invoke.texi

gcc库的原代码,对编程有很大帮助.

@item
In the preprocessor, comments convert to nothing at all, rather than
to a space.  This allows traditional token concatenation.

@item
In preprocessing directive, the @samp{#} symbol must appear as the first
character of a line.

@item
In the preprocessor, macro arguments are recognized within string
constants in a macro definition (and their values are stringified,
though without additional quote marks, when they appear in such a
context).  The preprocessor always considers a string constant to end
at a newline.

@item
@cindex detecting @w{@samp{-traditional}}
The predefined macro @code{__STDC__} is not defined when you use
@samp{-traditional}, but @code{__GNUC__} is (since the GNU extensions
which @code{__GNUC__} indicates are not affected by
@samp{-traditional}).  If you need to write header files that work
differently depending on whether @samp{-traditional} is in use, by
testing both of these predefined macros you can distinguish four
situations: GNU C, traditional GNU C, other ANSI C compilers, and other
old C compilers.  The predefined macro @code{__STDC_VERSION__} is also
not defined when you use @samp{-traditional}.  @xref{Standard
Predefined,,Standard Predefined Macros,cpp.info,The C Preprocessor},
for more discussion of these and other predefined macros.

@item
@cindex string constants vs newline
@cindex newline vs string constants
The preprocessor considers a string constant to end at a newline (unless
the newline is escaped with @samp{\}).  (Without @w{@samp{-traditional}},
string constants can contain the newline character as typed.)

@item -traditional-cpp
Attempt to support some aspects of traditional C preprocessors.
This includes the last five items in the table immediately above,
but none of the other effects of @samp{-traditional}.

@item -fcond-mismatch
Allow conditional expressions with mismatched types in the second and
third arguments.  The value of such an expression is void.

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

Note that this is equivalent to @samp{-fno-unsigned-char}, which is
the negative form of @samp{-funsigned-char}.  Likewise, the option
@samp{-fno-signed-char} is equivalent to @samp{-funsigned-char}.

@item -fsigned-bitfields
@itemx -funsigned-bitfields
@itemx -fno-signed-bitfields
@itemx -fno-unsigned-bitfields
These options control whether a bitfield is signed or unsigned, when the
declaration does not use either @code{signed} or @code{unsigned}.  By
default, such a bitfield is signed, because this is consistent: the
basic integer types such as @code{int} are signed types.

However, when @samp{-traditional} is used, bitfields are all unsigned
no matter what.

@item -fwritable-strings
Store string constants in the writable data segment and don't uniquize
them.  This is for compatibility with old programs which assume they can
write into string constants.  The option @samp{-traditional} also has
this effect.

Writing into string constants is a very bad idea; ``constants'' should
be constant.

@item -fallow-single-precision
Do not promote single precision math operations to double precision,
even when compiling with @samp{-traditional}.

Traditional K&R C promotes all floating point operations to double
precision, regardless of the sizes of the operands.   On the
architecture for which you are compiling, single precision may be faster
than double precision.   If you must use @samp{-traditional}, but want
to use single precision operations when the operands are single
precision, use this option.   This option has no effect when compiling
with ANSI or GNU C conventions (the default).

@end table

@node C++ Dialect Options
@section Options Controlling C++ Dialect

@cindex compiler options, C++
@cindex C++ options, command line
@cindex options, C++
This section describes the command-line options that are only meaningful
for C++ programs; but you can also use most of the GNU compiler options
regardless of what language your program is in.  For example, you
might compile a file @code{firstClass.C} like this:

@example
g++ -g -felide-constructors -O -c firstClass.C
@end example

@noindent
In this example, only @samp{-felide-constructors} is an option meant
only for C++ programs; you can use the other options with any
language supported by GNU CC.

Here is a list of options that are @emph{only} for compiling C++ programs:

@table @code
@item -fno-access-control
Turn off all access checking.  This switch is mainly useful for working
around bugs in the access control code.

@item -fall-virtual
Treat all possible member functions as virtual, implicitly.
All member functions (except for constructor functions and @code{new} or
@code{delete} member operators) are treated as virtual functions of the
class where they appear.

This does not mean that all calls to these member functions will be made
through the internal table of virtual functions.  Under some
circumstances, the compiler can determine that a call to a given virtual
function can be made directly; in these cases the calls are direct in
any case.

@item -fcheck-new
Check that the pointer returned by @code{operator new} is non-null
before attempting to modify the storage allocated.  The current Working
Paper requires that @code{operator new} never return a null pointer, so
this check is normally unnecessary.

@item -fconserve-space
Put uninitialized or runtime-initialized global variables into the
common segment, as C does.  This saves space in the executable at the
cost of not diagnosing duplicate definitions.  If you compile with this
flag and your program mysteriously crashes after @code{main()} has
completed, you may have an object that is being destroyed twice because
two definitions were merged.

@item -fdollars-in-identifiers
Accept @samp{$} in identifiers.  You can also explicitly prohibit use of
@samp{$} with the option @samp{-fno-dollars-in-identifiers}.  (GNU C++
allows @samp{$} by default on some target systems but not others.)
Traditional C allowed the character @samp{$} to form part of
identifiers.  However, ANSI C and C++ forbid @samp{$} in identifiers.

@item -fenum-int-equiv
Anachronistically permit implicit conversion of @code{int} to
enumeration types.  Current C++ allows conversion of @code{enum} to
@code{int}, but not the other way around.

@item -fexternal-templates
Cause template instantiations to obey @samp{#pragma interface} and
@samp{implementation}; template instances are emitted or not according
to the location of the template definition.  @xref{Template
Instantiation}, for more information.

@item -falt-external-templates
Similar to -fexternal-templates, but template instances are emitted or
not according to the place where they are first instantiated.
@xref{Template Instantiation}, for more information.

@item -ffor-scope
@item -fno-for-scope
If -ffor-scope is specified, the scope of variables declared in
a @i{for-init-statement} is limited to the @samp{for} loop itself,
as specified by the draft C++ standard.
If -fno-for-scope is specified, the scope of variables declared in
a @i{for-init-statement} extends to the end of the enclosing scope,
as was the case in old versions of gcc, and other (traditional)
implementations of C++.

The default if neither flag is given to follow the standard,
but to allow and give a warning for old-style code that would
otherwise be invalid, or have different behavior.

@item -fno-gnu-keywords
Do not recognize @code{classof}, @code{headof}, @code{signature},
@code{sigof} or @code{typeof} as a keyword, so that code can use these
words as identifiers.  You can use the keywords @code{__classof__},
@code{__headof__}, @code{__signature__}, @code{__sigof__}, and
@code{__typeof__} instead.  @samp{-ansi} implies
@samp{-fno-gnu-keywords}.

@item -fno-implicit-templates
Never emit code for templates which are instantiated implicitly (i.e. by
use); only emit code for explicit instantiations.  @xref{Template
Instantiation}, for more information.

@item -fhandle-signatures
Recognize the @code{signature} and @code{sigof} keywords for specifying
abstract types.  The default (@samp{-fno-handle-signatures}) is not to
recognize them.  @xref{C++ Signatures, Type Abstraction using
Signatures}.

@item -fhuge-objects
Support virtual function calls for objects that exceed the size
representable by a @samp{short int}.  Users should not use this flag by
default; if you need to use it, the compiler will tell you so.  If you
compile any of your code with this flag, you must compile @emph{all} of
your code with this flag (including libg++, if you use it).

This flag is not useful when compiling with -fvtable-thunks.

@item -fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
controlled by @samp{#pragma implementation}.  This will cause linker
errors if these functions are not inlined everywhere they are called.

@item -fmemoize-lookups
@itemx -fsave-memoized
Use heuristics to compile faster.  These heuristics are not enabled by
default, since they are only effective for certain input files.  Other
input files compile more slowly.

The first time the compiler must build a call to a member function (or
reference to a data member), it must (1) determine whether the class
implements member functions of that name; (2) resolve which member
function to call (which involves figuring out what sorts of type
conversions need to be made); and (3) check the visibility of the member
function to the caller.  All of this adds up to slower compilation.
Normally, the second time a call is made to that member function (or
reference to that data member), it must go through the same lengthy
process again.  This means that code like this:

@smallexample
cout << "This " << p << " has " << n << " legs.\n";
@end smallexample

@noindent
makes six passes through all three steps.  By using a software cache, a
``hit'' significantly reduces this cost.  Unfortunately, using the cache
introduces another layer of mechanisms which must be implemented, and so
incurs its own overhead.  @samp{-fmemoize-lookups} enables the software
cache.

Because access privileges (visibility) to members and member functions
may differ from one function context to the next, G++ may need to flush
the cache.  With the @samp{-fmemoize-lookups} flag, the cache is flushed
after every function that is compiled.  The @samp{-fsave-memoized} flag
enables the same software cache, but when the compiler determines that
the context of the last function compiled would yield the same access
privileges of the next function to compile, it preserves the cache.
This is most helpful when defining many member functions for the same
class: with the exception of member functions which are friends of other
classes, each member function has exactly the same access privileges as
every other, and the cache need not be flushed.

The code that implements these flags has rotted; you should probably
avoid using them.

@item -fstrict-prototype
Within an @samp{extern "C"} linkage specification, treat a function
declaration with no arguments, such as @samp{int foo ();}, as declaring
the function to take no arguments.  Normally, such a declaration means
that the function @code{foo} can take any combination of arguments, as
in C.  @samp{-pedantic} implies @samp{-fstrict-prototype} unless
overridden with @samp{-fno-strict-prototype}.

This flag no longer affects declarations with C++ linkage.

@item -fno-nonnull-objects
Don't assume that a reference is initialized to refer to a valid object.
Although the current C++ Working Paper prohibits null references, some
old code may rely on them, and you can use @samp{-fno-nonnull-objects}
to turn on checking.

At the moment, the compiler only does this checking for conversions to
virtual base classes.

@item -foperator-names
Recognize the operator name keywords @code{and}, @code{bitand},
@code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
synonyms for the symbols they refer to.  @samp{-ansi} implies
@samp{-foperator-names}.

@item -fthis-is-variable
Permit assignment to @code{this}.  The incorporation of user-defined
free store management into C++ has made assignment to @samp{this} an
anachronism.  Therefore, by default it is invalid to assign to
@code{this} within a class member function; that is, GNU C++ treats
@samp{this} in a member function of class @code{X} as a non-lvalue of
type @samp{X *}.  However, for backwards compatibility, you can make it
valid with @samp{-fthis-is-variable}.

@item -fvtable-thunks
Use @samp{thunks} to implement the virtual function dispatch table
(@samp{vtable}).  The traditional (cfront-style) approach to
implementing vtables was to store a pointer to the function and two
offsets for adjusting the @samp{this} pointer at the call site.  Newer
implementations store a single pointer to a @samp{thunk} function which
does any necessary adjustment and then calls the target function.

This option also enables a heuristic for controlling emission of
vtables; if a class has any non-inline virtual functions, the vtable
will be emitted in the translation unit containing the first one of
those.

@item -nostdinc++
Do not search for header files in the standard directories specific to
C++, but do still search the other standard directories.  (This option
is used when building libg++.)

@item -traditional
For C++ programs (in addition to the effects that apply to both C and
C++), this has the same effect as @samp{-fthis-is-variable}.
@xref{C Dialect Options,, Options Controlling C Dialect}.
@end table

In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:

@table @code
@item -fno-default-inline
Do not assume @samp{inline} for functions defined inside a class scope.
@xref{Optimize Options,,Options That Control Optimization}.

@item -Wenum-clash
@itemx -Woverloaded-virtual
@itemx -Wtemplate-debugging
Warnings that apply only to C++ programs.  @xref{Warning
Options,,Options to Request or Suppress Warnings}.

@item +e@var{n}