|
\fIx\fP+1 might depend on control modes set as a side effect of the
|
|
call to function \fIg\fP(), then the application shall
|
|
contain an appropriately placed invocation as follows:
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
\fB#pragma STDC FENV_ACCESS ON
|
|
\fP
|
|
.fi
|
|
.RE
|
|
.SH RATIONALE
|
|
.SS The fexcept_t Type
|
|
.LP
|
|
\fBfexcept_t\fP does not have to be an integer type. Its values must
|
|
be obtained by a call to \fIfegetexceptflag\fP(), and cannot be created
|
|
by logical operations from the exception
|
|
macros. An implementation might simply implement \fBfexcept_t\fP as
|
|
an \fBint\fP and use the representations reflected by the
|
|
exception macros, but is not required to; other representations might
|
|
contain extra information about the exceptions.
|
|
\fBfexcept_t\fP might be a \fBstruct\fP with a member for each exception
|
|
(that might hold the address of the first or last
|
|
floating-point instruction that caused that exception). The ISO/IEC\ 9899:1999
|
|
standard makes no claims about the internals of
|
|
an \fBfexcept_t\fP, and so the user cannot inspect it.
|
|
.SS Exception and Rounding Macros
|
|
.LP
|
|
Macros corresponding to unsupported modes and rounding directions
|
|
are not defined by the implementation and must not be defined
|
|
by the application. An application might use \fB#ifdef\fP to test
|
|
for this.
|
|
.SH FUTURE DIRECTIONS
|
|
.LP
|
|
None.
|
|
.SH SEE ALSO
|
|
.LP
|
|
The System Interfaces volume of IEEE\ Std\ 1003.1-2001, \fIfeclearexcept\fP(),
|
|
\fIfegetenv\fP(), \fIfegetexceptflag\fP(), \fIfegetround\fP(),
|
|
\fIfeholdexcept\fP(), \fIferaiseexcept\fP(), \fIfesetenv\fP(), \fIfesetexceptflag\fP(),
|
|
\fIfesetround\fP(),
|
|
\fIfetestexcept\fP(), \fIfeupdateenv\fP()
|
|
.SH COPYRIGHT
|
|
Portions of this text are reprinted and reproduced in electronic form
|
|
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
|
|
-- Portable Operating System Interface (POSIX), The Open Group Base
|
|
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
|
|
Electrical and Electronics Engineers, Inc and The Open Group. In the
|
|
event of any discrepancy between this version and the original IEEE and
|
|
The Open Group Standard, the original IEEE and The Open Group Standard
|
|
is the referee document. The original Standard can be obtained online at
|
|
http://www.opengroup.org/unix/online.html .
|