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* Gcc: (gcc).                           Using and Porting the GNU C compiler.
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INFO-DIR-SECTION Programming
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* gcc: (gcc).                  The GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the use and the internals of the GNU compiler.

   Published by the Free Software Foundation 59 Temple Place - Suite 330
Boston, MA 02111-1307 USA

   Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
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manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
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may be included in translations approved by the Free Software Foundation
instead of in the original English.


File: gcc.info,  Node: C++ Dialect Options,  Next: Warning Options,  Prev: C Dialect Options,  Up: Invoking GCC

Options Controlling C++ Dialect
===============================

   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 `firstClass.C' like this:

     g++ -g -frepo -O -c firstClass.C

In this example, only `-frepo' is an option meant only for C++
programs; you can use the other options with any language supported by
GCC.

   Here is a list of options that are *only* for compiling C++ programs:

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

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

     An alternative to using this option is to specify that your
     `operator new' does not throw any exceptions; if you declare it
     `throw()', g++ will check the return value.  See also `new
     (nothrow)'.

`-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
     `main()' has completed, you may have an object that is being
     destroyed twice because two definitions were merged.

     This option is no longer useful on most targets, now that support
     has been added for putting variables into BSS without making them
     common.

`-fdollars-in-identifiers'
     Accept `$' in identifiers.  You can also explicitly prohibit use of
     `$' with the option `-fno-dollars-in-identifiers'.  (GNU C allows
     `$' by default on most target systems, but there are a few
     exceptions.)  Traditional C allowed the character `$' to form part
     of identifiers.  However, ANSI C and C++ forbid `$' in identifiers.

`-fembedded-cxx'
     In compliance with the Embedded C++ specification, make the use of
     templates, exception handling, multiple inheritance, or RTTI
     illegal.  Attempts to use namespaces are also not allowed.  This
     makes the use of these keywords result in warnings by default:
     `template', `typename', `catch', `throw', `try', `using',
     `namespace', `dynamic_cast', `static_cast', `reinterpret_cast',
     `const_cast', and `typeid'.  To make the warnings for these things
     be given as errors, add the `-pedantic-errors' flag.

`-fno-elide-constructors'
     The C++ standard allows an implementation to omit creating a
     temporary which is only used to initialize another object of the
     same type.  Specifying this option disables that optimization, and
     forces g++ to call the copy constructor in all cases.

`-fexternal-templates'
     Cause template instantiations to obey `#pragma interface' and
     `implementation'; template instances are emitted or not according
     to the location of the template definition.  *Note Template
     Instantiation::, for more information.

     This option is deprecated.

`-falt-external-templates'
     Similar to -fexternal-templates, but template instances are
     emitted or not according to the place where they are first
     instantiated.  *Note Template Instantiation::, for more
     information.

     This option is deprecated.

`-ffor-scope'
`-fno-for-scope'
     If -ffor-scope is specified, the scope of variables declared in a
     for-init-statement is limited to the `for' loop itself, as
     specified by the draft C++ standard.  If -fno-for-scope is
     specified, the scope of variables declared in a 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.

`-fno-gnu-keywords'
     Do not recognize `classof', `headof', or `typeof' as a keyword, so
     that code can use these words as identifiers.  You can use the
     keywords `__classof__', `__headof__', and `__typeof__' instead.
     `-ansi' implies `-fno-gnu-keywords'.

`-fguiding-decls'
     Treat a function declaration with the same type as a potential
     function template instantiation as though it declares that
     instantiation, not a normal function.  If a definition is given
     for the function later in the translation unit (or another
     translation unit if the target supports weak symbols), that
     definition will be used; otherwise the template will be
     instantiated.  This behavior reflects the C++ language prior to
     September 1996, when guiding declarations were removed.

     This option implies `-fname-mangling-version-0', and will not work
     with other name mangling versions.  Like all options that change
     the ABI, all C++ code, *including libgcc.a* must be built with the
     same setting of this option.

`-fno-implicit-templates'
     Never emit code for templates which are instantiated implicitly
     (i.e. by use); only emit code for explicit instantiations.  *Note
     Template Instantiation::, for more information.

`-fhonor-std'
     Treat the `namespace std' as a namespace, instead of ignoring it.
     For compatibility with earlier versions of g++, the compiler will,
     by default, ignore `namespace-declarations', `using-declarations',
     `using-directives', and `namespace-names', if they involve `std'.

`-fhuge-objects'
     Support virtual function calls for objects that exceed the size
     representable by a `short int'.  Users should not use this flag by
     default; if you need to use it, the compiler will tell you so.

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

     Like all options that change the ABI, all C++ code, *including
     libgcc* must be built with the same setting of this option.

`-fno-implicit-templates'
     Never emit code for non-inline templates which are instantiated
     implicitly (i.e. by use); only emit code for explicit
     instantiations.  *Note Template Instantiation::, for more
     information.

`-fno-implicit-inline-templates'
     Don't emit code for implicit instantiations of inline templates,
     either.  The default is to handle inlines differently so that
     compiles with and without optimization will need the same set of
     explicit instantiations.

`-finit-priority'
     Support `__attribute__ ((init_priority (n)))' for controlling the
     order of initialization of file-scope objects.  On ELF targets,
     this requires GNU ld 2.10 or later.

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

`-fms-extensions'
     Disable pedwarns about constructs used in MFC, such as implicit
     int and getting a pointer to member function via non-standard
     syntax.

`-fname-mangling-version-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:

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

     Like all options that change the ABI, all C++ code, *including
     libgcc* must be built with the same setting of this option.

`-foperator-names'
     Recognize the operator name keywords `and', `bitand', `bitor',
     `compl', `not', `or' and `xor' as synonyms for the symbols they
     refer to.  `-ansi' implies `-foperator-names'.

`-fno-optional-diags'
     Disable diagnostics that the standard says a compiler does not
     need to issue.  Currently, the only such diagnostic issued by g++
     is the one for a name having multiple meanings within a class.

`-fpermissive'
     Downgrade messages about nonconformant code from errors to
     warnings.  By default, g++ effectively sets `-pedantic-errors'
     without `-pedantic'; this option reverses that.  This behavior and
     this option are superseded by `-pedantic', which works as it does
     for GNU C.

`-frepo'
     Enable automatic template instantiation.  This option also implies
     `-fno-implicit-templates'.  *Note Template Instantiation::, for
     more information.

`-fno-rtti'
     Disable generation of information about every class with virtual
     functions for use by the C++ runtime type identification features
     (`dynamic_cast' and `typeid').  If you don't use those parts of
     the language, you can save some space by using this flag.  Note
     that exception handling uses the same information, but it will
     generate it as needed.

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

     Specifying this option will also suppress implicit declarations of
     functions.

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

`-fsquangle'
`-fno-squangle'
     `-fsquangle' will enable a compressed form of name mangling for
     identifiers. In particular, it helps to shorten very long names by
     recognizing types and class names which occur more than once,
     replacing them with special short ID codes.  This option also
     requires any C++ libraries being used to be compiled with this
     option as well.  The compiler has this disabled (the equivalent of
     `-fno-squangle') by default.

     Like all options that change the ABI, all C++ code, *including
     libgcc.a* must be built with the same setting of this option.

`-ftemplate-depth-N'
     Set the maximum instantiation depth for template classes to 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.

`-fuse-cxa-atexit'
     Register destructors for objects with static storage duration with
     the `__cxa_atexit' function rather than the `atexit' function.
     This option is required for fully standards-compliant handling of
     static destructors, but will only work if your C library supports
     `__cxa_atexit'.

`-fvtable-thunks'
     Use `thunks' to implement the virtual function dispatch table
     (`vtable').  The traditional (cfront-style) approach to
     implementing vtables was to store a pointer to the function and two
     offsets for adjusting the `this' pointer at the call site.  Newer
     implementations store a single pointer to a `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.

     Like all options that change the ABI, all C++ code, *including
     libgcc.a* must be built with the same setting of this option.

`-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 the C++ library.)

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

`-fno-default-inline'
     Do not assume `inline' for functions defined inside a class scope.
     *Note Options That Control Optimization: Optimize Options.  Note
     that these functions will have linkage like inline functions; they
     just won't be inlined by default.

`-Wctor-dtor-privacy (C++ only)'
     Warn when a class seems unusable, because all the constructors or
     destructors in a class are private and the class has no friends or
     public static member functions.

`-Wnon-virtual-dtor (C++ only)'
     Warn when a class declares a non-virtual destructor that should
     probably be virtual, because it looks like the class will be used
     polymorphically.

`-Wreorder (C++ only)'
     Warn when the order of member initializers given in the code does
     not match the order in which they must be executed.  For instance:

          struct A {
            int i;
            int j;
            A(): j (0), i (1) { }
          };

     Here the compiler will warn that the member initializers for `i'
     and `j' will be rearranged to match the declaration order of the
     members.

   The following `-W...' options are not affected by `-Wall'.

`-Weffc++ (C++ only)'
     Warn about violations of various style guidelines from Scott
     Meyers' `Effective C++' books.  If you use this option, you should
     be aware that the standard library headers do not obey all of
     these guidelines; you can use `grep -v' to filter out those
     warnings.

`-Wno-deprecated (C++ only)'
     Do not warn about usage of deprecated features. *Note Deprecated
     Features::.

`-Wno-non-template-friend (C++ only)'
     Disable warnings when non-templatized friend functions are declared
     within a template. With the advent of explicit template
     specification support in g++, if the name of the friend is an
     unqualified-id (ie, `friend foo(int)'), the C++ language
     specification demands that the friend declare or define an
     ordinary, nontemplate function. (Section 14.5.3). Before g++
     implemented explicit specification, unqualified-ids could be
     interpreted as a particular specialization of a templatized
     function. Because this non-conforming behavior is no longer the
     default behavior for g++, `-Wnon-template-friend' allows the
     compiler to check existing code for potential trouble spots, and
     is on by default.  This new compiler behavior can also be turned
     off with the flag `-fguiding-decls', which activates the older,
     non-specification compiler code, or with
     `-Wno-non-template-friend' which keeps the conformant compiler
     code but disables the helpful warning.

`-Wold-style-cast (C++ only)'
     Warn if an old-style (C-style) cast is used within a C++ program.
     The new-style casts (`static_cast', `reinterpret_cast', and
     `const_cast') are less vulnerable to unintended effects.

`-Woverloaded-virtual (C++ only)'
     Warn when a derived class function declaration may be an error in
     defining a virtual function.  In a derived class, the definitions
     of virtual functions must match the type signature of a virtual
     function declared in the base class.  With this option, the
     compiler warns when you define a function with the same name as a
     virtual function, but with a type signature that does not match any
     declarations from the base class.

`-Wno-pmf-conversions (C++ only)'
     Disable the diagnostic for converting a bound pointer to member
     function to a plain pointer.

`-Wsign-promo (C++ only)'
     Warn when overload resolution chooses a promotion from unsigned or
     enumeral type to a signed type over a conversion to an unsigned
     type of the same size.  Previous versions of g++ would try to
     preserve unsignedness, but the standard mandates the current
     behavior.

`-Wsynth (C++ only)'
     Warn when g++'s synthesis behavior does not match that of cfront.
     For instance:

          struct A {
            operator int ();
            A& operator = (int);
          };
          
          main ()
          {
            A a,b;
            a = b;
          }

     In this example, g++ will synthesize a default `A& operator =
     (const A&);', while cfront will use the user-defined `operator ='.


File: gcc.info,  Node: Warning Options,  Next: Debugging Options,  Prev: C++ Dialect Options,  Up: Invoking GCC

Options to Request or Suppress 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 `-W',
for example `-Wimplicit' to request warnings on implicit declarations.
Each of these specific warning options also has a negative form
beginning `-Wno-' to turn off warnings; for example, `-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
GCC:

`-fsyntax-only'
     Check the code for syntax errors, but don't do anything beyond
     that.

`-pedantic'
     Issue all the warnings demanded by strict ANSI C and ISO C++;
     reject all programs that use forbidden extensions.

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

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

     This option is not intended to be useful; it exists only to satisfy
     pedants who would otherwise claim that GCC fails to support the
     ANSI standard.

     Some users try to use `-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 *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 `-pedantic'.  We
     don't have plans to support such a feature in the near future.

`-pedantic-errors'
     Like `-pedantic', except that errors are produced rather than
     warnings.

`-w'
     Inhibit all warning messages.

`-Wno-import'
     Inhibit warning messages about the use of `#import'.

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

`-Wcomment'
     Warn whenever a comment-start sequence `/*' appears in a `/*'
     comment, or whenever a Backslash-Newline appears in a `//' comment.

`-Wformat'
     Check calls to `printf' and `scanf', etc., to make sure that the
     arguments supplied have types appropriate to the format string
     specified.

`-Wimplicit-int'
     Warn when a declaration does not specify a type.

`-Wimplicit-function-declaration'
`-Werror-implicit-function-declaration'
     Give a warning (or error) whenever a function is used before being
     declared.

`-Wimplicit'
     Same as `-Wimplicit-int' and `-Wimplicit-function-'
     `declaration'.

`-Wmain'
     Warn if the type of `main' is suspicious.  `main' should be a
     function with external linkage, returning int, taking either zero
     arguments, two, or three arguments of appropriate types.

`-Wmultichar'
     Warn if a multicharacter constant (`'FOOF'') is used.  Usually they
     indicate a typo in the user's code, as they have
     implementation-defined values, and should not be used in portable
     code.

`-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.

     This option includes `-Wparentheses-else' below.

`-Wparentheses-else'
     Warn about constructions where there may be confusion to which
     `if' statement an `else' branch belongs.  Here is an example of
     such a case:

          {
            if (a)
              if (b)
                foo ();
            else
              bar ();
          }

     In C, every `else' branch belongs to the innermost possible `if'
     statement, which in this example is `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 `if' statement so there is no way the `else' could
     belong to the enclosing `if'.  The resulting code would look like
     this:

          {
            if (a)
              {
                if (b)
                  foo ();
                else
                  bar ();
              }
          }

`-Wreturn-type'
     Warn whenever a function is defined with a return-type that
     defaults to `int'.  Also warn about any `return' statement with no
     return-value in a function whose return-type is not `void'.

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

`-Wtrigraphs'
     Warn if any trigraphs are encountered (assuming they are enabled).

`-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 `-W' and `-Wunused'.

     To suppress this warning for an expression, simply cast it to
     void.  For unused variables, parameters and labels, use the
     `unused' attribute (*note Variable Attributes::.).

`-Wuninitialized'
     Warn if an automatic variable is used without first being
     initialized or if a variable may be clobbered by a `setjmp' call.

     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 `-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 `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 GCC 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:

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

     If the value of `y' is always 1, 2 or 3, then `x' is always
     initialized, but GCC doesn't know this.  Here is another common
     case:

          {
            int save_y;
            if (change_y) save_y = y, y = new_y;
            ...
            if (change_y) y = save_y;
          }

     This has no bug because `save_y' is used only if it is set.

     This option also includes `-Wlongjmp-clobbers' below.

`-Wlongjmp-clobbers'
     Warn when a nonvolatile automatic variable might be changed by a
     call to `longjmp'.  These warnings as well are possible only in
     optimizing compilation.

     This option also warns when a nonvolatile automatic variable might
     be changed by a call to `longjmp'.  These warnings as well are
     possible only in optimizing compilation.

     The compiler sees only the calls to `setjmp'.  It cannot know
     where `longjmp' will be called; in fact, a signal handler could
     call it at any point in the code.  As a result, you may get a
     warning even when there is in fact no problem because `longjmp'
     cannot in fact be called at the place which would cause a problem.

     Some spurious warnings can be avoided if you declare all the
     functions you use that never return as `noreturn'.  *Note Function
     Attributes::.

`-Wreorder (C++ only)'
     Warn when the order of member initializers given in the code does
     not match the order in which they must be executed.  For instance:

`-Wunknown-pragmas'
     Warn when a #pragma directive is encountered which is not
     understood by GCC.  If this command line option is used, warnings
     will even be issued for unknown pragmas in system header files.
     This is not the case if the warnings were only enabled by the
     `-Wall' command line option.

`-Wall'
     All of the above `-W' options combined.  This enables all the
     warnings about constructions that some users consider
     questionable, and that are easy to avoid (or modify to prevent the
     warning), even in conjunction with macros.

   The following `-W...' options are not implied by `-Wall'.  Some of
them warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid in
some cases, and there is no simple way to modify the code to suppress
the warning.

`-W'
     Print extra warning messages for these events:

        * A function can return either with or without a value.
          (Falling off the end of the function body is considered
          returning without a value.)  For example, this function would
          evoke such a warning:

               foo (a)
               {
                 if (a > 0)
                   return a;
               }

        * An expression-statement or the left-hand side of a comma
          expression contains no side effects.  To suppress the
          warning, cast the unused expression to void.  For example, an
          expression such as `x[i,j]' will cause a warning, but
          `x[(void)i,j]' will not.

        * An unsigned value is compared against zero with `<' or `<='.

        * A comparison like `x<=y<=z' appears; this is equivalent to
          `(x<=y ? 1 : 0) <= z', which is a different interpretation
          from that of ordinary mathematical notation.

        * Storage-class specifiers like `static' are not the first
          things in a declaration.  According to the C Standard, this
          usage is obsolescent.

        * If `-Wall' or `-Wunused' is also specified, warn about unused
          arguments.

        * A comparison between signed and unsigned values could produce
          an incorrect result when the signed value is converted to
          unsigned.  (But don't warn if `-Wno-sign-compare' is also
          specified.)

        * An aggregate has a partly bracketed initializer.  For
          example, the following code would evoke such a warning,
          because braces are missing around the initializer for `x.h':

               struct s { int f, g; };
               struct t { struct s h; int i; };
               struct t x = { 1, 2, 3 };

        * An aggregate has an initializer which does not initialize all
          members.  For example, the following code would cause such a
          warning, because `x.h' would be implicitly initialized to
          zero:

               struct s { int f, g, h; };
               struct s x = { 3, 4 };

`-Wfloat-equal'
     Warn if floating point values are used in equality comparisons.

     The idea behind this is that sometimes it is convenient (for the
     programmer) to consider floating-point values as approximations to
     infinitely precise real numbers.  If you are doing this, then you
     need to compute (by analysing the code, or in some other way) the
     maximum or likely maximum error that the computation introduces,
     and allow for it when performing comparisons (and when producing
     output, but that's a different problem).  In particular, instead
     of testing for equality, you would check to see whether the two
     values have ranges that overlap; and this is done with the
     relational operators, so equality comparisons are probably
     mistaken.

`-Wtraditional (C only)'
     Warn about certain constructs that behave differently in
     traditional and ANSI C.

        * Macro arguments occurring within string constants in the
          macro body.  These would substitute the argument in
          traditional C, but are part of the constant in ANSI C.

        * A function declared external in one block and then used after
          the end of the block.

        * A `switch' statement has an operand of type `long'.

        * A non-`static' function declaration follows a `static' one.
          This construct is not accepted by some traditional C
          compilers.

        * The ANSI type of an integer constant has a different width or
          signedness from its traditional type.  This warning is only
          issued if the base of the constant is ten.  I.e. hexadecimal
          or octal values, which typically represent bit patterns, are
          not warned about.

        * Usage of ANSI string concatenation is detected.

`-Wundef'
     Warn if an undefined identifier is evaluated in an `#if' directive.

`-Wshadow'
     Warn whenever a local variable shadows another local variable.

`-Wid-clash-LEN'
     Warn whenever two distinct identifiers match in the first LEN
     characters.  This may help you prepare a program that will compile
     with certain obsolete, brain-damaged compilers.

`-Wlarger-than-LEN'
     Warn whenever an object of larger than LEN bytes is defined.

`-Wpointer-arith'
     Warn about anything that depends on the "size of" a function type
     or of `void'.  GNU C assigns these types a size of 1, for
     convenience in calculations with `void *' pointers and pointers to
     functions.

`-Wbad-function-cast (C only)'
     Warn whenever a function call is cast to a non-matching type.  For
     example, warn if `int malloc()' is cast to `anything *'.

`-Wcast-qual'
     Warn whenever a pointer is cast so as to remove a type qualifier
     from the target type.  For example, warn if a `const char *' is
     cast to an ordinary `char *'.

`-Wcast-align'
     Warn whenever a pointer is cast such that the required alignment
     of the target is increased.  For example, warn if a `char *' is
     cast to an `int *' on machines where integers can only be accessed
     at two- or four-byte boundaries.

`-Wwrite-strings'
     Give string constants the type `const char[LENGTH]' so that
     copying the address of one into a non-`const' `char *' pointer
     will get a warning.  These warnings will help you find at compile
     time code that can try to write into a string constant, but only
     if you have been very careful about using `const' in declarations
     and prototypes.  Otherwise, it will just be a nuisance; this is
     why we did not make `-Wall' request these warnings.

`-Wconversion'
     Warn if a prototype causes a type conversion that is different
     from what would happen to the same argument in the absence of a
     prototype.  This includes conversions of fixed point to floating
     and vice versa, and conversions changing the width or signedness
     of a fixed point argument except when the same as the default
     promotion.

     Also, warn if a negative integer constant expression is implicitly
     converted to an unsigned type.  For example, warn about the
     assignment `x = -1' if `x' is unsigned.  But do not warn about
     explicit casts like `(unsigned) -1'.

`-Wsign-compare'
     Warn when a comparison between signed and unsigned values could
     produce an incorrect result when the signed value is converted to
     unsigned.  This warning is also enabled by `-W'; to get the other
     warnings of `-W' without this warning, use `-W -Wno-sign-compare'.

`-Waggregate-return'
     Warn if any functions that return structures or unions are defined
     or called.  (In languages where you can return an array, this also
     elicits a warning.)

`-Wstrict-prototypes (C only)'
     Warn if a function is declared or defined without specifying the
     argument types.  (An old-style function definition is permitted
     without a warning if preceded by a declaration which specifies the
     argument types.)

`-Wmissing-prototypes (C only)'
     Warn if a global function is defined without a previous prototype
     declaration.  This warning is issued even if the definition itself
     provides a prototype.  The aim is to detect global functions that
     fail to be declared in header files.

`-Wmissing-declarations'
     Warn if a global function is defined without a previous
     declaration.  Do so even if the definition itself provides a
     prototype.  Use this option to detect global functions that are
     not declared in header files.

`-Wmissing-noreturn'
     Warn about functions which might be candidates for attribute
     `noreturn'.  Note these are only possible candidates, not absolute
     ones.  Care should be taken to manually verify functions actually
     do not ever return before adding the `noreturn' attribute,
     otherwise subtle code generation bugs could be introduced.

`-Wpacked'
     Warn if a structure is given the packed attribute, but the packed
     attribute has no effect on the layout or size of the structure.
     Such structures may be mis-aligned for little benefit.  For
     instance, in this code, the variable `f.x' in `struct bar' will be
     misaligned even though `struct bar' does not itself have the
     packed attribute:

          struct foo {
            int x;
            char a, b, c, d;
          } __attribute__((packed));
          struct bar {
            char z;
            struct foo f;
          };

`-Wpadded'
     Warn if padding is included in a structure, either to align an
     element of the structure or to align the whole structure.
     Sometimes when this happens it is possible to rearrange the fields
     of the structure to reduce the padding and so make the structure
     smaller.

`-Wredundant-decls'
     Warn if anything is declared more than once in the same scope,
     even in cases where multiple declaration is valid and changes
     nothing.

`-Wnested-externs (C only)'
     Warn if an `extern' declaration is encountered within a function.

`-Wunreachable-code'
     Warn if the compiler detects that code will never be executed.

     This option is intended to warn when the compiler detects that at
     least a whole line of source code will never be executed, because
     some condition is never satisfied or because it is after a
     procedure that never returns.

     It is possible for this option to produce a warning even though
     there are circumstances under which part of the affected line can
     be executed, so care should be taken when removing
     apparently-unreachable code.

     For instance, when a function is inlined, a warning may mean that
     the line is unreachable in only one inlined copy of the function.

     This option is not made part of `-Wall' because in a debugging
     version of a program there is often substantial code which checks
     correct functioning of the program and is, hopefully, unreachable
     because the program does work.  Another common use of unreachable
     code is to provide behaviour which is selectable at compile-time.

`-Winline'
     Warn if a function can not be inlined and it was declared as
     inline.

`-Wlong-long'
     Warn if `long long' type is used.  This is default.  To inhibit
     the warning messages, use `-Wno-long-long'.  Flags `-Wlong-long'
     and `-Wno-long-long' are taken into account only when `-pedantic'
     flag is used.

`-Werror'
     Make all warnings into errors.

