invoke.texi

GCC编译器源代码

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 -fname-mangling-version-@var{n}
Control the way in which names are mangled.  Version 0 is compatible
with versions of g++ before 2.8.  Version 1 is the default.  Version 1
will allow correct mangling of function templates.  For example, 
version 0 mangling does not mangle foo<int, double> and foo<int, char>
given this declaration:

@example
template <class T, class U> void foo(T t);
@end example

@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 -ftemplate-depth-@var{n}
Set the maximum instantiation depth for template classes to @var{n}.
A limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than 17.

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

@item -Weffc++
Warn about violation of some style rules from Effective C++ by Scott Myers.

@item +e@var{n}
Control how virtual function definitions are used, in a fashion
compatible with @code{cfront} 1.x.  @xref{Code Gen Options,,Options for
Code Generation Conventions}.
@end table

@node Warning Options
@section Options to Request or Suppress Warnings
@cindex options to control warnings
@cindex warning messages
@cindex messages, warning
@cindex suppressing warnings

Warnings are diagnostic messages that report constructions which
are not inherently erroneous but which are risky or suggest there
may have been an error.

You can request many specific warnings with options beginning @samp{-W},
for example @samp{-Wimplicit} to request warnings on implicit
declarations.  Each of these specific warning options also has a
negative form beginning @samp{-Wno-} to turn off warnings;
for example, @samp{-Wno-implicit}.  This manual lists only one of the
two forms, whichever is not the default.

These options control the amount and kinds of warnings produced by GNU
CC:

@table @code
@cindex syntax checking
@item -fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.

@item -pedantic
Issue all the warnings demanded by strict ANSI standard C; reject
all programs that use forbidden extensions.

Valid ANSI standard C programs should compile properly with or without
this option (though a rare few will require @samp{-ansi}).  However,
without this option, certain GNU extensions and traditional C features
are supported as well.  With this option, they are rejected.

@samp{-pedantic} does not cause warning messages for use of the
alternate keywords whose names begin and end with @samp{__}.  Pedantic
warnings are also disabled in the expression that follows
@code{__extension__}.  However, only system header files should use
these escape routes; application programs should avoid them.
@xref{Alternate Keywords}.

This option is not intended to be @i{useful}; it exists only to satisfy
pedants who would otherwise claim that GNU CC fails to support the ANSI
standard.

Some users try to use @samp{-pedantic} to check programs for strict ANSI
C conformance.  They soon find that it does not do quite what they want:
it finds some non-ANSI practices, but not all---only those for which
ANSI C @emph{requires} a diagnostic.

A feature to report any failure to conform to ANSI C might be useful in
some instances, but would require considerable additional work and would
be quite different from @samp{-pedantic}.  We recommend, rather, that
users take advantage of the extensions of GNU C and disregard the
limitations of other compilers.  Aside from certain supercomputers and
obsolete small machines, there is less and less reason ever to use any
other C compiler other than for bootstrapping GNU CC.

@item -pedantic-errors
Like @samp{-pedantic}, except that errors are produced rather than
warnings.

@item -w
Inhibit all warning messages.

@item -Wno-import
Inhibit warning messages about the use of @samp{#import}.

@item -Wchar-subscripts
Warn if an array subscript has type @code{char}.  This is a common cause
of error, as programmers often forget that this type is signed on some
machines.

@item -Wcomment
Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
comment, or whenever a Backslash-Newline appears in a @samp{//} comment.

@item -Wformat
Check calls to @code{printf} and @code{scanf}, etc., to make sure that
the arguments supplied have types appropriate to the format string
specified.

@item -Wimplicit-int
Warn when a declaration does not specify a type.

@item -Wimplicit-function-declarations
Warn whenever a function is used before being declared.

@item -Wimplicit
Same as @samp{-Wimplicit-int} @samp{-Wimplicit-function-declaration}.

@item -Wmain
Warn if the type of @samp{main} is suspicious.  @samp{main} should be a
function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types.
  
@item -Wparentheses
Warn if parentheses are omitted in certain contexts, such
as when there is an assignment in a context where a truth value
is expected, or when operators are nested whose precedence people
often get confused about.

Also warn about constructions where there may be confusion to which
@code{if} statement an @code{else} branch belongs.  Here is an example of
such a case:

@smallexample
@{
  if (a)
    if (b)
      foo ();
  else
    bar ();
@}
@end smallexample

In C, every @code{else} branch belongs to the innermost possible @code{if}
statement, which in this example is @code{if (b)}.  This is often not
what the programmer expected, as illustrated in the above example by
indentation the programmer chose.  When there is the potential for this
confusion, GNU C will issue a warning when this flag is specified.
To eliminate the warning, add explicit braces around the innermost
@code{if} statement so there is no way the @code{else} could belong to
the enclosing @code{if}.  The resulting code would look like this:

@smallexample
@{
  if (a)
    @{
      if (b)
        foo ();
      else
        bar ();
    @}
@}
@end smallexample

@item -Wreturn-type
Warn whenever a function is defined with a return-type that defaults
to @code{int}.  Also warn about any @code{return} statement with no
return-value in a function whose return-type is not @code{void}.

@item -Wswitch
Warn whenever a @code{switch} statement has an index of enumeral type
and lacks a @code{case} for one or more of the named codes of that
enumeration.  (The presence of a @code{default} label prevents this
warning.)  @code{case} labels outside the enumeration range also
provoke warnings when this option is used.

@item -Wtrigraphs
Warn if any trigraphs are encountered (assuming they are enabled).

@item -Wunused
Warn whenever a variable is unused aside from its declaration,
whenever a function is declared static but never defined, whenever a
label is declared but not used, and whenever a statement computes a
result that is explicitly not used.

In order to get a warning about an unused function parameter, you must
specify both @samp{-W} and @samp{-Wunused}.

To suppress this warning for an expression, simply cast it to void.  For
unused variables and parameters, use the @samp{unused} attribute
(@pxref{Variable Attributes}).

@item -Wuninitialized
An automatic variable is used without first being initialized.

These warnings are possible only in optimizing compilation,
because they require data flow information that is computed only
when optimizing.  If you don't specify @samp{-O}, you simply won't
get these warnings.

These warnings occur only for variables that are candidates for
register allocation.  Therefore, they do not occur for a variable that
is declared @code{volatile}, or whose address is taken, or whose size
is other than 1, 2, 4 or 8 bytes.  Also, they do not occur for
structures, unions or arrays, even when they are in registers.

Note that there may be no warning about a variable that is used only
to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the warnings
are printed.

These warnings are made optional because GNU CC is not smart
enough to see all the reasons why the code might be correct
despite appearing to have an error.  Here is one example of how
this can happen:

@smallexample
@{
  int x;
  switch (y)
    @{
    case 1: x = 1;
      break;
    case 2: x = 4;
      break;
    case 3: x = 5;
    @}
  foo (x);
@}
@end smallexample

@noindent
If the value of @code{y} is always 1, 2 or 3, then @code{x} is
always initialized, but GNU CC doesn't know this.  Here is
another common case:

@smallexample
@{
  int save_y;
  if (change_y) save_y = y, y = new_y;
  @dots{}
  if (change_y) y = save_y;
@}
@end smallexample

@noindent
This has no bug because @code{save_y} is used only if it is set.

Some spurious warnings can be avoided if you declare all the functions
you use that never return as @code{noreturn}.  @xref{Function
Attributes}.

@item -Wreorder (C++ only)
@cindex reordering, warning