invoke.texi

GUN开源阻止下的编译器GCC

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 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
Warn whenever a function or parameter is implicitly declared.

@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.

@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.

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 -Wenum-clash
@cindex enumeration clash warnings
@cindex warning for enumeration conversions
Warn about conversion between different enumeration types.
(C++ only).

@item -Wreorder (C++ only)
@cindex reordering, warning
@cindex warning for reordering of member initializers
Warn when the order of member initializers given in the code does not
match the order in which they must be executed.  For instance:

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

Here the compiler will warn that the member initializers for @samp{i}
and @samp{j} will be rearranged to match the declaration order of the
members.

@item -Wtemplate-debugging
@cindex template debugging
When using templates in a C++ program, warn if debugging is not yet
fully available (C++ only).

@item -Wall
All of the above @samp{-W} options combined.  These are all the
options which pertain to usage that we recommend avoiding and that we
believe is easy to avoid, even in conjunction with macros.
@end table

The remaining @samp{-W@dots{}} options are not implied by @samp{-Wall}
because they warn about constructions that we consider reasonable to
use, on occasion, in clean programs.

@table @code
@item -W
Print extra warning messages for these events:

@itemize @bullet
@cindex @code{longjmp} warnings
@item
A nonvolatile automatic variable might be changed by a call to
@code{longjmp}.  These warnings as well are possible only in
optimizing compilation.

The compiler sees only the calls to @code{setjmp}.  It cannot know
where @code{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 @code{longjmp} cannot
in fact be called at the place which would cause a problem.

@item
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:

@smallexample
@group
foo (a)
@{
  if (a > 0)
    return a;
@}
@end group
@end smallexample

@item
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 @samp{x[i,j]} will cause a warning,
but @samp{x[(void)i,j]} will not.

@item
An unsigned value is compared against zero with @samp{<} or @samp{<=}.

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

@item
Storage-class specifiers like @code{static} are not the first things in
a declaration.  According to the C Standard, this usage is obsolescent.

@item
If @samp{-Wall} or @samp{-Wunused} is also specified, warn about unused
arguments.

@item
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 @code{x.h}:

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

@item -Wtraditional
Warn about certain constructs that behave differently in traditional and
ANSI C.

@itemize @bullet
@item
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.

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

@item
A @code{switch} statement has an operand of type @code{long}.
@end itemize

@item -Wshadow
Warn whenever a local variable shadows another local variable.

@item -Wid-clash-@var{len}
Warn whenever two distinct identifiers match in the first @var{len}
characters.  This may help you prepare a program that will compile
with certain obsolete, brain-damaged compilers.

@item -Wlarger-than-@var{len}
Warn whenever an object of larger than @var{len} bytes is defined.

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

@item -Wbad-function-cast
Warn whenever a function call is cast to a non-matching type.
For example, warn if @code{int malloc()} is cast to @code{anything *}.

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

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

@item -Wwrite-strings
Give string constants the type @code{const char[@var{length}]} so that
copying the address of one into a non-@code{const} @code{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