_construct' and `-
(void) .cxx_destruct' methods.
`-fobjc-direct-dispatch'
Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.
`-fobjc-exceptions'
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java. This
option is unavailable in conjunction with the NeXT runtime on Mac
OS X 10.2 and earlier.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The `@throw' statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a `@catch' block, the
`@throw' may appear without an argument (as shown above), in which
case the object caught by the `@catch' will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme. When an object is thrown, it will be
caught by the nearest `@catch' clause capable of handling objects
of that type, analogously to how `catch' blocks work in C++ and
Java. A `@catch(id ...)' clause (as shown above) may also be
provided to catch any and all Objective-C exceptions not caught by
previous `@catch' clauses (if any).
The `@finally' clause, if present, will be executed upon exit from
the immediately preceding `@try ... @catch' section. This will
happen regardless of whether any exceptions are thrown, caught or
rethrown inside the `@try ... @catch' section, analogously to the
behavior of the `finally' clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with
`NS_HANDLER'-style idioms provided by the `NSException'
class, the new exceptions can only be used on Mac OS X 10.3
(Panther) and later systems, due to additional functionality
needed in the (NeXT) Objective-C runtime.
* As mentioned above, the new exceptions do not support handling
types other than Objective-C objects. Furthermore, when
used from Objective-C++, the Objective-C exception model does
not interoperate with C++ exceptions at this time. This
means you cannot `@throw' an exception from Objective-C and
`catch' it in C++, or vice versa (i.e., `throw ... @catch').
The `-fobjc-exceptions' switch also enables the use of
synchronization blocks for thread-safe execution:
@synchronized (ObjCClass *guard) {
...
}
Upon entering the `@synchronized' block, a thread of execution
shall first check whether a lock has been placed on the
corresponding `guard' object by another thread. If it has, the
current thread shall wait until the other thread relinquishes its
lock. Once `guard' becomes available, the current thread will
place its own lock on it, execute the code contained in the
`@synchronized' block, and finally relinquish the lock (thereby
making `guard' available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be
marked `@synchronized'. Note that throwing exceptions out of
`@synchronized' blocks is allowed, and will cause the guarding
object to be unlocked properly.
`-fobjc-gc'
Enable garbage collection (GC) in Objective-C and Objective-C++
programs.
`-freplace-objc-classes'
Emit a special marker instructing `ld(1)' not to statically link in
the resulting object file, and allow `dyld(1)' to load it in at
run time instead. This is used in conjunction with the
Fix-and-Continue debugging mode, where the object file in question
may be recompiled and dynamically reloaded in the course of
program execution, without the need to restart the program itself.
Currently, Fix-and-Continue functionality is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
`-fzero-link'
When compiling for the NeXT runtime, the compiler ordinarily
replaces calls to `objc_getClass("...")' (when the name of the
class is known at compile time) with static class references that
get initialized at load time, which improves run-time performance.
Specifying the `-fzero-link' flag suppresses this behavior and
causes calls to `objc_getClass("...")' to be retained. This is
useful in Zero-Link debugging mode, since it allows for individual
class implementations to be modified during program execution.
`-gen-decls'
Dump interface declarations for all classes seen in the source
file to a file named `SOURCENAME.decl'.
`-Wassign-intercept (Objective-C and Objective-C++ only)'
Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.
`-Wno-protocol (Objective-C and Objective-C++ only)'
If a class is declared to implement a protocol, a warning is
issued for every method in the protocol that is not implemented by
the class. The default behavior is to issue a warning for every
method not explicitly implemented in the class, even if a method
implementation is inherited from the superclass. If you use the
`-Wno-protocol' option, then methods inherited from the superclass
are considered to be implemented, and no warning is issued for
them.
`-Wselector (Objective-C and Objective-C++ only)'
Warn if multiple methods of different types for the same selector
are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a
check is performed for each selector appearing in a
`@selector(...)' expression, and a corresponding method for that
selector has been found during compilation. Because these checks
scan the method table only at the end of compilation, these
warnings are not produced if the final stage of compilation is not
reached, for example because an error is found during compilation,
or because the `-fsyntax-only' option is being used.
`-Wstrict-selector-match (Objective-C and Objective-C++ only)'
Warn if multiple methods with differing argument and/or return
types are found for a given selector when attempting to send a
message using this selector to a receiver of type `id' or `Class'.
When this flag is off (which is the default behavior), the
compiler will omit such warnings if any differences found are
confined to types which share the same size and alignment.
`-Wundeclared-selector (Objective-C and Objective-C++ only)'
Warn if a `@selector(...)' expression referring to an undeclared
selector is found. A selector is considered undeclared if no
method with that name has been declared before the
`@selector(...)' expression, either explicitly in an `@interface'
or `@protocol' declaration, or implicitly in an `@implementation'
section. This option always performs its checks as soon as a
`@selector(...)' expression is found, while `-Wselector' only
performs its checks in the final stage of compilation. This also
enforces the coding style convention that methods and selectors
must be declared before being used.
`-print-objc-runtime-info'
Generate C header describing the largest structure that is passed
by value, if any.
File: gcc.info, Node: Language Independent Options, Next: Warning Options, Prev: Objective-C and Objective-C++ Dialect Options, Up: Invoking GCC
3.7 Options to Control Diagnostic Messages Formatting
=====================================================
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g. its width, ...). The options described
below can be used to control the diagnostic messages formatting
algorithm, e.g. how many characters per line, how often source location
information should be reported. Right now, only the C++ front end can
honor these options. However it is expected, in the near future, that
the remaining front ends would be able to digest them correctly.
`-fmessage-length=N'
Try to format error messages so that they fit on lines of about N
characters. The default is 72 characters for `g++' and 0 for the
rest of the front ends supported by GCC. If N is zero, then no
line-wrapping will be done; each error message will appear on a
single line.
`-fdiagnostics-show-location=once'
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit _once_ source location information; that
is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation
lines. This is the default behavior.
`-fdiagnostics-show-location=every-line'
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking
a message which is too long to fit on a single line.
`-fdiagnostics-show-option'
This option instructs the diagnostic machinery to add text to each
diagnostic emitted, which indicates which command line option
directly controls that diagnostic, when such an option is known to
the diagnostic machinery.
`-Wcoverage-mismatch'
Warn if feedback profiles do not match when using the
`-fprofile-use' option. If a source file was changed between
`-fprofile-gen' and `-fprofile-use', the files with the profile
feedback can fail to match the source file and GCC can not use the
profile feedback information. By default, GCC emits an error
message in this case. The option `-Wcoverage-mismatch' emits a
warning instead of an error. GCC does not use appropriate
feedback profiles, so using this option can result in poorly
optimized code. This option is useful only in the case of very
minor changes such as bug fixes to an existing code-base.
File: gcc.info, Node: Warning Options, Next: Debugging Options, Prev: Language Independent Options, Up: Invoking GCC
3.8 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.
The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.
`-fsyntax-only'
Check the code for syntax errors, but don't do anything beyond
that.
`-w'
Inhibit all warning messages.
`-Werror'
Make all warnings into errors.
`-Werror='
Make the specified warning into an error. The specifier for a
warning is appended, for example `-Werror=switch' turns the
warnings controlled by `-Wswitch' into errors. This switch takes a
negative form, to be used to negate `-Werror' for specific
warnings, for example `-Wno-error=switch' makes `-Wswitch'
warnings not be errors, even when `-Werror' is in effect. You can
use the `-fdiagnostics-show-option' option to have each
controllable warning amended with the option which controls it, to
determine what to use with this option.
Note that specifying `-Werror='FOO automatically implies `-W'FOO.
However, `-Wno-error='FOO does not imply anything.
`-Wfatal-errors'
This option causes the compiler to abort compilation on the first
error occurred rather than trying to keep going and printing
further error messages.
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. For further, language-specific options also refer to *note
C++ Dialect Options:: and *note Objective-C and Objective-C++ Dialect
Options::.
`-pedantic'
Issue all the warnings demanded by strict ISO C and ISO C++;
reject all programs that use forbidden extensions, and some other
programs that do not follow ISO C and ISO C++. For ISO C, follows
the version of the ISO C standard specified by any `-std' option
used.
Valid ISO C and ISO C++ programs should compile properly with or
without this option (though a rare few will require `-ansi' or a
`-std' option specifying the required version of ISO C). 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::.
Some users try to use `-pedantic' to check programs for strict ISO
C conformance. They soon find that it does not do quite what they
want: it finds some non-ISO practices, but not all--only those for
which ISO C _requires_ a diagnostic, and some others for which
diagnostics have been added.
A feature to report any failure to conform to ISO 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.
Where the standard specified with `-std' represents a GNU extended
dialect of C, such as `gnu89' or `gnu99', there is a corresponding
"base standard", the version of ISO C on which the GNU extended
dialect is based. Warnings from `-pedantic' are given where they
are required by the base standard. (It would not make sense for
such warnings to be given only for features not in the specified
GNU C dialect, since by definition the GNU dialects of C include
all features the compiler supports with the given option, and
there would be nothing to warn about.)
`-pedantic-errors'
Like `-pedantic', except that errors are produced rather than
warnings.
`-Wall'
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. This also
enables some language-specific warnings described in *note C++
Dialect Options:: and *note Objective-C and Objective-C++ Dialect
Options::.
`-Wall' turns on the following warning flags:
-Waddress
-Warray-bounds (only with `-O2')
-Wc++0x-compat
-Wchar-subscripts
-Wimplicit-int
-Wimplicit-function-declaration
-Wcomment
-Wformat
-Wmain (only for C/ObjC and unless `-ffreestanding')
-Wmissing-braces
-Wnonnull
-Wparentheses
-Wpointer-sign
-Wreorder
-Wreturn-type
-Wsequence-point
-Wsign-compare (only in C++)
-Wstrict-aliasing
-Wstrict-overflow=1
-Wswitch
-Wtrigraphs
-Wuninitialized (only with `-O1' and above)
-Wunknown-pragmas
-Wunused-function
-Wunused-label
-Wunused-value
-Wunused-variable
Note that some warning flags 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. Some of them are enabled by `-Wextra' but
many of them must be enabled individually.
`-Wextra'
This enables some extra warning flags that are not enabled by
`-Wall'. (This option used to be called `-W'. The older name is
still supported, but the newer name is more descriptive.)
-Wclobbered
-Wempty-body
-Wignored-qualifiers
-Wmissing-field-initializers
-Wmissing-parameter-type (C only)
-Wold-style-declaration (C only)
-Woverride-init
-Wsign-compare
-Wtype-limits
-Wuninitialized (only with `-O1' and above)
-Wunused-parameter (only with `-Wunused' or `-Wall')
The option `-Wextra' also prints warning messages for the
following cases:
* A pointer is compared against integer zero with `<', `<=',
`>', or `>='.
* (C++ only) An enumerator and a non-enumerator both appear in a
conditional expression.
* (C++ only) A non-static reference or non-static `const' member
appears in a class without constructors.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array which has been declared
`register'.
* (C++ only) Taking the address of a variable which has been
declared `register'.
* (C++ only) A base class is not initialized in a derived
class' copy constructor.
`-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. This warning is enabled by `-Wall'.
`-Wcomment'
Warn whenever a comment-start sequence `/*' appears in a `/*'
comment, or whenever a Backslash-Newline appears in a `//' comment.
This warning is enabled by `-Wall'.
`-Wformat'
Check calls to `printf' and `scanf', etc., to make sure that the
arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others
specified by format attributes (*note Function Attributes::), in
the `printf', `scanf', `strftime' and `strfmon' (an X/Open
extension, not in the C standard) families (or other
target-specific families). Which functions are checked without
format attributes having been specified depends on the standard
version selected, and such checks of functions without the
attribute specified are disabled by `-ffreestanding' or
`-fno-builtin'.
The formats are checked against the format features supported by
GNU libc version 2.2. These include all ISO C90 and C99 features,
as well as features from the Single Unix Specification and some
BSD and GNU extensions. Other library implementations may not
support all these features; GCC does not support warning about
features that go beyond a particular library's limitations.
However, if `-pedantic' is used with `-Wformat', warnings will be
given about format features not in the selected standard version
(but not for `strfmon' formats, since those are not in any version
of the C standard). *Note Options Controlling C Dialect: C
Dialect Options.
Since `-Wformat' also checks for null format arguments for several
functions, `-Wformat' also implies `-Wnonnull'.
`-Wformat' is included in `-Wall'. For more control over some
aspects of format checking, the options `-Wformat-y2k',
`-Wno-format-extra-args', `-Wno-format-zero-length',
`-Wformat-nonliteral', `-Wformat-security', and `-Wformat=2' are
available, but are not included in `-Wall'.
`-Wformat-y2k'
If `-Wformat' is specified, also warn about `strftime' formats
which may yield only a two-digit year.
`-Wno-format-extra-args'
If `-Wformat' is specified, do not warn about excess arguments to a
`printf' or `scanf' format function. The C standard specifies
that such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with `$' operand number specifications, normally
warnings are still given, since the implementation could not know
what type to pass to `va_arg' to skip the unused arguments.
However, in the case of `scanf' formats, this option will suppress
the warning if the unused arguments are all pointers, since the
Single Unix Specification says that such unused arguments are
allowed.
`-Wno-format-zero-length (C and Objective-C only)'
If `-Wformat' is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.
`-Wformat-nonliteral'
If `-Wformat' is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a `va_list'.
`-Wformat-security'
If `-Wformat' is specified, also warn about uses of format
functions that represent possible security problems. At present,
this warns about calls to `printf' and `scanf' functions where the
format string is not a string literal and there are no format
arguments, as in `printf (foo);'. This may be a security hole if
the format string came from untrusted input and contains `%n'.
(This is currently a subset of what `-Wformat-nonliteral' warns
about, but in future warnings may be added to `-Wformat-security'
that are not included in `-Wformat-nonliteral'.)
`-Wformat=2'
Enable `-Wformat' plus format checks not included in `-Wformat'.
Currently equivalent to `-Wformat -Wformat-nonliteral
-Wformat-security -Wformat-y2k'.
`-Wnonnull (C and Objective-C only)'
Warn about passing a null pointer for arguments marked as
requiring a non-null value by the `nonnull' function attribute.
`-Wnonnull' is included in `-Wall' and `-Wformat'. It can be
disabled with the `-Wno-nonnull' option.
`-Winit-self (C, C++, Objective-C and Objective-C++ only)'
Warn about uninitialized variables which are initialized with
themselves. Note this option can only be used with the
`-Wuninitialized' option, which in turn only works with `-O1' and
above.
For example, GCC will warn about `i' being uninitialized in the
following snippet only when `-Winit-self' has been specified:
int f()
{
int i = i;
return i;
}
`-Wimplicit-int (C and Objective-C only)'
Warn when a declaration does not specify a type. This warning is
enabled by `-Wall'.
`-Wimplicit-function-declaration (C and Objective-C only)'
Give a warning whenever a function is used before being declared.
In C99 mode (`-std=c99' or `-std=gnu99'), this warning is enabled
by default and it is made into an error by `-pedantic-errors'.
This warning is also enabled by `-Wall'.
`-Wimplicit'
Same as `-Wimplicit-int' and `-Wimplicit-function-declaration'.
This warning is enabled by `-Wall'.
`-Wignored-qualifiers (C and C++ only)'
Warn if the return type of a function has a type qualifier such as
`const'. For ISO C such a type qualifier has no effect, since the
value returned by a function is not an lvalue. For C++, the
warning is only emitted for scalar types or `void'. ISO C
prohibits qualified `void' return types on function definitions,
so such return types always receive a warning even without this
option.
This warning is also enabled by `-Wextra'.
`-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. This
warning is enabled by `-Wall'.
`-Wmissing-braces'
Warn if an aggregate or union initializer is not fully bracketed.
In the following example, the initializer for `a' is not fully
bracketed, but that for `b' is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by `-Wall'.
`-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)'
Warn if a user-supplied include directory does not exist.
`-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 if 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.
Also 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/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, GCC 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 ();
}
}
This warning is enabled by `-Wall'.
`-Wsequence-point'
Warn about code that may have undefined semantics because of
violations of sequence point rules in the C and C++ standards.
The C and C++ standards defines the order in which expressions in
a C/C++ program are evaluated in terms of "sequence points", which
represent a partial ordering between the execution of parts of the
program: those executed before the sequence point, and those
executed after it. These occur after the evaluation of a full
expression (one which is not part of a larger expression), after
the evaluation of the first operand of a `&&', `||', `? :' or `,'
(comma) operator, before a function is called (but after the
evaluation of its arguments and the expression denoting the called
function), and in certain other places. Other than as expressed
by the sequence point rules, the order of evaluation of
subexpressions of an expression is not specified. All these rules
describe only a partial order rather than a total order, since,
for example, if two functions are called within one expression
with no sequence point between them, the order in which the
functions are called is not specified. However, the standards
committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to
the values of objects take effect. Programs whose behavior
depends on this have undefined behavior; the C and C++ standards
specify that "Between the previous and next sequence point an
object shall have its stored value modified at most once by the
evaluation of an expression. Furthermore, the prior value shall
be read only to determine the value to be stored.". If a program
breaks these rules, the results on any particular implementation
are entirely unpredictable.
Examples of code with undefined behavior are `a = a++;', `a[n] =
b[n++]' and `a[i++] = i;'. Some more complicated cases are not
diagnosed by this option, and it may give an occasional false
positive result, but in general it has been found fairly effective
at detecting this sort of problem in programs.
The standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle
cases. Links to discussions of the problem, including proposed
formal definitions, may be found on the GCC readings page, at
`http://gcc.gnu.org/readings.html'.
This warning is enabled by `-Wall' for C and C++.
`-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'
(falling off the end of the function body is considered returning
without a value), and about a `return' statement with a expression
in a function whose return-type is `void'.
For C++, a function without return type always produces a
diagnostic message, even when `-Wno-return-type' is specified.
The only exceptions are `main' and functions defined in system
headers.
This warning is enabled by `-Wall'.
`-Wswitch'
Warn whenever a `switch' statement has an index of enumerated 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. This warning is
enabled by `-Wall'.
`-Wswitch-default'
Warn whenever a `switch' statement does not have a `default' case.
`-Wswitch-enum'
Warn whenever a `switch' statement has an index of enumerated type
and lacks a `case' for one or more of the named codes of that
enumeration. `case' labels outside the enumeration range also
provoke warnings when this option is used.
`-Wtrigraphs'
Warn if any trigraphs are encountered that might change the
meaning of the program (trigraphs within comments are not warned
about). This warning is enabled by `-Wall'.
`-Wunused-function'
Warn whenever a static function is declared but not defined or a
non-inline static function is unused. This warning is enabled by
`-Wall'.
`-Wunused-label'
Warn whenever a label is declared but not used. This warning is
enabled by `-Wall'.
To suppress this warning use the `unused' attribute (*note
Variable Attributes::).
`-Wunused-parameter'
Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the `unused' attribute (*note
Variable Attributes::).
`-Wunused-variable'
Warn whenever a local variable or non-constant static variable is
unused aside from its declaration. This warning is enabled by
`-Wall'.
To suppress this warning use the `unused' attribute (*note
Variable Attributes::).
`-Wunused-value'
Warn whenever a statement computes a result that is explicitly not
used. To suppress this warning cast the unused expression to
`void'. This includes an expression-statement or the left-hand
side of a comma expression that contains no side effects. For
example, an expression such as `x[i,j]' will cause a warning, while
`x[(void)i,j]' will not.
This warning is enabled by `-Wall'.
`-Wunused'
All the above `-Wunused' options combined.
In order to get a warning about an unused function parameter, you
must either specify `-Wextra -Wunused' (note that `-Wall' implies
`-Wunused'), or separately specify `-Wunused-parameter'.
`-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 do not specify `-O', you will not get
these warnings. Instead, GCC will issue a warning about
`-Wuninitialized' requiring `-O'.
If you want to warn about code which uses the uninitialized value
of the variable in its own initializer, use the `-Winit-self'
option.
These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables which are uninitialized or clobbered as a whole. They do
not occur for variables or elements declared `volatile'. Because
these warnings depend on optimization, the exact variables or
elements for which there are warnings will depend on the precise
optimization options and version of GCC used.
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 warns when a non-volatile 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::.
This warning is enabled by `-Wall' or `-Wextra' in optimizing
compilations (`-O1' and above).
`-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.
`-Wno-pragmas'
Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas. See also
`-Wunknown-pragmas'.
`-Wstrict-aliasing'
This option is only active when `-fstrict-aliasing' is active. It
warns about code which might break the strict aliasing rules that
the compiler is using for optimization. The warning does not
catch all cases, but does attempt to catch the more common
pitfalls. It is included in `-Wall'. It is equivalent to
`-Wstrict-aliasing=3'
`-Wstrict-aliasing=n'
This option is only active when `-fstrict-aliasing' is active. It
warns about code which might break the strict aliasing rules that
the compiler is using for optimization. Higher levels correspond
to higher accuracy (fewer false positives). Higher levels also
correspond to more effort, similar to the way -O works.
`-Wstrict-aliasing' is equivalent to `-Wstrict-aliasing=n', with
n=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful
when higher levels do not warn but -fstrict-aliasing still breaks
the code, as it has very few false negatives. However, it has
many false positives. Warns for all pointer conversions between
possibly incompatible types, even if never dereferenced. Runs in
the frontend only.
Level 2: Aggressive, quick, not too precise. May still have many
false positives (not as many as level 1 though), and few false
negatives (but possibly more than level 1). Unlike level 1, it
only warns when an address is taken. Warns about incomplete
types. Runs in the frontend only.
Level 3 (default for `-Wstrict-aliasing'): Should have very few
false positives and few false negatives. Slightly slower than
levels 1 or 2 when optimization is enabled. Takes care of the
common punn+dereference pattern in the frontend:
`*(int*)&some_float'. If optimization is enabled, it also runs in
the backend, where it deals with multiple statement cases using
flow-sensitive points-to information. Only warns when the
converted pointer is dereferenced. Does not warn about incomplete
types.
`-Wstrict-overflow'
`-Wstrict-overflow=N'
This option is only active when `-fstrict-overflow' is active. It
warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does
not warn about all cases where the code might overflow: it only
warns about cases where the compiler implements some optimization.
Thus this warning depends on the optimization level.
An optimization which assumes that signed overflow does not occur
is perfectly safe if the values of the variables involved are such
that overflow never does, in fact, occur. Therefore this warning
can easily give a false positive: a warning about code which is not
actually a problem. To help focus on important issues, several
warning levels are defined. No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a
loop will require, in particular when determining whether a loop
will be executed at all.
`-Wstrict-overflow=1'
Warn about cases which are both questionable and easy to
avoid. For example: `x + 1 > x'; with `-fstrict-overflow',
the compiler will simplify this to `1'. This level of
`-Wstrict-overflow' is enabled by `-Wall'; higher levels are
not, and must be explicitly requested.
`-Wstrict-overflow=2'
Also warn about other cases where a comparison is simplified
to a constant. For example: `abs (x) >= 0'. This can only be
simplified when `-fstrict-overflow' is in effect, because
`abs (INT_MIN)' overflows to `INT_MIN', which is less than
zero. `-Wstrict-overflow' (with no level) is the same as
`-Wstrict-overflow=2'.
`-Wstrict-overflow=3'
Also warn about other cases where a comparison is simplified.
For example: `x + 1 > 1' will be simplified to `x > 0'.
`-Wstrict-overflow=4'
Also warn about other simplifications not covered by the
above cases. For example: `(x * 10) / 5' will be simplified
to `x * 2'.
`-Wstrict-overflow=5'
Also warn about cases where the compiler reduces the
magnitude of a constant involved in a comparison. For
example: `x + 2 > y' will be simplified to `x + 1 >= y'.
This is reported only at the highest warning level because
this simplification applies to many comparisons, so this
warning level will give a very large number of false
positives.
`-Warray-bounds'
This option is only active when `-ftree-vrp' is active (default
for -O2 and above). It warns about subscripts to arrays that are
always out of bounds. This warning is enabled by `-Wall'.
`-Wno-div-by-zero'
Do not warn about compile-time integer division by zero. Floating
point division by zero is not warned about, as it can be a
legitimate way of obtaining infinities and NaNs.
`-Wsystem-headers'
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the
assumption that they usually do not indicate real problems and
would only make the compiler output harder to read. Using this
command line option tells GCC to emit warnings from system headers
as if they occurred in user code. However, note that using
`-Wall' in conjunction with this option will _not_ warn about
unknown pragmas in system headers--for that, `-Wunknown-pragmas'
must also be used.
`-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 analyzing 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 and Objective-C only)'
Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have
no traditional C equivalent, and/or problematic constructs which
should be avoided.
* Macro parameters that appear within string literals in the
macro body. In traditional C macro replacement takes place
within string literals, but does not in ISO C.
* In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider a line to be a
directive if the `#' appeared in column 1 on the line.
Therefore `-Wtraditional' warns about directives that
traditional C understands but would ignore because the `#'
does not appear as the first character on the line. It also
suggests you hide directives like `#pragma' not understood by
traditional C by indenting them. Some traditional
implementations would not recognize `#elif', so it suggests
avoiding it altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The `U' integer constant suffix, or the `F' or `L' floating
point constant suffixes. (Traditional C does support the `L'
suffix on integer constants.) Note, these suffixes appear in
macros defined in the system headers of most modern systems,
e.g. the `_MIN'/`_MAX' macros in `<limits.h>'. Use of these
macros in user code might normally lead to spurious warnings,
however GCC's integrated preprocessor has enough context to
avoid warning in these cases.
* 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 ISO 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 ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a
separate namespace for labels.
* Initialization of unions. If the initializer is zero, the
warning is omitted. This is done under the assumption that
the zero initializer in user code appears conditioned on e.g.
`__STDC__' to avoid missing initializer warnings and relies
on default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating point values
and vice versa. The absence of these prototypes when
compiling with traditional C would cause serious problems.
This is a subset of the possible conversion warnings, for the
full set use `-Wtraditional-conversion'.
* Use of ISO C style function definitions. This warning
intentionally is _not_ issued for prototype declarations or
variadic functions because these ISO C features will appear
in your code when using libiberty's traditional C
compatibility macros, `PARAMS' and `VPARAMS'. This warning
is also bypassed for nested functions because that feature is
already a GCC extension and thus not relevant to traditional
C compatibility.
`-Wtraditional-conversion (C and Objective-C only)'
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.
`-Wdeclaration-after-statement (C and Objective-C only)'
Warn when a declaration is found after a statement in a block.
This construct, known from C++, was introduced with ISO C99 and is
by default allowed in GCC. It is not supported by ISO C90 and was
not supported by GCC versions before GCC 3.0. *Note Mixed
Declarations::.
`-Wundef'
Warn if an undefined identifier is evaluated in an `#if' directive.
`-Wno-endif-labels'
Do not warn whenever an `#else' or an `#endif' are followed by
text.
`-Wshadow'
Warn whenever a local variable shadows another local variable,
parameter or global variable or whenever a built-in function is
shadowed.
`-Wlarger-than-LEN'
Warn whenever an object of larger than LEN bytes is defined.
`-Wunsafe-loop-optimizations'
Warn if the loop cannot be optimized because the compiler could not
assume anything on the bounds of the loop indices. With
`-funsafe-loop-optimizations' warn if the compiler made such
assumptions.
`-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. In C++, warn also when an arithmetic operation involves
`NULL'. This warning is also enabled by `-pedantic'.
`-Wtype-limits'
Warn if a comparison is always true or always false due to the
limited range of the data type, but do not warn for constant
expressions. For example, warn if an unsigned variable is
compared against zero with `<' or `>='. This warning is also
enabled by `-Wextra'.
`-Wbad-function-cast (C and Objective-C only)'
Warn whenever a function call is cast to a non-matching type. For
example, warn if `int malloc()' is cast to `anything *'.
`-Wc++-compat (C and Objective-C only)'
Warn about ISO C constructs that are outside of the common subset
of ISO C and ISO C++, e.g. request for implicit conversion from
`void *' to a pointer to non-`void' type.
`-Wc++0x-compat (C++ and Objective-C++ only)'
Warn about C++ constructs whose meaning differs between ISO C++
1998 and ISO C++ 200x, e.g., identifiers in ISO C++ 1998 that will
become keywords in ISO C++ 200x. This warning is enabled by
`-Wall'.
`-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'
When compiling C, 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; when compiling
C++, warn about the deprecated conversion from string literals to
`char *'. This warning, by default, is enabled for C++ programs.
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.
`-Wclobbered'
Warn for variables that might be changed by `longjmp' or `vfork'.
This warning is also enabled by `-Wextra'.
`-Wconversion'
Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like `abs (x)' when `x' is
`double'; conversions between signed and unsigned, like `unsigned
ui = -1'; and conversions to smaller types, like `sqrtf (M_PI)'.
Do not warn for explicit casts like `abs ((int) x)' and `ui =
(unsigned) -1', or if the value is not changed by the conversion
like in `abs (2.0)'. Warnings about conversions between signed
and unsigned integers can be disabled by using
`-Wno-sign-conversion'.
For C++, also warn for conversions between `NULL' and non-pointer
types; confusing overload resolution for user-defined conversions;
and conversions that will never use a type conversion operator:
conversions to `void', the same type, a base class or a reference
to them. Warnings about conversions between signed and unsigned
integers are disabled by default in C++ unless `-Wsign-conversion'
is explicitly enabled.
`-Wempty-body'
Warn if an empty body occurs in an `if', `else' or `do while'
statement. Additionally, in C++, warn when an empty body occurs
in a `while' or `for' statement with no whitespacing before the
semicolon. This warning is also enabled by `-Wextra'.
`-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 `-Wextra'; to get the
other warnings of `-Wextra' without this warning, use `-Wextra
-Wno-sign-compare'.
`-Wsign-conversion'
Warn for implicit conversions that may change the sign of an
integer value, like assigning a signed integer expression to an
unsigned integer variable. An explicit cast silences the warning.
In C, this option is enabled also by `-Wconversion'.
`-Waddress'
Warn about suspicious uses of memory addresses. These include using
the address of a function in a conditional expression, such as
`void func(void); if (func)', and comparisons against the memory
address of a string literal, such as `if (x == "abc")'. Such uses
typically indicate a programmer error: the address of a function
always evaluates to true, so their use in a conditional usually
indicate that the programmer forgot the parentheses in a function
call; and comparisons against string literals result in unspecified
behavior and are not portable in C, so they usually indicate that
the programmer intended to use `strcmp'. This warning is enabled
by `-Wall'.
`-Wlogical-op'
Warn about suspicious uses of logical operators in expressions.
This includes using logical operators in contexts where a bit-wise
operator is likely to be expected.
`-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.)
`-Wno-attributes'
Do not warn if an unexpected `__attribute__' is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This will not stop errors for incorrect use of supported
attributes.
`-Wstrict-prototypes (C and Objective-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.)
`-Wold-style-declaration (C and Objective-C only)'
Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
`static' are not the first things in a declaration. This warning
is also enabled by `-Wextra'.
`-Wold-style-definition (C and Objective-C only)'
Warn if an old-style function definition is used. A warning is
given even if there is a previous prototype.
`-Wmissing-parameter-type (C and Objective-C only)'
A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
This warning is also enabled by `-Wextra'.
`-Wmissing-prototypes (C and Objective-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. In C++, no warnings are issued for
function templates, or for inline functions, or for functions in
anonymous namespaces.
`-Wmissing-field-initializers'
Warn if a structure's initializer has some fields missing. For
example, the following code would cause such a warning, because
`x.h' is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the
following modification would not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in `-Wextra'. To get other `-Wextra'
warnings without this one, use `-Wextra
-Wno-missing-field-initializers'.
`-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. You
will not get a warning for `main' in hosted C environments.
`-Wmissing-format-attribute'
Warn about function pointers which might be candidates for `format'
attributes. Note these are only possible candidates, not absolute
ones. GCC will guess that function pointers with `format'
attributes that are used in assignment, initialization, parameter
passing or return statements should have a corresponding `format'
attribute in the resulting type. I.e. the left-hand side of the
assignment or initialization, the type of the parameter variable,
or the return type of the containing function respectively should
also have a `format' attribute to avoid the warning.
GCC will also warn about function definitions which might be
candidates for `format' attributes. Again, these are only
possible candidates. GCC will guess that `format' attributes
might be appropriate for any function that calls a function like
`vprintf' or `vscanf', but this might not always be the case, and
some functions for which `format' attributes are appropriate may
not be detected.
`-Wno-multichar'
Do not 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.
`-Wnormalized=<none|id|nfc|nfkc>'
In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters. However, sometimes when
characters outside the basic ASCII character set are used, you can
have two different character sequences that look the same. To
avoid confusion, the ISO 10646 standard sets out some
"normalization rules" which when applied ensure that two sequences
that look the same are turned into the same sequence. GCC can
warn you if you are using identifiers which have not been
normalized; this option controls that warning.
There are four levels of warning that GCC supports. The default is
`-Wnormalized=nfc', which warns about any identifier which is not
in the ISO 10646 "C" normalized form, "NFC". NFC is the
recommended form for most uses.
Unfortunately, there are some characters which ISO C and ISO C++
allow in identifiers that when turned into NFC aren't allowable as
identifiers. That is, there's no way to use these symbols in
portable ISO C or C++ and have all your identifiers in NFC.
`-Wnormalized=id' suppresses the warning for these characters. It
is hoped that future versions of the standards involved will
correct this, which is why this option is not the default.
You can switch the warning off for all characters by writing
`-Wnormalized=none'. You would only want to do this if you were
using some other normalization scheme (like "D"), because
otherwise you can easily create bugs that are literally impossible
to see.
Some characters in ISO 10646 have distinct meanings but look
identical in some fonts or display methodologies, especially once
formatting has been applied. For instance `\u207F', "SUPERSCRIPT
LATIN SMALL LETTER N", will display just like a regular `n' which
has been placed in a superscript. ISO 10646 defines the "NFKC"
normalization scheme to convert all these into a standard form as
well, and GCC will warn if your code is not in NFKC if you use
`-Wnormalized=nfkc'. This warning is comparable to warning about
every identifier that contains the letter O because it might be
confused with the digit 0, and so is not the default, but may be
useful as a local coding convention if the programming environment
is unable to be fixed to display these characters distinctly.
`-Wno-deprecated-declarations'
Do not warn about uses of functions (*note Function Attributes::),
variables (*note Variable Attributes::), and types (*note Type
Attributes::) marked as deprecated by using the `deprecated'
attribute.
`-Wno-overflow'
Do not warn about compile-time overflow in constant expressions.
`-Woverride-init (C and Objective-C only)'
Warn if an initialized field without side effects is overridden
when using designated initializers (*note Designated Initializers:
Designated Inits.).
This warning is included in `-Wextra'. To get other `-Wextra'
warnings without this one, use `-Wextra -Wno-override-init'.
`-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 and Objective-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 behavior which is selectable at compile-time.
`-Winline'
Warn if a function can not be inlined and it was declared as
inline. Even with this option, the compiler will not warn about
failures to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or
not to inline a function. For example, the compiler takes into
account the size of the function being inlined and the amount of
inlining that has already been done in the current function.
Therefore, seemingly insignificant changes in the source program
can cause the warnings produced by `-Winline' to appear or
disappear.
`-Wno-invalid-offsetof (C++ and Objective-C++ only)'
Suppress warnings from applying the `offsetof' macro to a non-POD
type. According to the 1998 ISO C++ standard, applying `offsetof'
to a non-POD type is undefined. In existing C++ implementations,
however, `offsetof' typically gives meaningful results even when
applied to certain kinds of non-POD types. (Such as a simple
`struct' that fails to be a POD type only by virtue of having a
constructor.) This flag is for users who are aware that they are
writing nonportable code and who have deliberately chosen to
ignore the warning about it.
The restrictions on `offsetof' may be relaxed in a future version
of the C++ standard.
`-Wno-int-to-pointer-cast (C and Objective-C only)'
Suppress warnings from casts to pointer type of an integer of a
different size.
`-Wno-pointer-to-int-cast (C and Objective-C only)'
Suppress warnings from casts from a pointer to an integer type of a
different size.
`-Winvalid-pch'
Warn if a precompiled header (*note Precompiled Headers::) is
found in the search path but can't be used.
`-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.
`-Wvariadic-macros'
Warn if variadic macros are used in pedantic ISO C90 mode, or the
GNU alternate syntax when in pedantic ISO C99 mode. This is
default. To inhibit the warning messages, use
`-Wno-variadic-macros'.
`-Wvla'
Warn if variable length array is used in the code. `-Wno-vla'
will prevent the `-pedantic' warning of the variable length array.
`-Wvolatile-register-var'
Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate