implement-c.texi

理解和实践操作系统的一本好书

@c Copyright (C) 2001, 2002, 2003, 2004, 2006 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node C Implementation
@chapter C Implementation-defined behavior
@cindex implementation-defined behavior, C language

A conforming implementation of ISO C is required to document its
choice of behavior in each of the areas that are designated
``implementation defined''.  The following lists all such areas,
along with the section numbers from the ISO/IEC 9899:1990 and ISO/IEC
9899:1999 standards.  Some areas are only implementation-defined in
one version of the standard.

Some choices depend on the externally determined ABI for the platform
(including standard character encodings) which GCC follows; these are
listed as ``determined by ABI'' below.  @xref{Compatibility, , Binary
Compatibility}, and @uref{http://gcc.gnu.org/readings.html}.  Some
choices are documented in the preprocessor manual.
@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.  Some choices are made by the
library and operating system (or other environment when compiling for
a freestanding environment); refer to their documentation for details.

@menu
* Translation implementation::
* Environment implementation::
* Identifiers implementation::
* Characters implementation::
* Integers implementation::
* Floating point implementation::
* Arrays and pointers implementation::
* Hints implementation::
* Structures unions enumerations and bit-fields implementation::
* Qualifiers implementation::
* Declarators implementation::
* Statements implementation::
* Preprocessing directives implementation::
* Library functions implementation::
* Architecture implementation::
* Locale-specific behavior implementation::
@end menu

@node Translation implementation
@section Translation

@itemize @bullet
@item
@cite{How a diagnostic is identified (C90 3.7, C99 3.10, C90 and C99 5.1.1.3).}

Diagnostics consist of all the output sent to stderr by GCC@.

@item
@cite{Whether each nonempty sequence of white-space characters other than
new-line is retained or replaced by one space character in translation
phase 3 (C90 and C99 5.1.1.2).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@end itemize

@node Environment implementation
@section Environment

The behavior of most of these points are dependent on the implementation
of the C library, and are not defined by GCC itself.

@itemize @bullet
@item
@cite{The mapping between physical source file multibyte characters
and the source character set in translation phase 1 (C90 and C99 5.1.1.2).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@end itemize

@node Identifiers implementation
@section Identifiers

@itemize @bullet
@item
@cite{Which additional multibyte characters may appear in identifiers
and their correspondence to universal character names (C99 6.4.2).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{The number of significant initial characters in an identifier
(C90 6.1.2, C90 and C99 5.2.4.1, C99 6.4.2).}

For internal names, all characters are significant.  For external names,
the number of significant characters are defined by the linker; for
almost all targets, all characters are significant.

@item
@cite{Whether case distinctions are significant in an identifier with
external linkage (C90 6.1.2).}

This is a property of the linker.  C99 requires that case distinctions
are always significant in identifiers with external linkage and
systems without this property are not supported by GCC@.

@end itemize

@node Characters implementation
@section Characters

@itemize @bullet
@item
@cite{The number of bits in a byte (C90 3.4, C99 3.6).}

Determined by ABI@.

@item
@cite{The values of the members of the execution character set (C90
and C99 5.2.1).}

Determined by ABI@.

@item
@cite{The unique value of the member of the execution character set produced
for each of the standard alphabetic escape sequences (C90 and C99 5.2.2).}

Determined by ABI@.

@item
@cite{The value of a @code{char} object into which has been stored any
character other than a member of the basic execution character set
(C90 6.1.2.5, C99 6.2.5).}

Determined by ABI@.

@item
@cite{Which of @code{signed char} or @code{unsigned char} has the same
range, representation, and behavior as ``plain'' @code{char} (C90
6.1.2.5, C90 6.2.1.1, C99 6.2.5, C99 6.3.1.1).}

@opindex fsigned-char
@opindex funsigned-char
Determined by ABI@.  The options @option{-funsigned-char} and
@option{-fsigned-char} change the default.  @xref{C Dialect Options, ,
Options Controlling C Dialect}.

@item
@cite{The mapping of members of the source character set (in character
constants and string literals) to members of the execution character
set (C90 6.1.3.4, C99 6.4.4.4, C90 and C99 5.1.1.2).}

Determined by ABI@.

@item
@cite{The value of an integer character constant containing more than one
character or containing a character or escape sequence that does not map
to a single-byte execution character (C90 6.1.3.4, C99 6.4.4.4).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{The value of a wide character constant containing more than one
multibyte character, or containing a multibyte character or escape
sequence not represented in the extended execution character set (C90
6.1.3.4, C99 6.4.4.4).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{The current locale used to convert a wide character constant consisting
of a single multibyte character that maps to a member of the extended
execution character set into a corresponding wide character code (C90
6.1.3.4, C99 6.4.4.4).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{The current locale used to convert a wide string literal into
corresponding wide character codes (C90 6.1.4, C99 6.4.5).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{The value of a string literal containing a multibyte character or escape
sequence not represented in the execution character set (C90 6.1.4, C99 6.4.5).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.
@end itemize

@node Integers implementation
@section Integers

@itemize @bullet
@item
@cite{Any extended integer types that exist in the implementation (C99 6.2.5).}

GCC does not support any extended integer types.
@c The __mode__ attribute might create types of precisions not
@c otherwise supported, but the syntax isn't right for use everywhere
@c the standard type names might be used.  Predefined typedefs should
@c be used if any extended integer types are to be defined.  The
@c __int128_t and __uint128_t typedefs are not extended integer types
@c as they are generally longer than the ABI-specified intmax_t.

@item
@cite{Whether signed integer types are represented using sign and magnitude,
two's complement, or one's complement, and whether the extraordinary value
is a trap representation or an ordinary value (C99 6.2.6.2).}

GCC supports only two's complement integer types, and all bit patterns
are ordinary values.

@item
@cite{The rank of any extended integer type relative to another extended
integer type with the same precision (C99 6.3.1.1).}

GCC does not support any extended integer types.
@c If it did, there would only be one of each precision and signedness.

@item
@cite{The result of, or the signal raised by, converting an integer to a
signed integer type when the value cannot be represented in an object of
that type (C90 6.2.1.2, C99 6.3.1.3).}

For conversion to a type of width @math{N}, the value is reduced
modulo @math{2^N} to be within range of the type; no signal is raised.

@item
@cite{The results of some bitwise operations on signed integers (C90
6.3, C99 6.5).}

Bitwise operators act on the representation of the value including
both the sign and value bits, where the sign bit is considered
immediately above the highest-value value bit.  Signed @samp{>>} acts
on negative numbers by sign extension.

GCC does not use the latitude given in C99 only to treat certain
aspects of signed @samp{<<} as undefined, but this is subject to
change.

@item
@cite{The sign of the remainder on integer division (C90 6.3.5).}

GCC always follows the C99 requirement that the result of division is
truncated towards zero.

@end itemize

@node Floating point implementation
@section Floating point

@itemize @bullet
@item
@cite{The accuracy of the floating-point operations and of the library
functions in @code{<math.h>} and @code{<complex.h>} that return floating-point
results (C90 and C99 5.2.4.2.2).}

The accuracy is unknown.

@item
@cite{The rounding behaviors characterized by non-standard values
of @code{FLT_ROUNDS} @gol
(C90 and C99 5.2.4.2.2).}

GCC does not use such values.

@item
@cite{The evaluation methods characterized by non-standard negative
values of @code{FLT_EVAL_METHOD} (C99 5.2.4.2.2).}

GCC does not use such values.

@item
@cite{The direction of rounding when an integer is converted to a
floating-point number that cannot exactly represent the original
value (C90 6.2.1.3, C99 6.3.1.4).}

C99 Annex F is followed.

@item
@cite{The direction of rounding when a floating-point number is
converted to a narrower floating-point number (C90 6.2.1.4, C99
6.3.1.5).}

C99 Annex F is followed.

@item
@cite{How the nearest representable value or the larger or smaller
representable value immediately adjacent to the nearest representable
value is chosen for certain floating constants (C90 6.1.3.1, C99
6.4.4.2).}

C99 Annex F is followed.

@item
@cite{Whether and how floating expressions are contracted when not
disallowed by the @code{FP_CONTRACT} pragma (C99 6.5).}

Expressions are currently only contracted if
@option{-funsafe-math-optimizations} or @option{-ffast-math} are used.
This is subject to change.

@item
@cite{The default state for the @code{FENV_ACCESS} pragma (C99 7.6.1).}

This pragma is not implemented, but the default is to ``off'' unless
@option{-frounding-math} is used in which case it is ``on''.

@item
@cite{Additional floating-point exceptions, rounding modes, environments,
and classifications, and their macro names (C99 7.6, C99 7.12).}

This is dependent on the implementation of the C library, and is not
defined by GCC itself.

@item
@cite{The default state for the @code{FP_CONTRACT} pragma (C99 7.12.2).}

This pragma is not implemented.  Expressions are currently only
contracted if @option{-funsafe-math-optimizations} or
@option{-ffast-math} are used.  This is subject to change.

@item
@cite{Whether the ``inexact'' floating-point exception can be raised
when the rounded result actually does equal the mathematical result
in an IEC 60559 conformant implementation (C99 F.9).}

This is dependent on the implementation of the C library, and is not
defined by GCC itself.

@item
@cite{Whether the ``underflow'' (and ``inexact'') floating-point