invoke.texi

早期freebsd实现

@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:

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

@item
An expression-statement contains no side effects.

@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
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}:

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

@item -Wenum-clash
Warn about conversion between different enumeration types (C++ only).

@item -Wimplicit
Warn whenever a function or parameter is implicitly declared.

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

If you want to prevent a warning for a particular variable, you can use
this macro:

@example
#define USE(var) \
  static void * use_##var = (&use_##var, (void *) &var)

USE (string);
@end example

@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 -Wcomment
Warn whenever a comment-start sequence @samp{/*} appears in a comment.

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

@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 -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 -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:

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

@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:

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

@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 as
@code{volatile} all the functions you use that never return.
@xref{Function Attributes}.

@item -Wparentheses
Warn if parentheses are omitted in certain contexts.

@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 -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 -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 -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
only if you have been very careful about using @code{const} in
declarations and prototypes.  Otherwise, it will just be a nuisance;
this is why we did not make @samp{-Wall} request these warnings.

@item -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.

@item -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.)

@item -Wstrict-prototypes
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.)

@item -Wmissing-prototypes
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.

@item -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.

@item -Wnested-externs
Warn if an @code{extern} declaration is encountered within an function.

@item -Winline
Warn if a function can not be inlined, and either it was declared as inline,
or else the @samp{-finline-functions} option was given.

@item -Werror
Make all warnings into errors.
@end table

@node Debugging Options
@section Options for Debugging Your Program or GNU CC
@cindex options, debugging
@cindex debugging information options

GNU CC has various special options that are used for debugging
either your program or GCC:

@table @code
@item -g
Produce debugging information in the operating system's native format
(stabs, COFF, XCOFF, or DWARF).  GDB can work with this debugging
information.

On most systems that use stabs format, @samp{-g} enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but will probably make other debuggers
crash or
refuse to read the program.  If you want to control for certain whether
to generate the extra information, use @samp{-gstabs+}, @samp{-gstabs},
@samp{-gxcoff+}, @samp{-gxcoff}, @samp{-gdwarf+}, or @samp{-gdwarf}
(see below).

Unlike most other C compilers, GNU CC allows you to use @samp{-g} with
@samp{-O}.  The shortcuts taken by optimized code may occasionally
produce surprising results: some variables you declared may not exist
at all; flow of control may briefly move where you did not expect it;
some statements may not be executed because they compute constant
results or their values were already at hand; some statements may
execute in different places because they were moved out of loops.

Nevertheless it proves possible to debug optimized output.  This makes
it reasonable to use the optimizer for programs that might have bugs.

The following options are useful when GNU CC is generated with the
capability for more than one debugging format.

@item -ggdb
Produce debugging information in the native format (if that is supported),
including GDB extensions if at all possible.

@item -gstabs
Produce debugging information in stabs format (if that is supported),
without GDB extensions.  This is the format used by DBX on most BSD
systems.

@item -gstabs+
Produce debugging information in stabs format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB).  The
use of these extensions is likely to make other debuggers crash or
refuse to read the program.

@item -gcoff
Produce debugging information in COFF format (if that is supported).
This is the format used by SDB on most System V systems prior to
System V Release 4.

@item -gxcoff
Produce debugging information in XCOFF format (if that is supported).
This is the format used by the DBX debugger on IBM RS/6000 systems.

@item -gxcoff+
Produce debugging information in XCOFF format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB).  The
use of these extensions is likely to make other debuggers crash or
refuse to read the program.

@item -gdwarf
Produce debugging information in DWARF format (if that is supported).
This is the format used by SDB on most System V Release 4 systems.

@item -gdwarf+
Produce debugging information in DWARF format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB).  The
use of these extensions is likely to make other debuggers crash or
refuse to read the program.