invoke.texi

理解和实践操作系统的一本好书

@node C Dialect Options
@section Options Controlling C Dialect
@cindex dialect options
@cindex language dialect options
@cindex options, dialect

The following options control the dialect of C (or languages derived
from C, such as C++, Objective-C and Objective-C++) that the compiler
accepts:

@table @gcctabopt
@cindex ANSI support
@cindex ISO support
@item -ansi
@opindex ansi
In C mode, this is equivalent to @samp{-std=c89}. In C++ mode, it is
equivalent to @samp{-std=c++98}.

This turns off certain features of GCC that are incompatible with ISO
C90 (when compiling C code), or of standard C++ (when compiling C++ code),
such as the @code{asm} and @code{typeof} keywords, and
predefined macros such as @code{unix} and @code{vax} that identify the
type of system you are using.  It also enables the undesirable and
rarely used ISO trigraph feature.  For the C compiler,
it disables recognition of C++ style @samp{//} comments as well as
the @code{inline} keyword.

The alternate keywords @code{__asm__}, @code{__extension__},
@code{__inline__} and @code{__typeof__} continue to work despite
@option{-ansi}.  You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that might be included
in compilations done with @option{-ansi}.  Alternate predefined macros
such as @code{__unix__} and @code{__vax__} are also available, with or
without @option{-ansi}.

The @option{-ansi} option does not cause non-ISO programs to be
rejected gratuitously.  For that, @option{-pedantic} is required in
addition to @option{-ansi}.  @xref{Warning Options}.

The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
option is used.  Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that the
ISO standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.

Functions that would normally be built in but do not have semantics
defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
functions when @option{-ansi} is used.  @xref{Other Builtins,,Other
built-in functions provided by GCC}, for details of the functions
affected.

@item -std=
@opindex std
Determine the language standard. @xref{Standards,,Language Standards
Supported by GCC}, for details of these standard versions.  This option
is currently only supported when compiling C or C++. 

The compiler can accept several base standards, such as @samp{c89} or
@samp{c++98}, and GNU dialects of those standards, such as
@samp{gnu89} or @samp{gnu++98}.  By specifing a base standard, the
compiler will accept all programs following that standard and those
using GNU extensions that do not contradict it.  For example,
@samp{-std=c89} turns off certain features of GCC that are
incompatible with ISO C90, such as the @code{asm} and @code{typeof}
keywords, but not other GNU extensions that do not have a meaning in
ISO C90, such as omitting the middle term of a @code{?:}
expression. On the other hand, by specifing a GNU dialect of a
standard, all features the compiler support are enabled, even when
those features change the meaning of the base standard and some
strict-conforming programs may be rejected.  The particular standard
is used by @option{-pedantic} to identify which features are GNU
extensions given that version of the standard. For example
@samp{-std=gnu89 -pedantic} would warn about C++ style @samp{//}
comments, while @samp{-std=gnu99 -pedantic} would not.

A value for this option must be provided; possible values are

@table @samp
@item c89
@itemx iso9899:1990
Support all ISO C90 programs (certain GNU extensions that conflict
with ISO C90 are disabled). Same as @option{-ansi} for C code.

@item iso9899:199409
ISO C90 as modified in amendment 1.

@item c99
@itemx c9x
@itemx iso9899:1999
@itemx iso9899:199x
ISO C99.  Note that this standard is not yet fully supported; see
@w{@uref{http://gcc.gnu.org/gcc-4.3/c99status.html}} for more information.  The
names @samp{c9x} and @samp{iso9899:199x} are deprecated.

@item gnu89
GNU dialect of ISO C90 (including some C99 features). This
is the default for C code.

@item gnu99
@itemx gnu9x
GNU dialect of ISO C99.  When ISO C99 is fully implemented in GCC,
this will become the default.  The name @samp{gnu9x} is deprecated.

@item c++98
The 1998 ISO C++ standard plus amendments. Same as @option{-ansi} for
C++ code.

@item gnu++98
GNU dialect of @option{-std=c++98}.  This is the default for
C++ code.

@item c++0x
The working draft of the upcoming ISO C++0x standard. This option
enables experimental features that are likely to be included in
C++0x. The working draft is constantly changing, and any feature that is
enabled by this flag may be removed from future versions of GCC if it is
not part of the C++0x standard.

@item gnu++0x
GNU dialect of @option{-std=c++0x}. This option enables
experimental features that may be removed in future versions of GCC.
@end table

@item -fgnu89-inline
@opindex fgnu89-inline
The option @option{-fgnu89-inline} tells GCC to use the traditional
GNU semantics for @code{inline} functions when in C99 mode.
@xref{Inline,,An Inline Function is As Fast As a Macro}.  This option
is accepted and ignored by GCC versions 4.1.3 up to but not including
4.3.  In GCC versions 4.3 and later it changes the behavior of GCC in
C99 mode.  Using this option is roughly equivalent to adding the
@code{gnu_inline} function attribute to all inline functions
(@pxref{Function Attributes}).

The option @option{-fno-gnu89-inline} explicitly tells GCC to use the
C99 semantics for @code{inline} when in C99 or gnu99 mode (i.e., it
specifies the default behavior).  This option was first supported in
GCC 4.3.  This option is not supported in C89 or gnu89 mode.

The preprocessor macros @code{__GNUC_GNU_INLINE__} and
@code{__GNUC_STDC_INLINE__} may be used to check which semantics are
in effect for @code{inline} functions.  @xref{Common Predefined
Macros,,,cpp,The C Preprocessor}.

@item -aux-info @var{filename}
@opindex aux-info
Output to the given filename prototyped declarations for all functions
declared and/or defined in a translation unit, including those in header
files.  This option is silently ignored in any language other than C@.

Besides declarations, the file indicates, in comments, the origin of
each declaration (source file and line), whether the declaration was
implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
@samp{O} for old, respectively, in the first character after the line
number and the colon), and whether it came from a declaration or a
definition (@samp{C} or @samp{F}, respectively, in the following
character).  In the case of function definitions, a K&R-style list of
arguments followed by their declarations is also provided, inside
comments, after the declaration.

@item -fno-asm
@opindex fno-asm
Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
keyword, so that code can use these words as identifiers.  You can use
the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
instead.  @option{-ansi} implies @option{-fno-asm}.

In C++, this switch only affects the @code{typeof} keyword, since
@code{asm} and @code{inline} are standard keywords.  You may want to
use the @option{-fno-gnu-keywords} flag instead, which has the same
effect.  In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
switch only affects the @code{asm} and @code{typeof} keywords, since
@code{inline} is a standard keyword in ISO C99.

@item -fno-builtin
@itemx -fno-builtin-@var{function}
@opindex fno-builtin
@cindex built-in functions
Don't recognize built-in functions that do not begin with
@samp{__builtin_} as prefix.  @xref{Other Builtins,,Other built-in
functions provided by GCC}, for details of the functions affected,
including those which are not built-in functions when @option{-ansi} or
@option{-std} options for strict ISO C conformance are used because they
do not have an ISO standard meaning.

GCC normally generates special code to handle certain built-in functions
more efficiently; for instance, calls to @code{alloca} may become single
instructions that adjust the stack directly, and calls to @code{memcpy}
may become inline copy loops.  The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library.  In addition,
when a function is recognized as a built-in function, GCC may use
information about that function to warn about problems with calls to
that function, or to generate more efficient code, even if the
resulting code still contains calls to that function.  For example,
warnings are given with @option{-Wformat} for bad calls to
@code{printf}, when @code{printf} is built in, and @code{strlen} is
known not to modify global memory.

With the @option{-fno-builtin-@var{function}} option
only the built-in function @var{function} is
disabled.  @var{function} must not begin with @samp{__builtin_}.  If a
function is named this is not built-in in this version of GCC, this
option is ignored.  There is no corresponding
@option{-fbuiltin-@var{function}} option; if you wish to enable
built-in functions selectively when using @option{-fno-builtin} or
@option{-ffreestanding}, you may define macros such as:

@smallexample
#define abs(n)          __builtin_abs ((n))
#define strcpy(d, s)    __builtin_strcpy ((d), (s))
@end smallexample

@item -fhosted
@opindex fhosted
@cindex hosted environment

Assert that compilation takes place in a hosted environment.  This implies
@option{-fbuiltin}.  A hosted environment is one in which the
entire standard library is available, and in which @code{main} has a return
type of @code{int}.  Examples are nearly everything except a kernel.
This is equivalent to @option{-fno-freestanding}.

@item -ffreestanding
@opindex ffreestanding
@cindex hosted environment

Assert that compilation takes place in a freestanding environment.  This
implies @option{-fno-builtin}.  A freestanding environment
is one in which the standard library may not exist, and program startup may
not necessarily be at @code{main}.  The most obvious example is an OS kernel.
This is equivalent to @option{-fno-hosted}.

@xref{Standards,,Language Standards Supported by GCC}, for details of
freestanding and hosted environments.

@item -fopenmp
@opindex fopenmp
@cindex openmp parallel
Enable handling of OpenMP directives @code{#pragma omp} in C/C++ and
@code{!$omp} in Fortran.  When @option{-fopenmp} is specified, the
compiler generates parallel code according to the OpenMP Application
Program Interface v2.5 @w{@uref{http://www.openmp.org/}}.  This option
implies @option{-pthread}, and thus is only supported on targets that
have support for @option{-pthread}.

@item -fms-extensions
@opindex fms-extensions
Accept some non-standard constructs used in Microsoft header files.

Some cases of unnamed fields in structures and unions are only
accepted with this option.  @xref{Unnamed Fields,,Unnamed struct/union
fields within structs/unions}, for details.

@item -trigraphs
@opindex trigraphs
Support ISO C trigraphs.  The @option{-ansi} option (and @option{-std}
options for strict ISO C conformance) implies @option{-trigraphs}.

@item -no-integrated-cpp
@opindex no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling.  This
option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
@option{-B} option.  The user supplied compilation step can then add in
an additional preprocessing step after normal preprocessing but before
compiling.  The default is to use the integrated cpp (internal cpp)

The semantics of this option will change if "cc1", "cc1plus", and
"cc1obj" are merged.

@cindex traditional C language
@cindex C language, traditional
@item -traditional
@itemx -traditional-cpp
@opindex traditional-cpp
@opindex traditional
Formerly, these options caused GCC to attempt to emulate a pre-standard
C compiler.  They are now only supported with the @option{-E} switch.
The preprocessor continues to support a pre-standard mode.  See the GNU
CPP manual for details.

@item -fcond-mismatch
@opindex fcond-mismatch
Allow conditional expressions with mismatched types in the second and
third arguments.  The value of such an expression is void.  This option
is not supported for C++.

@item -flax-vector-conversions
@opindex flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of
elements and/or incompatible element types.  This option should not be
used for new code.

@item -funsigned-char
@opindex funsigned-char
Let the type @code{char} be unsigned, like @code{unsigned char}.

Each kind of machine has a default for what @code{char} should
be.  It is either like @code{unsigned char} by default or like
@code{signed char} by default.

Ideally, a portable program should always use @code{signed char} or
@code{unsigned char} when it depends on the signedness of an object.
But many programs have been written to use plain @code{char} and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for.  This option, and its inverse, let you
make such a program work with the opposite default.

The type @code{char} is always a distinct type from each of
@code{signed char} or @code{unsigned char}, even though its behavior
is always just like one of those two.

@item -fsigned-char
@opindex fsigned-char
Let the type @code{char} be signed, like @code{signed char}.

Note