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<a name="Warning-Options"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="Debugging-Options.html#Debugging-Options">Debugging Options</a>,
Previous:&nbsp;<a rel="previous" accesskey="p" href="Language-Independent-Options.html#Language-Independent-Options">Language Independent Options</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="Invoking-GCC.html#Invoking-GCC">Invoking GCC</a>
<hr>
</div>
<h3 class="section">3.8 Options to Request or Suppress Warnings</h3>
<p><a name="index-options-to-control-warnings-243"></a><a name="index-warning-messages-244"></a><a name="index-messages_002c-warning-245"></a><a name="index-suppressing-warnings-246"></a>
Warnings are diagnostic messages that report constructions that
are not inherently erroneous but that are risky or suggest there
may have been an error.
<p>The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.
<a name="index-syntax-checking-247"></a>
<dl><dt><code>-fsyntax-only</code><dd><a name="index-fsyntax_002donly-248"></a>Check the code for syntax errors, but don't do anything beyond that.
<br><dt><code>-fmax-errors=</code><var>n</var><dd><a name="index-fmax_002derrors-249"></a>Limits the maximum number of error messages to <var>n</var>, at which point
GCC bails out rather than attempting to continue processing the source
code. If <var>n</var> is 0 (the default), there is no limit on the number
of error messages produced. If <samp><span class="option">-Wfatal-errors</span></samp> is also
specified, then <samp><span class="option">-Wfatal-errors</span></samp> takes precedence over this
option.
<br><dt><code>-w</code><dd><a name="index-w-250"></a>Inhibit all warning messages.
<br><dt><code>-Werror</code><dd><a name="index-Werror-251"></a><a name="index-Wno_002derror-252"></a>Make all warnings into errors.
<br><dt><code>-Werror=</code><dd><a name="index-Werror_003d-253"></a><a name="index-Wno_002derror_003d-254"></a>Make the specified warning into an error. The specifier for a warning
is appended; for example <samp><span class="option">-Werror=switch</span></samp> turns the warnings
controlled by <samp><span class="option">-Wswitch</span></samp> into errors. This switch takes a
negative form, to be used to negate <samp><span class="option">-Werror</span></samp> for specific
warnings; for example <samp><span class="option">-Wno-error=switch</span></samp> makes
<samp><span class="option">-Wswitch</span></samp> warnings not be errors, even when <samp><span class="option">-Werror</span></samp>
is in effect.
<p>The warning message for each controllable warning includes the
option that controls the warning. That option can then be used with
<samp><span class="option">-Werror=</span></samp> and <samp><span class="option">-Wno-error=</span></samp> as described above.
(Printing of the option in the warning message can be disabled using the
<samp><span class="option">-fno-diagnostics-show-option</span></samp> flag.)
<p>Note that specifying <samp><span class="option">-Werror=</span></samp><var>foo</var> automatically implies
<samp><span class="option">-W</span></samp><var>foo</var>. However, <samp><span class="option">-Wno-error=</span></samp><var>foo</var> does not
imply anything.
<br><dt><code>-Wfatal-errors</code><dd><a name="index-Wfatal_002derrors-255"></a><a name="index-Wno_002dfatal_002derrors-256"></a>This option causes the compiler to abort compilation on the first error
occurred rather than trying to keep going and printing further error
messages.
</dl>
<p>You can request many specific warnings with options beginning with
&lsquo;<samp><span class="samp">-W</span></samp>&rsquo;, for example <samp><span class="option">-Wimplicit</span></samp> to request warnings on
implicit declarations. Each of these specific warning options also
has a negative form beginning &lsquo;<samp><span class="samp">-Wno-</span></samp>&rsquo; to turn off warnings; for
example, <samp><span class="option">-Wno-implicit</span></samp>. This manual lists only one of the
two forms, whichever is not the default. For further
language-specific options also refer to <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options">C++ Dialect Options</a> and
<a href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options">Objective-C and Objective-C++ Dialect Options</a>.
<p>When an unrecognized warning option is requested (e.g.,
<samp><span class="option">-Wunknown-warning</span></samp>), GCC emits a diagnostic stating
that the option is not recognized. However, if the <samp><span class="option">-Wno-</span></samp> form
is used, the behavior is slightly different: no diagnostic is
produced for <samp><span class="option">-Wno-unknown-warning</span></samp> unless other diagnostics
are being produced. This allows the use of new <samp><span class="option">-Wno-</span></samp> options
with old compilers, but if something goes wrong, the compiler
warns that an unrecognized option is present.
<dl>
<dt><code>-Wpedantic</code><dt><code>-pedantic</code><dd><a name="index-pedantic-257"></a><a name="index-Wpedantic-258"></a>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 <samp><span class="option">-std</span></samp> option used.
<p>Valid ISO C and ISO C++ programs should compile properly with or without
this option (though a rare few require <samp><span class="option">-ansi</span></samp> or a
<samp><span class="option">-std</span></samp> 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.
<p><samp><span class="option">-Wpedantic</span></samp> does not cause warning messages for use of the
alternate keywords whose names begin and end with &lsquo;<samp><span class="samp">__</span></samp>&rsquo;. Pedantic
warnings are also disabled in the expression that follows
<code>__extension__</code>. However, only system header files should use
these escape routes; application programs should avoid them.
See <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>.
<p>Some users try to use <samp><span class="option">-Wpedantic</span></samp> 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&mdash;only those for which
ISO C <em>requires</em> a diagnostic, and some others for which
diagnostics have been added.
<p>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 <samp><span class="option">-Wpedantic</span></samp>. We don't have plans to
support such a feature in the near future.
<p>Where the standard specified with <samp><span class="option">-std</span></samp> represents a GNU
extended dialect of C, such as &lsquo;<samp><span class="samp">gnu90</span></samp>&rsquo; or &lsquo;<samp><span class="samp">gnu99</span></samp>&rsquo;, there is a
corresponding <dfn>base standard</dfn>, the version of ISO C on which the GNU
extended dialect is based. Warnings from <samp><span class="option">-Wpedantic</span></samp> are given
where they are required by the base standard. (It does 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.)
<br><dt><code>-pedantic-errors</code><dd><a name="index-pedantic_002derrors-259"></a>Like <samp><span class="option">-Wpedantic</span></samp>, except that errors are produced rather than
warnings.
<br><dt><code>-Wall</code><dd><a name="index-Wall-260"></a><a name="index-Wno_002dall-261"></a>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 <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options">C++ Dialect Options</a> and <a href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options">Objective-C and Objective-C++ Dialect Options</a>.
<p><samp><span class="option">-Wall</span></samp> turns on the following warning flags:
<pre class="smallexample"> -Waddress
-Warray-bounds <span class="roman">(only with</span> <samp><span class="option">-O2</span></samp><span class="roman">)</span>
-Wc++11-compat
-Wchar-subscripts
-Wenum-compare <span class="roman">(in C/ObjC; this is on by default in C++)</span>
-Wimplicit-int <span class="roman">(C and Objective-C only)</span>
-Wimplicit-function-declaration <span class="roman">(C and Objective-C only)</span>
-Wcomment
-Wformat
-Wmain <span class="roman">(only for C/ObjC and unless</span> <samp><span class="option">-ffreestanding</span></samp><span class="roman">)</span>
-Wmaybe-uninitialized
-Wmissing-braces <span class="roman">(only for C/ObjC)</span>
-Wnonnull
-Wparentheses
-Wpointer-sign
-Wreorder
-Wreturn-type
-Wsequence-point
-Wsign-compare <span class="roman">(only in C++)</span>
-Wstrict-aliasing
-Wstrict-overflow=1
-Wswitch
-Wtrigraphs
-Wuninitialized
-Wunknown-pragmas
-Wunused-function
-Wunused-label
-Wunused-value
-Wunused-variable
-Wvolatile-register-var
</pre>
<p>Note that some warning flags are not implied by <samp><span class="option">-Wall</span></samp>. 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 <samp><span class="option">-Wextra</span></samp> but many of
them must be enabled individually.
<br><dt><code>-Wextra</code><dd><a name="index-W-262"></a><a name="index-Wextra-263"></a><a name="index-Wno_002dextra-264"></a>This enables some extra warning flags that are not enabled by
<samp><span class="option">-Wall</span></samp>. (This option used to be called <samp><span class="option">-W</span></samp>. The older
name is still supported, but the newer name is more descriptive.)
<pre class="smallexample"> -Wclobbered
-Wempty-body
-Wignored-qualifiers
-Wmissing-field-initializers
-Wmissing-parameter-type <span class="roman">(C only)</span>
-Wold-style-declaration <span class="roman">(C only)</span>
-Woverride-init
-Wsign-compare
-Wtype-limits
-Wuninitialized
-Wunused-parameter <span class="roman">(only with</span> <samp><span class="option">-Wunused</span></samp> <span class="roman">or</span> <samp><span class="option">-Wall</span></samp><span class="roman">)</span>
-Wunused-but-set-parameter <span class="roman">(only with</span> <samp><span class="option">-Wunused</span></samp> <span class="roman">or</span> <samp><span class="option">-Wall</span></samp><span class="roman">)</span>
</pre>
<p>The option <samp><span class="option">-Wextra</span></samp> also prints warning messages for the
following cases:
<ul>
<li>A pointer is compared against integer zero with &lsquo;<samp><span class="samp">&lt;</span></samp>&rsquo;, &lsquo;<samp><span class="samp">&lt;=</span></samp>&rsquo;,
&lsquo;<samp><span class="samp">&gt;</span></samp>&rsquo;, or &lsquo;<samp><span class="samp">&gt;=</span></samp>&rsquo;.
<li>(C++ only) An enumerator and a non-enumerator both appear in a
conditional expression.
<li>(C++ only) Ambiguous virtual bases.
<li>(C++ only) Subscripting an array that has been declared &lsquo;<samp><span class="samp">register</span></samp>&rsquo;.
<li>(C++ only) Taking the address of a variable that has been declared
&lsquo;<samp><span class="samp">register</span></samp>&rsquo;.
<li>(C++ only) A base class is not initialized in a derived class's copy
constructor.
</ul>
<br><dt><code>-Wchar-subscripts</code><dd><a name="index-Wchar_002dsubscripts-265"></a><a name="index-Wno_002dchar_002dsubscripts-266"></a>Warn if an array subscript has type <code>char</code>. This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wcomment</code><dd><a name="index-Wcomment-267"></a><a name="index-Wno_002dcomment-268"></a>Warn whenever a comment-start sequence &lsquo;<samp><span class="samp">/*</span></samp>&rsquo; appears in a &lsquo;<samp><span class="samp">/*</span></samp>&rsquo;
comment, or whenever a Backslash-Newline appears in a &lsquo;<samp><span class="samp">//</span></samp>&rsquo; comment.
This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wno-coverage-mismatch</code><dd><a name="index-Wno_002dcoverage_002dmismatch-269"></a>Warn if feedback profiles do not match when using the
<samp><span class="option">-fprofile-use</span></samp> option.
If a source file is changed between compiling with <samp><span class="option">-fprofile-gen</span></samp> and
with <samp><span class="option">-fprofile-use</span></samp>, the files with the profile feedback can fail
to match the source file and GCC cannot use the profile feedback
information. By default, this warning is enabled and is treated as an
error. <samp><span class="option">-Wno-coverage-mismatch</span></samp> can be used to disable the
warning or <samp><span class="option">-Wno-error=coverage-mismatch</span></samp> can be used to
disable the error. Disabling the error for this warning can result in
poorly optimized code and is useful only in the
case of very minor changes such as bug fixes to an existing code-base.
Completely disabling the warning is not recommended.
<br><dt><code>-Wno-cpp</code><dd><span class="roman">(C, Objective-C, C++, Objective-C++ and Fortran only)</span>
<p>Suppress warning messages emitted by <code>#warning</code> directives.
<br><dt><code>-Wdouble-promotion </code><span class="roman">(C, C++, Objective-C and Objective-C++ only)</span><dd><a name="index-Wdouble_002dpromotion-270"></a><a name="index-Wno_002ddouble_002dpromotion-271"></a>Give a warning when a value of type <code>float</code> is implicitly
promoted to <code>double</code>. CPUs with a 32-bit &ldquo;single-precision&rdquo;
floating-point unit implement <code>float</code> in hardware, but emulate
<code>double</code> in software. On such a machine, doing computations
using <code>double</code> values is much more expensive because of the
overhead required for software emulation.
<p>It is easy to accidentally do computations with <code>double</code> because
floating-point literals are implicitly of type <code>double</code>. For
example, in:
<pre class="smallexample"> float area(float radius)
{
return 3.14159 * radius * radius;
}
</pre>
<p>the compiler performs the entire computation with <code>double</code>
because the floating-point literal is a <code>double</code>.
<br><dt><code>-Wformat</code><dt><code>-Wformat=</code><var>n</var><dd><a name="index-Wformat-272"></a><a name="index-Wno_002dformat-273"></a><a name="index-ffreestanding-274"></a><a name="index-fno_002dbuiltin-275"></a><a name="index-Wformat_003d-276"></a>Check calls to <code>printf</code> and <code>scanf</code>, 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 (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>), in the <code>printf</code>,
<code>scanf</code>, <code>strftime</code> and <code>strfmon</code> (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
<samp><span class="option">-ffreestanding</span></samp> or <samp><span class="option">-fno-builtin</span></samp>.
<p>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 <samp><span class="option">-Wpedantic</span></samp> is used
with <samp><span class="option">-Wformat</span></samp>, warnings are given about format features not
in the selected standard version (but not for <code>strfmon</code> formats,
since those are not in any version of the C standard). See <a href="C-Dialect-Options.html#C-Dialect-Options">Options Controlling C Dialect</a>.
<dl>
<dt><code>-Wformat=1</code><dt><code>-Wformat</code><dd>Option <samp><span class="option">-Wformat</span></samp> is equivalent to <samp><span class="option">-Wformat=1</span></samp>, and
<samp><span class="option">-Wno-format</span></samp> is equivalent to <samp><span class="option">-Wformat=0</span></samp>. Since
<samp><span class="option">-Wformat</span></samp> also checks for null format arguments for several
functions, <samp><span class="option">-Wformat</span></samp> also implies <samp><span class="option">-Wnonnull</span></samp>. Some
aspects of this level of format checking can be disabled by the
options: <samp><span class="option">-Wno-format-contains-nul</span></samp>,
<samp><span class="option">-Wno-format-extra-args</span></samp>, and <samp><span class="option">-Wno-format-zero-length</span></samp>.
<samp><span class="option">-Wformat</span></samp> is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wno-format-contains-nul</code><dd><a name="index-Wno_002dformat_002dcontains_002dnul-277"></a><a name="index-Wformat_002dcontains_002dnul-278"></a>If <samp><span class="option">-Wformat</span></samp> is specified, do not warn about format strings that
contain NUL bytes.
<br><dt><code>-Wno-format-extra-args</code><dd><a name="index-Wno_002dformat_002dextra_002dargs-279"></a><a name="index-Wformat_002dextra_002dargs-280"></a>If <samp><span class="option">-Wformat</span></samp> is specified, do not warn about excess arguments to a
<code>printf</code> or <code>scanf</code> format function. The C standard specifies
that such arguments are ignored.
<p>Where the unused arguments lie between used arguments that are
specified with &lsquo;<samp><span class="samp">$</span></samp>&rsquo; operand number specifications, normally
warnings are still given, since the implementation could not know what
type to pass to <code>va_arg</code> to skip the unused arguments. However,
in the case of <code>scanf</code> formats, this option suppresses the
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
<br><dt><code>-Wno-format-zero-length</code><dd><a name="index-Wno_002dformat_002dzero_002dlength-281"></a><a name="index-Wformat_002dzero_002dlength-282"></a>If <samp><span class="option">-Wformat</span></samp> is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.
<br><dt><code>-Wformat=2</code><dd>Enable <samp><span class="option">-Wformat</span></samp> plus additional format checks. Currently
equivalent to <samp><span class="option">-Wformat -Wformat-nonliteral -Wformat-security
-Wformat-y2k</span></samp>.
<br><dt><code>-Wformat-nonliteral</code><dd><a name="index-Wformat_002dnonliteral-283"></a><a name="index-Wno_002dformat_002dnonliteral-284"></a>If <samp><span class="option">-Wformat</span></samp> 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 <code>va_list</code>.
<br><dt><code>-Wformat-security</code><dd><a name="index-Wformat_002dsecurity-285"></a><a name="index-Wno_002dformat_002dsecurity-286"></a>If <samp><span class="option">-Wformat</span></samp> is specified, also warn about uses of format
functions that represent possible security problems. At present, this
warns about calls to <code>printf</code> and <code>scanf</code> functions where the
format string is not a string literal and there are no format arguments,
as in <code>printf (foo);</code>. This may be a security hole if the format
string came from untrusted input and contains &lsquo;<samp><span class="samp">%n</span></samp>&rsquo;. (This is
currently a subset of what <samp><span class="option">-Wformat-nonliteral</span></samp> warns about, but
in future warnings may be added to <samp><span class="option">-Wformat-security</span></samp> that are not
included in <samp><span class="option">-Wformat-nonliteral</span></samp>.)
<br><dt><code>-Wformat-y2k</code><dd><a name="index-Wformat_002dy2k-287"></a><a name="index-Wno_002dformat_002dy2k-288"></a>If <samp><span class="option">-Wformat</span></samp> is specified, also warn about <code>strftime</code>
formats that may yield only a two-digit year.
</dl>
<br><dt><code>-Wnonnull</code><dd><a name="index-Wnonnull-289"></a><a name="index-Wno_002dnonnull-290"></a>Warn about passing a null pointer for arguments marked as
requiring a non-null value by the <code>nonnull</code> function attribute.
<p><samp><span class="option">-Wnonnull</span></samp> is included in <samp><span class="option">-Wall</span></samp> and <samp><span class="option">-Wformat</span></samp>. It
can be disabled with the <samp><span class="option">-Wno-nonnull</span></samp> option.
<br><dt><code>-Winit-self </code><span class="roman">(C, C++, Objective-C and Objective-C++ only)</span><dd><a name="index-Winit_002dself-291"></a><a name="index-Wno_002dinit_002dself-292"></a>Warn about uninitialized variables that are initialized with themselves.
Note this option can only be used with the <samp><span class="option">-Wuninitialized</span></samp> option.
<p>For example, GCC warns about <code>i</code> being uninitialized in the
following snippet only when <samp><span class="option">-Winit-self</span></samp> has been specified:
<pre class="smallexample"> int f()
{
int i = i;
return i;
}
</pre>
<p>This warning is enabled by <samp><span class="option">-Wall</span></samp> in C++.
<br><dt><code>-Wimplicit-int </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wimplicit_002dint-293"></a><a name="index-Wno_002dimplicit_002dint-294"></a>Warn when a declaration does not specify a type.
This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wimplicit-function-declaration </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wimplicit_002dfunction_002ddeclaration-295"></a><a name="index-Wno_002dimplicit_002dfunction_002ddeclaration-296"></a>Give a warning whenever a function is used before being declared. In
C99 mode (<samp><span class="option">-std=c99</span></samp> or <samp><span class="option">-std=gnu99</span></samp>), this warning is
enabled by default and it is made into an error by
<samp><span class="option">-pedantic-errors</span></samp>. This warning is also enabled by
<samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wimplicit </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wimplicit-297"></a><a name="index-Wno_002dimplicit-298"></a>Same as <samp><span class="option">-Wimplicit-int</span></samp> and <samp><span class="option">-Wimplicit-function-declaration</span></samp>.
This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wignored-qualifiers </code><span class="roman">(C and C++ only)</span><dd><a name="index-Wignored_002dqualifiers-299"></a><a name="index-Wno_002dignored_002dqualifiers-300"></a>Warn if the return type of a function has a type qualifier
such as <code>const</code>. 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 <code>void</code>.
ISO C prohibits qualified <code>void</code> return types on function
definitions, so such return types always receive a warning
even without this option.
<p>This warning is also enabled by <samp><span class="option">-Wextra</span></samp>.
<br><dt><code>-Wmain</code><dd><a name="index-Wmain-301"></a><a name="index-Wno_002dmain-302"></a>Warn if the type of &lsquo;<samp><span class="samp">main</span></samp>&rsquo; is suspicious. &lsquo;<samp><span class="samp">main</span></samp>&rsquo; 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 default in C++ and is enabled by either <samp><span class="option">-Wall</span></samp>
or <samp><span class="option">-Wpedantic</span></samp>.
<br><dt><code>-Wmissing-braces</code><dd><a name="index-Wmissing_002dbraces-303"></a><a name="index-Wno_002dmissing_002dbraces-304"></a>Warn if an aggregate or union initializer is not fully bracketed. In
the following example, the initializer for &lsquo;<samp><span class="samp">a</span></samp>&rsquo; is not fully
bracketed, but that for &lsquo;<samp><span class="samp">b</span></samp>&rsquo; is fully bracketed. This warning is
enabled by <samp><span class="option">-Wall</span></samp> in C.
<pre class="smallexample"> int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
</pre>
<p>This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wmissing-include-dirs </code><span class="roman">(C, C++, Objective-C and Objective-C++ only)</span><dd><a name="index-Wmissing_002dinclude_002ddirs-305"></a><a name="index-Wno_002dmissing_002dinclude_002ddirs-306"></a>Warn if a user-supplied include directory does not exist.
<br><dt><code>-Wparentheses</code><dd><a name="index-Wparentheses-307"></a><a name="index-Wno_002dparentheses-308"></a>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.
<p>Also warn if a comparison like &lsquo;<samp><span class="samp">x&lt;=y&lt;=z</span></samp>&rsquo; appears; this is
equivalent to &lsquo;<samp><span class="samp">(x&lt;=y ? 1 : 0) &lt;= z</span></samp>&rsquo;, which is a different
interpretation from that of ordinary mathematical notation.
<p>Also warn about constructions where there may be confusion to which
<code>if</code> statement an <code>else</code> branch belongs. Here is an example of
such a case:
<pre class="smallexample"> {
if (a)
if (b)
foo ();
else
bar ();
}
</pre>
<p>In C/C++, every <code>else</code> branch belongs to the innermost possible
<code>if</code> statement, which in this example is <code>if (b)</code>. 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 issues a warning when this flag
is specified. To eliminate the warning, add explicit braces around
the innermost <code>if</code> statement so there is no way the <code>else</code>
can belong to the enclosing <code>if</code>. The resulting code
looks like this:
<pre class="smallexample"> {
if (a)
{
if (b)
foo ();
else
bar ();
}
}
</pre>
<p>Also warn for dangerous uses of the GNU extension to
<code>?:</code> with omitted middle operand. When the condition
in the <code>?</code>: operator is a boolean expression, the omitted value is
always 1. Often programmers expect it to be a value computed
inside the conditional expression instead.
<p>This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wsequence-point</code><dd><a name="index-Wsequence_002dpoint-309"></a><a name="index-Wno_002dsequence_002dpoint-310"></a>Warn about code that may have undefined semantics because of violations
of sequence point rules in the C and C++ standards.
<p>The C and C++ standards define the order in which expressions in a C/C++
program are evaluated in terms of <dfn>sequence points</dfn>, 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
<code>&amp;&amp;</code>, <code>||</code>, <code>? :</code> or <code>,</code> (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.
<p>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 &ldquo;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.&rdquo;. If a program breaks these rules, the results on any
particular implementation are entirely unpredictable.
<p>Examples of code with undefined behavior are <code>a = a++;</code>, <code>a[n]
= b[n++]</code> and <code>a[i++] = i;</code>. 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.
<p>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
<a href="http://gcc.gnu.org/readings.html">http://gcc.gnu.org/readings.html</a>.
<p>This warning is enabled by <samp><span class="option">-Wall</span></samp> for C and C++.
<br><dt><code>-Wno-return-local-addr</code><dd><a name="index-Wno_002dreturn_002dlocal_002daddr-311"></a><a name="index-Wreturn_002dlocal_002daddr-312"></a>Do not warn about returning a pointer (or in C++, a reference) to a
variable that goes out of scope after the function returns.
<br><dt><code>-Wreturn-type</code><dd><a name="index-Wreturn_002dtype-313"></a><a name="index-Wno_002dreturn_002dtype-314"></a>Warn whenever a function is defined with a return type that defaults
to <code>int</code>. Also warn about any <code>return</code> statement with no
return value in a function whose return type is not <code>void</code>
(falling off the end of the function body is considered returning
without a value), and about a <code>return</code> statement with an
expression in a function whose return type is <code>void</code>.
<p>For C++, a function without return type always produces a diagnostic
message, even when <samp><span class="option">-Wno-return-type</span></samp> is specified. The only
exceptions are &lsquo;<samp><span class="samp">main</span></samp>&rsquo; and functions defined in system headers.
<p>This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wswitch</code><dd><a name="index-Wswitch-315"></a><a name="index-Wno_002dswitch-316"></a>Warn whenever a <code>switch</code> statement has an index of enumerated type
and lacks a <code>case</code> for one or more of the named codes of that
enumeration. (The presence of a <code>default</code> label prevents this
warning.) <code>case</code> labels outside the enumeration range also
provoke warnings when this option is used (even if there is a
<code>default</code> label).
This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wswitch-default</code><dd><a name="index-Wswitch_002ddefault-317"></a><a name="index-Wno_002dswitch_002ddefault-318"></a>Warn whenever a <code>switch</code> statement does not have a <code>default</code>
case.
<br><dt><code>-Wswitch-enum</code><dd><a name="index-Wswitch_002denum-319"></a><a name="index-Wno_002dswitch_002denum-320"></a>Warn whenever a <code>switch</code> statement has an index of enumerated type
and lacks a <code>case</code> for one or more of the named codes of that
enumeration. <code>case</code> labels outside the enumeration range also
provoke warnings when this option is used. The only difference
between <samp><span class="option">-Wswitch</span></samp> and this option is that this option gives a
warning about an omitted enumeration code even if there is a
<code>default</code> label.
<br><dt><code>-Wsync-nand </code><span class="roman">(C and C++ only)</span><dd><a name="index-Wsync_002dnand-321"></a><a name="index-Wno_002dsync_002dnand-322"></a>Warn when <code>__sync_fetch_and_nand</code> and <code>__sync_nand_and_fetch</code>
built-in functions are used. These functions changed semantics in GCC 4.4.
<br><dt><code>-Wtrigraphs</code><dd><a name="index-Wtrigraphs-323"></a><a name="index-Wno_002dtrigraphs-324"></a>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 <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wunused-but-set-parameter</code><dd><a name="index-Wunused_002dbut_002dset_002dparameter-325"></a><a name="index-Wno_002dunused_002dbut_002dset_002dparameter-326"></a>Warn whenever a function parameter is assigned to, but otherwise unused
(aside from its declaration).
<p>To suppress this warning use the &lsquo;<samp><span class="samp">unused</span></samp>&rsquo; attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
<p>This warning is also enabled by <samp><span class="option">-Wunused</span></samp> together with
<samp><span class="option">-Wextra</span></samp>.
<br><dt><code>-Wunused-but-set-variable</code><dd><a name="index-Wunused_002dbut_002dset_002dvariable-327"></a><a name="index-Wno_002dunused_002dbut_002dset_002dvariable-328"></a>Warn whenever a local variable is assigned to, but otherwise unused
(aside from its declaration).
This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<p>To suppress this warning use the &lsquo;<samp><span class="samp">unused</span></samp>&rsquo; attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
<p>This warning is also enabled by <samp><span class="option">-Wunused</span></samp>, which is enabled
by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wunused-function</code><dd><a name="index-Wunused_002dfunction-329"></a><a name="index-Wno_002dunused_002dfunction-330"></a>Warn whenever a static function is declared but not defined or a
non-inline static function is unused.
This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wunused-label</code><dd><a name="index-Wunused_002dlabel-331"></a><a name="index-Wno_002dunused_002dlabel-332"></a>Warn whenever a label is declared but not used.
This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<p>To suppress this warning use the &lsquo;<samp><span class="samp">unused</span></samp>&rsquo; attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
<br><dt><code>-Wunused-local-typedefs </code><span class="roman">(C, Objective-C, C++ and Objective-C++ only)</span><dd><a name="index-Wunused_002dlocal_002dtypedefs-333"></a>Warn when a typedef locally defined in a function is not used.
This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wunused-parameter</code><dd><a name="index-Wunused_002dparameter-334"></a><a name="index-Wno_002dunused_002dparameter-335"></a>Warn whenever a function parameter is unused aside from its declaration.
<p>To suppress this warning use the &lsquo;<samp><span class="samp">unused</span></samp>&rsquo; attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
<br><dt><code>-Wno-unused-result</code><dd><a name="index-Wunused_002dresult-336"></a><a name="index-Wno_002dunused_002dresult-337"></a>Do not warn if a caller of a function marked with attribute
<code>warn_unused_result</code> (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>) does not use
its return value. The default is <samp><span class="option">-Wunused-result</span></samp>.
<br><dt><code>-Wunused-variable</code><dd><a name="index-Wunused_002dvariable-338"></a><a name="index-Wno_002dunused_002dvariable-339"></a>Warn whenever a local variable or non-constant static variable is unused
aside from its declaration.
This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<p>To suppress this warning use the &lsquo;<samp><span class="samp">unused</span></samp>&rsquo; attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
<br><dt><code>-Wunused-value</code><dd><a name="index-Wunused_002dvalue-340"></a><a name="index-Wno_002dunused_002dvalue-341"></a>Warn whenever a statement computes a result that is explicitly not
used. To suppress this warning cast the unused expression to
&lsquo;<samp><span class="samp">void</span></samp>&rsquo;. 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 &lsquo;<samp><span class="samp">x[i,j]</span></samp>&rsquo; causes a warning, while
&lsquo;<samp><span class="samp">x[(void)i,j]</span></samp>&rsquo; does not.
<p>This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wunused</code><dd><a name="index-Wunused-342"></a><a name="index-Wno_002dunused-343"></a>All the above <samp><span class="option">-Wunused</span></samp> options combined.
<p>In order to get a warning about an unused function parameter, you must
either specify <samp><span class="option">-Wextra -Wunused</span></samp> (note that <samp><span class="option">-Wall</span></samp> implies
<samp><span class="option">-Wunused</span></samp>), or separately specify <samp><span class="option">-Wunused-parameter</span></samp>.
<br><dt><code>-Wuninitialized</code><dd><a name="index-Wuninitialized-344"></a><a name="index-Wno_002duninitialized-345"></a>Warn if an automatic variable is used without first being initialized
or if a variable may be clobbered by a <code>setjmp</code> call. In C++,
warn if a non-static reference or non-static &lsquo;<samp><span class="samp">const</span></samp>&rsquo; member
appears in a class without constructors.
<p>If you want to warn about code that uses the uninitialized value of the
variable in its own initializer, use the <samp><span class="option">-Winit-self</span></samp> option.
<p>These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables that are uninitialized or clobbered as a whole. They do
not occur for variables or elements declared <code>volatile</code>. Because
these warnings depend on optimization, the exact variables or elements
for which there are warnings depends on the precise optimization
options and version of GCC used.
<p>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.
<br><dt><code>-Wmaybe-uninitialized</code><dd><a name="index-Wmaybe_002duninitialized-346"></a><a name="index-Wno_002dmaybe_002duninitialized-347"></a>For an automatic variable, if there exists a path from the function
entry to a use of the variable that is initialized, but there exist
some other paths for which the variable is not initialized, the compiler
emits a warning if it cannot prove the uninitialized paths are not
executed at run time. These warnings are made optional because GCC is
not smart enough to see all the reasons why the code might be correct
in spite of appearing to have an error. Here is one example of how
this can happen:
<pre class="smallexample"> {
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
</pre>
<p class="noindent">If the value of <code>y</code> is always 1, 2 or 3, then <code>x</code> is
always initialized, but GCC doesn't know this. To suppress the
warning, you need to provide a default case with assert(0) or
similar code.
<p><a name="index-g_t_0040code_007blongjmp_007d-warnings-348"></a>This option also warns when a non-volatile automatic variable might be
changed by a call to <code>longjmp</code>. These warnings as well are possible
only in optimizing compilation.
<p>The compiler sees only the calls to <code>setjmp</code>. It cannot know
where <code>longjmp</code> 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</code> cannot
in fact be called at the place that would cause a problem.
<p>Some spurious warnings can be avoided if you declare all the functions
you use that never return as <code>noreturn</code>. See <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>.
<p>This warning is enabled by <samp><span class="option">-Wall</span></samp> or <samp><span class="option">-Wextra</span></samp>.
<br><dt><code>-Wunknown-pragmas</code><dd><a name="index-Wunknown_002dpragmas-349"></a><a name="index-Wno_002dunknown_002dpragmas-350"></a><a name="index-warning-for-unknown-pragmas-351"></a><a name="index-unknown-pragmas_002c-warning-352"></a><a name="index-pragmas_002c-warning-of-unknown-353"></a>Warn when a <code>#pragma</code> directive is encountered that is not understood by
GCC. If this command-line option is used, warnings are even issued
for unknown pragmas in system header files. This is not the case if
the warnings are only enabled by the <samp><span class="option">-Wall</span></samp> command-line option.
<br><dt><code>-Wno-pragmas</code><dd><a name="index-Wno_002dpragmas-354"></a><a name="index-Wpragmas-355"></a>Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas. See also
<samp><span class="option">-Wunknown-pragmas</span></samp>.
<br><dt><code>-Wstrict-aliasing</code><dd><a name="index-Wstrict_002daliasing-356"></a><a name="index-Wno_002dstrict_002daliasing-357"></a>This option is only active when <samp><span class="option">-fstrict-aliasing</span></samp> is active.
It warns about code that 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 <samp><span class="option">-Wall</span></samp>.
It is equivalent to <samp><span class="option">-Wstrict-aliasing=3</span></samp>
<br><dt><code>-Wstrict-aliasing=n</code><dd><a name="index-Wstrict_002daliasing_003dn-358"></a>This option is only active when <samp><span class="option">-fstrict-aliasing</span></samp> is active.
It warns about code that 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 <samp><span class="option">-O</span></samp>
works.
<samp><span class="option">-Wstrict-aliasing</span></samp> is equivalent to <samp><span class="option">-Wstrict-aliasing=3</span></samp>.
<p>Level 1: Most aggressive, quick, least accurate.
Possibly useful when higher levels
do not warn but <samp><span class="option">-fstrict-aliasing</span></samp> 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 front end only.
<p>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 front end only.
<p>Level 3 (default for <samp><span class="option">-Wstrict-aliasing</span></samp>):
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 pun+dereference pattern in the front end:
<code>*(int*)&amp;some_float</code>.
If optimization is enabled, it also runs in the back end, 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.
<br><dt><code>-Wstrict-overflow</code><dt><code>-Wstrict-overflow=</code><var>n</var><dd><a name="index-Wstrict_002doverflow-359"></a><a name="index-Wno_002dstrict_002doverflow-360"></a>This option is only active when <samp><span class="option">-fstrict-overflow</span></samp> 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.
<p>An optimization that 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 that 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
requires, in particular when determining whether a loop will be
executed at all.
<dl>
<dt><code>-Wstrict-overflow=1</code><dd>Warn about cases that are both questionable and easy to avoid. For
example, with <samp><span class="option">-fstrict-overflow</span></samp>, the compiler simplifies
<code>x + 1 &gt; x</code> to <code>1</code>. This level of
<samp><span class="option">-Wstrict-overflow</span></samp> is enabled by <samp><span class="option">-Wall</span></samp>; higher levels
are not, and must be explicitly requested.
<br><dt><code>-Wstrict-overflow=2</code><dd>Also warn about other cases where a comparison is simplified to a
constant. For example: <code>abs (x) &gt;= 0</code>. This can only be
simplified when <samp><span class="option">-fstrict-overflow</span></samp> is in effect, because
<code>abs (INT_MIN)</code> overflows to <code>INT_MIN</code>, which is less than
zero. <samp><span class="option">-Wstrict-overflow</span></samp> (with no level) is the same as
<samp><span class="option">-Wstrict-overflow=2</span></samp>.
<br><dt><code>-Wstrict-overflow=3</code><dd>Also warn about other cases where a comparison is simplified. For
example: <code>x + 1 &gt; 1</code> is simplified to <code>x &gt; 0</code>.
<br><dt><code>-Wstrict-overflow=4</code><dd>Also warn about other simplifications not covered by the above cases.
For example: <code>(x * 10) / 5</code> is simplified to <code>x * 2</code>.
<br><dt><code>-Wstrict-overflow=5</code><dd>Also warn about cases where the compiler reduces the magnitude of a
constant involved in a comparison. For example: <code>x + 2 &gt; y</code> is
simplified to <code>x + 1 &gt;= y</code>. This is reported only at the
highest warning level because this simplification applies to many
comparisons, so this warning level gives a very large number of
false positives.
</dl>
<br><dt><code>-Wsuggest-attribute=</code><span class="roman">[</span><code>pure</code><span class="roman">|</span><code>const</code><span class="roman">|</span><code>noreturn</code><span class="roman">|</span><code>format</code><span class="roman">]</span><dd><a name="index-Wsuggest_002dattribute_003d-361"></a><a name="index-Wno_002dsuggest_002dattribute_003d-362"></a>Warn for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.
<dl>
<dt><code>-Wsuggest-attribute=pure</code><dt><code>-Wsuggest-attribute=const</code><dt><code>-Wsuggest-attribute=noreturn</code><dd><a name="index-Wsuggest_002dattribute_003dpure-363"></a><a name="index-Wno_002dsuggest_002dattribute_003dpure-364"></a><a name="index-Wsuggest_002dattribute_003dconst-365"></a><a name="index-Wno_002dsuggest_002dattribute_003dconst-366"></a><a name="index-Wsuggest_002dattribute_003dnoreturn-367"></a><a name="index-Wno_002dsuggest_002dattribute_003dnoreturn-368"></a>
Warn about functions that might be candidates for attributes
<code>pure</code>, <code>const</code> or <code>noreturn</code>. The compiler only warns for
functions visible in other compilation units or (in the case of <code>pure</code> and
<code>const</code>) if it cannot prove that the function returns normally. A function
returns normally if it doesn't contain an infinite loop or return abnormally
by throwing, calling <code>abort()</code> or trapping. This analysis requires option
<samp><span class="option">-fipa-pure-const</span></samp>, which is enabled by default at <samp><span class="option">-O</span></samp> and
higher. Higher optimization levels improve the accuracy of the analysis.
<br><dt><code>-Wsuggest-attribute=format</code><dt><code>-Wmissing-format-attribute</code><dd><a name="index-Wsuggest_002dattribute_003dformat-369"></a><a name="index-Wmissing_002dformat_002dattribute-370"></a><a name="index-Wno_002dsuggest_002dattribute_003dformat-371"></a><a name="index-Wno_002dmissing_002dformat_002dattribute-372"></a><a name="index-Wformat-373"></a><a name="index-Wno_002dformat-374"></a>
Warn about function pointers that might be candidates for <code>format</code>
attributes. Note these are only possible candidates, not absolute ones.
GCC guesses that function pointers with <code>format</code> attributes that
are used in assignment, initialization, parameter passing or return
statements should have a corresponding <code>format</code> 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 <code>format</code>
attribute to avoid the warning.
<p>GCC also warns about function definitions that might be
candidates for <code>format</code> attributes. Again, these are only
possible candidates. GCC guesses that <code>format</code> attributes
might be appropriate for any function that calls a function like
<code>vprintf</code> or <code>vscanf</code>, but this might not always be the
case, and some functions for which <code>format</code> attributes are
appropriate may not be detected.
</dl>
<br><dt><code>-Warray-bounds</code><dd><a name="index-Wno_002darray_002dbounds-375"></a><a name="index-Warray_002dbounds-376"></a>This option is only active when <samp><span class="option">-ftree-vrp</span></samp> is active
(default for <samp><span class="option">-O2</span></samp> and above). It warns about subscripts to arrays
that are always out of bounds. This warning is enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wno-div-by-zero</code><dd><a name="index-Wno_002ddiv_002dby_002dzero-377"></a><a name="index-Wdiv_002dby_002dzero-378"></a>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.
<br><dt><code>-Wsystem-headers</code><dd><a name="index-Wsystem_002dheaders-379"></a><a name="index-Wno_002dsystem_002dheaders-380"></a><a name="index-warnings-from-system-headers-381"></a><a name="index-system-headers_002c-warnings-from-382"></a>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 <samp><span class="option">-Wall</span></samp> in conjunction with this
option does <em>not</em> warn about unknown pragmas in system
headers&mdash;for that, <samp><span class="option">-Wunknown-pragmas</span></samp> must also be used.
<br><dt><code>-Wtrampolines</code><dd><a name="index-Wtrampolines-383"></a><a name="index-Wno_002dtrampolines-384"></a> Warn about trampolines generated for pointers to nested functions.
<p>A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and
is used to call the nested function indirectly. For some targets, it
is made up of data only and thus requires no special treatment. But,
for most targets, it is made up of code and thus requires the stack
to be made executable in order for the program to work properly.
<br><dt><code>-Wfloat-equal</code><dd><a name="index-Wfloat_002dequal-385"></a><a name="index-Wno_002dfloat_002dequal-386"></a>Warn if floating-point values are used in equality comparisons.
<p>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
should 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.
<br><dt><code>-Wtraditional </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wtraditional-387"></a><a name="index-Wno_002dtraditional-388"></a>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 that should be avoided.
<ul>
<li>Macro parameters that appear within string literals in the macro body.
In traditional C macro replacement takes place within string literals,
but in ISO C it does not.
<li>In traditional C, some preprocessor directives did not exist.
Traditional preprocessors only considered a line to be a directive
if the &lsquo;<samp><span class="samp">#</span></samp>&rsquo; appeared in column 1 on the line. Therefore
<samp><span class="option">-Wtraditional</span></samp> warns about directives that traditional C
understands but ignores because the &lsquo;<samp><span class="samp">#</span></samp>&rsquo; does not appear as the
first character on the line. It also suggests you hide directives like
&lsquo;<samp><span class="samp">#pragma</span></samp>&rsquo; not understood by traditional C by indenting them. Some
traditional implementations do not recognize &lsquo;<samp><span class="samp">#elif</span></samp>&rsquo;, so this option
suggests avoiding it altogether.
<li>A function-like macro that appears without arguments.
<li>The unary plus operator.
<li>The &lsquo;<samp><span class="samp">U</span></samp>&rsquo; integer constant suffix, or the &lsquo;<samp><span class="samp">F</span></samp>&rsquo; or &lsquo;<samp><span class="samp">L</span></samp>&rsquo; floating-point
constant suffixes. (Traditional C does support the &lsquo;<samp><span class="samp">L</span></samp>&rsquo; suffix on integer
constants.) Note, these suffixes appear in macros defined in the system
headers of most modern systems, e.g. the &lsquo;<samp><span class="samp">_MIN</span></samp>&rsquo;/&lsquo;<samp><span class="samp">_MAX</span></samp>&rsquo; macros in <code>&lt;limits.h&gt;</code>.
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.
<li>A function declared external in one block and then used after the end of
the block.
<li>A <code>switch</code> statement has an operand of type <code>long</code>.
<li>A non-<code>static</code> function declaration follows a <code>static</code> one.
This construct is not accepted by some traditional C compilers.
<li>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.
<li>Usage of ISO string concatenation is detected.
<li>Initialization of automatic aggregates.
<li>Identifier conflicts with labels. Traditional C lacks a separate
namespace for labels.
<li>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. <code>__STDC__</code> to avoid missing
initializer warnings and relies on default initialization to zero in the
traditional C case.
<li>Conversions by prototypes between fixed/floating-point values and vice
versa. The absence of these prototypes when compiling with traditional
C causes serious problems. This is a subset of the possible
conversion warnings; for the full set use <samp><span class="option">-Wtraditional-conversion</span></samp>.
<li>Use of ISO C style function definitions. This warning intentionally is
<em>not</em> issued for prototype declarations or variadic functions
because these ISO C features appear in your code when using
libiberty's traditional C compatibility macros, <code>PARAMS</code> and
<code>VPARAMS</code>. This warning is also bypassed for nested functions
because that feature is already a GCC extension and thus not relevant to
traditional C compatibility.
</ul>
<br><dt><code>-Wtraditional-conversion </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wtraditional_002dconversion-389"></a><a name="index-Wno_002dtraditional_002dconversion-390"></a>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.
<br><dt><code>-Wdeclaration-after-statement </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wdeclaration_002dafter_002dstatement-391"></a><a name="index-Wno_002ddeclaration_002dafter_002dstatement-392"></a>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. See <a href="Mixed-Declarations.html#Mixed-Declarations">Mixed Declarations</a>.
<br><dt><code>-Wundef</code><dd><a name="index-Wundef-393"></a><a name="index-Wno_002dundef-394"></a>Warn if an undefined identifier is evaluated in an &lsquo;<samp><span class="samp">#if</span></samp>&rsquo; directive.
<br><dt><code>-Wno-endif-labels</code><dd><a name="index-Wno_002dendif_002dlabels-395"></a><a name="index-Wendif_002dlabels-396"></a>Do not warn whenever an &lsquo;<samp><span class="samp">#else</span></samp>&rsquo; or an &lsquo;<samp><span class="samp">#endif</span></samp>&rsquo; are followed by text.
<br><dt><code>-Wshadow</code><dd><a name="index-Wshadow-397"></a><a name="index-Wno_002dshadow-398"></a>Warn whenever a local variable or type declaration shadows another variable,
parameter, type, or class member (in C++), or whenever a built-in function
is shadowed. Note that in C++, the compiler warns if a local variable
shadows an explicit typedef, but not if it shadows a struct/class/enum.
<br><dt><code>-Wlarger-than=</code><var>len</var><dd><a name="index-Wlarger_002dthan_003d_0040var_007blen_007d-399"></a><a name="index-Wlarger_002dthan_002d_0040var_007blen_007d-400"></a>Warn whenever an object of larger than <var>len</var> bytes is defined.
<br><dt><code>-Wframe-larger-than=</code><var>len</var><dd><a name="index-Wframe_002dlarger_002dthan-401"></a>Warn if the size of a function frame is larger than <var>len</var> bytes.
The computation done to determine the stack frame size is approximate
and not conservative.
The actual requirements may be somewhat greater than <var>len</var>
even if you do not get a warning. In addition, any space allocated
via <code>alloca</code>, variable-length arrays, or related constructs
is not included by the compiler when determining
whether or not to issue a warning.
<br><dt><code>-Wno-free-nonheap-object</code><dd><a name="index-Wno_002dfree_002dnonheap_002dobject-402"></a><a name="index-Wfree_002dnonheap_002dobject-403"></a>Do not warn when attempting to free an object that was not allocated
on the heap.
<br><dt><code>-Wstack-usage=</code><var>len</var><dd><a name="index-Wstack_002dusage-404"></a>Warn if the stack usage of a function might be larger than <var>len</var> bytes.
The computation done to determine the stack usage is conservative.
Any space allocated via <code>alloca</code>, variable-length arrays, or related
constructs is included by the compiler when determining whether or not to
issue a warning.
<p>The message is in keeping with the output of <samp><span class="option">-fstack-usage</span></samp>.
<ul>
<li>If the stack usage is fully static but exceeds the specified amount, it's:
<pre class="smallexample"> warning: stack usage is 1120 bytes
</pre>
<li>If the stack usage is (partly) dynamic but bounded, it's:
<pre class="smallexample"> warning: stack usage might be 1648 bytes
</pre>
<li>If the stack usage is (partly) dynamic and not bounded, it's:
<pre class="smallexample"> warning: stack usage might be unbounded
</pre>
</ul>
<br><dt><code>-Wunsafe-loop-optimizations</code><dd><a name="index-Wunsafe_002dloop_002doptimizations-405"></a><a name="index-Wno_002dunsafe_002dloop_002doptimizations-406"></a>Warn if the loop cannot be optimized because the compiler cannot
assume anything on the bounds of the loop indices. With
<samp><span class="option">-funsafe-loop-optimizations</span></samp> warn if the compiler makes
such assumptions.
<br><dt><code>-Wno-pedantic-ms-format </code><span class="roman">(MinGW targets only)</span><dd><a name="index-Wno_002dpedantic_002dms_002dformat-407"></a><a name="index-Wpedantic_002dms_002dformat-408"></a>When used in combination with <samp><span class="option">-Wformat</span></samp>
and <samp><span class="option">-pedantic</span></samp> without GNU extensions, this option
disables the warnings about non-ISO <code>printf</code> / <code>scanf</code> format
width specifiers <code>I32</code>, <code>I64</code>, and <code>I</code> used on Windows targets,
which depend on the MS runtime.
<br><dt><code>-Wpointer-arith</code><dd><a name="index-Wpointer_002darith-409"></a><a name="index-Wno_002dpointer_002darith-410"></a>Warn about anything that depends on the &ldquo;size of&rdquo; a function type or
of <code>void</code>. GNU C assigns these types a size of 1, for
convenience in calculations with <code>void *</code> pointers and pointers
to functions. In C++, warn also when an arithmetic operation involves
<code>NULL</code>. This warning is also enabled by <samp><span class="option">-Wpedantic</span></samp>.
<br><dt><code>-Wtype-limits</code><dd><a name="index-Wtype_002dlimits-411"></a><a name="index-Wno_002dtype_002dlimits-412"></a>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
&lsquo;<samp><span class="samp">&lt;</span></samp>&rsquo; or &lsquo;<samp><span class="samp">&gt;=</span></samp>&rsquo;. This warning is also enabled by
<samp><span class="option">-Wextra</span></samp>.
<br><dt><code>-Wbad-function-cast </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wbad_002dfunction_002dcast-413"></a><a name="index-Wno_002dbad_002dfunction_002dcast-414"></a>Warn whenever a function call is cast to a non-matching type.
For example, warn if <code>int malloc()</code> is cast to <code>anything *</code>.
<br><dt><code>-Wc++-compat </code><span class="roman">(C and Objective-C only)</span><dd>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
<code>void *</code> to a pointer to non-<code>void</code> type.
<br><dt><code>-Wc++11-compat </code><span class="roman">(C++ and Objective-C++ only)</span><dd>Warn about C++ constructs whose meaning differs between ISO C++ 1998
and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords
in ISO C++ 2011. This warning turns on <samp><span class="option">-Wnarrowing</span></samp> and is
enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wcast-qual</code><dd><a name="index-Wcast_002dqual-415"></a><a name="index-Wno_002dcast_002dqual-416"></a>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 *</code> is cast
to an ordinary <code>char *</code>.
<p>Also warn when making a cast that introduces a type qualifier in an
unsafe way. For example, casting <code>char **</code> to <code>const char **</code>
is unsafe, as in this example:
<pre class="smallexample"> /* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
</pre>
<br><dt><code>-Wcast-align</code><dd><a name="index-Wcast_002dalign-417"></a><a name="index-Wno_002dcast_002dalign-418"></a>Warn whenever a pointer is cast such that the required alignment of the
target is increased. For example, warn if a <code>char *</code> is cast to
an <code>int *</code> on machines where integers can only be accessed at
two- or four-byte boundaries.
<br><dt><code>-Wwrite-strings</code><dd><a name="index-Wwrite_002dstrings-419"></a><a name="index-Wno_002dwrite_002dstrings-420"></a>When compiling C, give string constants the type <code>const
char[</code><var>length</var><code>]</code> so that copying the address of one into a
non-<code>const</code> <code>char *</code> pointer produces a warning. These
warnings 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</code> in declarations and prototypes. Otherwise, it is
just a nuisance. This is why we did not make <samp><span class="option">-Wall</span></samp> request
these warnings.
<p>When compiling C++, warn about the deprecated conversion from string
literals to <code>char *</code>. This warning is enabled by default for C++
programs.
<br><dt><code>-Wclobbered</code><dd><a name="index-Wclobbered-421"></a><a name="index-Wno_002dclobbered-422"></a>Warn for variables that might be changed by &lsquo;<samp><span class="samp">longjmp</span></samp>&rsquo; or
&lsquo;<samp><span class="samp">vfork</span></samp>&rsquo;. This warning is also enabled by <samp><span class="option">-Wextra</span></samp>.
<br><dt><code>-Wconversion</code><dd><a name="index-Wconversion-423"></a><a name="index-Wno_002dconversion-424"></a>Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like <code>abs (x)</code> when
<code>x</code> is <code>double</code>; conversions between signed and unsigned,
like <code>unsigned ui = -1</code>; and conversions to smaller types, like
<code>sqrtf (M_PI)</code>. Do not warn for explicit casts like <code>abs
((int) x)</code> and <code>ui = (unsigned) -1</code>, or if the value is not
changed by the conversion like in <code>abs (2.0)</code>. Warnings about
conversions between signed and unsigned integers can be disabled by
using <samp><span class="option">-Wno-sign-conversion</span></samp>.
<p>For C++, also warn for confusing overload resolution for user-defined
conversions; and conversions that never use a type conversion
operator: conversions to <code>void</code>, 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
<samp><span class="option">-Wsign-conversion</span></samp> is explicitly enabled.
<br><dt><code>-Wno-conversion-null </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wconversion_002dnull-425"></a><a name="index-Wno_002dconversion_002dnull-426"></a>Do not warn for conversions between <code>NULL</code> and non-pointer
types. <samp><span class="option">-Wconversion-null</span></samp> is enabled by default.
<br><dt><code>-Wzero-as-null-pointer-constant </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wzero_002das_002dnull_002dpointer_002dconstant-427"></a><a name="index-Wno_002dzero_002das_002dnull_002dpointer_002dconstant-428"></a>Warn when a literal '0' is used as null pointer constant. This can
be useful to facilitate the conversion to <code>nullptr</code> in C++11.
<br><dt><code>-Wuseless-cast </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wuseless_002dcast-429"></a><a name="index-Wno_002duseless_002dcast-430"></a>Warn when an expression is casted to its own type.
<br><dt><code>-Wempty-body</code><dd><a name="index-Wempty_002dbody-431"></a><a name="index-Wno_002dempty_002dbody-432"></a>Warn if an empty body occurs in an &lsquo;<samp><span class="samp">if</span></samp>&rsquo;, &lsquo;<samp><span class="samp">else</span></samp>&rsquo; or &lsquo;<samp><span class="samp">do
while</span></samp>&rsquo; statement. This warning is also enabled by <samp><span class="option">-Wextra</span></samp>.
<br><dt><code>-Wenum-compare</code><dd><a name="index-Wenum_002dcompare-433"></a><a name="index-Wno_002denum_002dcompare-434"></a>Warn about a comparison between values of different enumerated types.
In C++ enumeral mismatches in conditional expressions are also
diagnosed and the warning is enabled by default. In C this warning is
enabled by <samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wjump-misses-init </code><span class="roman">(C, Objective-C only)</span><dd><a name="index-Wjump_002dmisses_002dinit-435"></a><a name="index-Wno_002djump_002dmisses_002dinit-436"></a>Warn if a <code>goto</code> statement or a <code>switch</code> statement jumps
forward across the initialization of a variable, or jumps backward to a
label after the variable has been initialized. This only warns about
variables that are initialized when they are declared. This warning is
only supported for C and Objective-C; in C++ this sort of branch is an
error in any case.
<p><samp><span class="option">-Wjump-misses-init</span></samp> is included in <samp><span class="option">-Wc++-compat</span></samp>. It
can be disabled with the <samp><span class="option">-Wno-jump-misses-init</span></samp> option.
<br><dt><code>-Wsign-compare</code><dd><a name="index-Wsign_002dcompare-437"></a><a name="index-Wno_002dsign_002dcompare-438"></a><a name="index-warning-for-comparison-of-signed-and-unsigned-values-439"></a><a name="index-comparison-of-signed-and-unsigned-values_002c-warning-440"></a><a name="index-signed-and-unsigned-values_002c-comparison-warning-441"></a>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 <samp><span class="option">-Wextra</span></samp>; to get the other warnings
of <samp><span class="option">-Wextra</span></samp> without this warning, use <samp><span class="option">-Wextra -Wno-sign-compare</span></samp>.
<br><dt><code>-Wsign-conversion</code><dd><a name="index-Wsign_002dconversion-442"></a><a name="index-Wno_002dsign_002dconversion-443"></a>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 <samp><span class="option">-Wconversion</span></samp>.
<br><dt><code>-Wsizeof-pointer-memaccess</code><dd><a name="index-Wsizeof_002dpointer_002dmemaccess-444"></a><a name="index-Wno_002dsizeof_002dpointer_002dmemaccess-445"></a>Warn for suspicious length parameters to certain string and memory built-in
functions if the argument uses <code>sizeof</code>. This warning warns e.g.
about <code>memset (ptr, 0, sizeof (ptr));</code> if <code>ptr</code> is not an array,
but a pointer, and suggests a possible fix, or about
<code>memcpy (&amp;foo, ptr, sizeof (&amp;foo));</code>. This warning is enabled by
<samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Waddress</code><dd><a name="index-Waddress-446"></a><a name="index-Wno_002daddress-447"></a>Warn about suspicious uses of memory addresses. These include using
the address of a function in a conditional expression, such as
<code>void func(void); if (func)</code>, and comparisons against the memory
address of a string literal, such as <code>if (x == "abc")</code>. 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 <code>strcmp</code>. This warning is enabled by
<samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wlogical-op</code><dd><a name="index-Wlogical_002dop-448"></a><a name="index-Wno_002dlogical_002dop-449"></a>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.
<br><dt><code>-Waggregate-return</code><dd><a name="index-Waggregate_002dreturn-450"></a><a name="index-Wno_002daggregate_002dreturn-451"></a>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.)
<br><dt><code>-Wno-aggressive-loop-optimizations</code><dd><a name="index-Wno_002daggressive_002dloop_002doptimizations-452"></a><a name="index-Waggressive_002dloop_002doptimizations-453"></a>Warn if in a loop with constant number of iterations the compiler detects
undefined behavior in some statement during one or more of the iterations.
<br><dt><code>-Wno-attributes</code><dd><a name="index-Wno_002dattributes-454"></a><a name="index-Wattributes-455"></a>Do not warn if an unexpected <code>__attribute__</code> is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This does not stop errors for incorrect use of supported
attributes.
<br><dt><code>-Wno-builtin-macro-redefined</code><dd><a name="index-Wno_002dbuiltin_002dmacro_002dredefined-456"></a><a name="index-Wbuiltin_002dmacro_002dredefined-457"></a>Do not warn if certain built-in macros are redefined. This suppresses
warnings for redefinition of <code>__TIMESTAMP__</code>, <code>__TIME__</code>,
<code>__DATE__</code>, <code>__FILE__</code>, and <code>__BASE_FILE__</code>.
<br><dt><code>-Wstrict-prototypes </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wstrict_002dprototypes-458"></a><a name="index-Wno_002dstrict_002dprototypes-459"></a>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 that specifies the argument
types.)
<br><dt><code>-Wold-style-declaration </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wold_002dstyle_002ddeclaration-460"></a><a name="index-Wno_002dold_002dstyle_002ddeclaration-461"></a>Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
<code>static</code> are not the first things in a declaration. This warning
is also enabled by <samp><span class="option">-Wextra</span></samp>.
<br><dt><code>-Wold-style-definition </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wold_002dstyle_002ddefinition-462"></a><a name="index-Wno_002dold_002dstyle_002ddefinition-463"></a>Warn if an old-style function definition is used. A warning is given
even if there is a previous prototype.
<br><dt><code>-Wmissing-parameter-type </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wmissing_002dparameter_002dtype-464"></a><a name="index-Wno_002dmissing_002dparameter_002dtype-465"></a>A function parameter is declared without a type specifier in K&amp;R-style
functions:
<pre class="smallexample"> void foo(bar) { }
</pre>
<p>This warning is also enabled by <samp><span class="option">-Wextra</span></samp>.
<br><dt><code>-Wmissing-prototypes </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wmissing_002dprototypes-466"></a><a name="index-Wno_002dmissing_002dprototypes-467"></a>Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. Use this option to detect global functions
that do not have a matching prototype declaration in a header file.
This option is not valid for C++ because all function declarations
provide prototypes and a non-matching declaration will declare an
overload rather than conflict with an earlier declaration.
Use <samp><span class="option">-Wmissing-declarations</span></samp> to detect missing declarations in C++.
<br><dt><code>-Wmissing-declarations</code><dd><a name="index-Wmissing_002ddeclarations-468"></a><a name="index-Wno_002dmissing_002ddeclarations-469"></a>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 functions with previous
non-prototype declarations; use <samp><span class="option">-Wmissing-prototype</span></samp> to detect
missing prototypes. In C++, no warnings are issued for function templates,
or for inline functions, or for functions in anonymous namespaces.
<br><dt><code>-Wmissing-field-initializers</code><dd><a name="index-Wmissing_002dfield_002dinitializers-470"></a><a name="index-Wno_002dmissing_002dfield_002dinitializers-471"></a><a name="index-W-472"></a><a name="index-Wextra-473"></a><a name="index-Wno_002dextra-474"></a>Warn if a structure's initializer has some fields missing. For
example, the following code causes such a warning, because
<code>x.h</code> is implicitly zero:
<pre class="smallexample"> struct s { int f, g, h; };
struct s x = { 3, 4 };
</pre>
<p>This option does not warn about designated initializers, so the following
modification does not trigger a warning:
<pre class="smallexample"> struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
</pre>
<p>This warning is included in <samp><span class="option">-Wextra</span></samp>. To get other <samp><span class="option">-Wextra</span></samp>
warnings without this one, use <samp><span class="option">-Wextra -Wno-missing-field-initializers</span></samp>.
<br><dt><code>-Wno-multichar</code><dd><a name="index-Wno_002dmultichar-475"></a><a name="index-Wmultichar-476"></a>Do not warn if a multicharacter constant (&lsquo;<samp><span class="samp">'FOOF'</span></samp>&rsquo;) 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.
<br><dt><code>-Wnormalized=&lt;none|id|nfc|nfkc&gt;</code><dd><a name="index-Wnormalized_003d-477"></a><a name="index-NFC-478"></a><a name="index-NFKC-479"></a><a name="index-character-set_002c-input-normalization-480"></a>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 <dfn>normalization rules</dfn> 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 that
have not been normalized; this option controls that warning.
<p>There are four levels of warning supported by GCC. The default is
<samp><span class="option">-Wnormalized=nfc</span></samp>, which warns about any identifier that is
not in the ISO 10646 &ldquo;C&rdquo; normalized form, <dfn>NFC</dfn>. NFC is the
recommended form for most uses.
<p>Unfortunately, there are some characters allowed in identifiers by
ISO C and ISO C++ that, when turned into NFC, are not allowed in
identifiers. That is, there's no way to use these symbols in portable
ISO C or C++ and have all your identifiers in NFC.
<samp><span class="option">-Wnormalized=id</span></samp> 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.
<p>You can switch the warning off for all characters by writing
<samp><span class="option">-Wnormalized=none</span></samp>. You should only do this if you
are using some other normalization scheme (like &ldquo;D&rdquo;), because
otherwise you can easily create bugs that are literally impossible to see.
<p>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 <code>\u207F</code>, &ldquo;SUPERSCRIPT LATIN SMALL
LETTER N&rdquo;, displays just like a regular <code>n</code> that has been
placed in a superscript. ISO 10646 defines the <dfn>NFKC</dfn>
normalization scheme to convert all these into a standard form as
well, and GCC warns if your code is not in NFKC if you use
<samp><span class="option">-Wnormalized=nfkc</span></samp>. 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
cannot be fixed to display these characters distinctly.
<br><dt><code>-Wno-deprecated</code><dd><a name="index-Wno_002ddeprecated-481"></a><a name="index-Wdeprecated-482"></a>Do not warn about usage of deprecated features. See <a href="Deprecated-Features.html#Deprecated-Features">Deprecated Features</a>.
<br><dt><code>-Wno-deprecated-declarations</code><dd><a name="index-Wno_002ddeprecated_002ddeclarations-483"></a><a name="index-Wdeprecated_002ddeclarations-484"></a>Do not warn about uses of functions (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>),
variables (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>), and types (see <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>) marked as deprecated by using the <code>deprecated</code>
attribute.
<br><dt><code>-Wno-overflow</code><dd><a name="index-Wno_002doverflow-485"></a><a name="index-Woverflow-486"></a>Do not warn about compile-time overflow in constant expressions.
<br><dt><code>-Woverride-init </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Woverride_002dinit-487"></a><a name="index-Wno_002doverride_002dinit-488"></a><a name="index-W-489"></a><a name="index-Wextra-490"></a><a name="index-Wno_002dextra-491"></a>Warn if an initialized field without side effects is overridden when
using designated initializers (see <a href="Designated-Inits.html#Designated-Inits">Designated Initializers</a>).
<p>This warning is included in <samp><span class="option">-Wextra</span></samp>. To get other
<samp><span class="option">-Wextra</span></samp> warnings without this one, use <samp><span class="option">-Wextra
-Wno-override-init</span></samp>.
<br><dt><code>-Wpacked</code><dd><a name="index-Wpacked-492"></a><a name="index-Wno_002dpacked-493"></a>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 <code>f.x</code> in <code>struct bar</code>
is misaligned even though <code>struct bar</code> does not itself
have the packed attribute:
<pre class="smallexample"> struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
</pre>
<br><dt><code>-Wpacked-bitfield-compat</code><dd><a name="index-Wpacked_002dbitfield_002dcompat-494"></a><a name="index-Wno_002dpacked_002dbitfield_002dcompat-495"></a>The 4.1, 4.2 and 4.3 series of GCC ignore the <code>packed</code> attribute
on bit-fields of type <code>char</code>. This has been fixed in GCC 4.4 but
the change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4.
For example there is no longer a 4-bit padding between field <code>a</code>
and <code>b</code> in this structure:
<pre class="smallexample"> struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
</pre>
<p>This warning is enabled by default. Use
<samp><span class="option">-Wno-packed-bitfield-compat</span></samp> to disable this warning.
<br><dt><code>-Wpadded</code><dd><a name="index-Wpadded-496"></a><a name="index-Wno_002dpadded-497"></a>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.
<br><dt><code>-Wredundant-decls</code><dd><a name="index-Wredundant_002ddecls-498"></a><a name="index-Wno_002dredundant_002ddecls-499"></a>Warn if anything is declared more than once in the same scope, even in
cases where multiple declaration is valid and changes nothing.
<br><dt><code>-Wnested-externs </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wnested_002dexterns-500"></a><a name="index-Wno_002dnested_002dexterns-501"></a>Warn if an <code>extern</code> declaration is encountered within a function.
<br><dt><code>-Wno-inherited-variadic-ctor</code><dd><a name="index-Winherited_002dvariadic_002dctor-502"></a><a name="index-Wno_002dinherited_002dvariadic_002dctor-503"></a>Suppress warnings about use of C++11 inheriting constructors when the
base class inherited from has a C variadic constructor; the warning is
on by default because the ellipsis is not inherited.
<br><dt><code>-Winline</code><dd><a name="index-Winline-504"></a><a name="index-Wno_002dinline-505"></a>Warn if a function that is declared as inline cannot be inlined.
Even with this option, the compiler does not warn about failures to
inline functions declared in system headers.
<p>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 <samp><span class="option">-Winline</span></samp> to appear or disappear.
<br><dt><code>-Wno-invalid-offsetof </code><span class="roman">(C++ and Objective-C++ only)</span><dd><a name="index-Wno_002dinvalid_002doffsetof-506"></a><a name="index-Winvalid_002doffsetof-507"></a>Suppress warnings from applying the &lsquo;<samp><span class="samp">offsetof</span></samp>&rsquo; macro to a non-POD
type. According to the 1998 ISO C++ standard, applying &lsquo;<samp><span class="samp">offsetof</span></samp>&rsquo;
to a non-POD type is undefined. In existing C++ implementations,
however, &lsquo;<samp><span class="samp">offsetof</span></samp>&rsquo; typically gives meaningful results even when
applied to certain kinds of non-POD types (such as a simple
&lsquo;<samp><span class="samp">struct</span></samp>&rsquo; 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.
<p>The restrictions on &lsquo;<samp><span class="samp">offsetof</span></samp>&rsquo; may be relaxed in a future version
of the C++ standard.
<br><dt><code>-Wno-int-to-pointer-cast</code><dd><a name="index-Wno_002dint_002dto_002dpointer_002dcast-508"></a><a name="index-Wint_002dto_002dpointer_002dcast-509"></a>Suppress warnings from casts to pointer type of an integer of a
different size. In C++, casting to a pointer type of smaller size is
an error. <samp><span class="option">Wint-to-pointer-cast</span></samp> is enabled by default.
<br><dt><code>-Wno-pointer-to-int-cast </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wno_002dpointer_002dto_002dint_002dcast-510"></a><a name="index-Wpointer_002dto_002dint_002dcast-511"></a>Suppress warnings from casts from a pointer to an integer type of a
different size.
<br><dt><code>-Winvalid-pch</code><dd><a name="index-Winvalid_002dpch-512"></a><a name="index-Wno_002dinvalid_002dpch-513"></a>Warn if a precompiled header (see <a href="Precompiled-Headers.html#Precompiled-Headers">Precompiled Headers</a>) is found in
the search path but can't be used.
<br><dt><code>-Wlong-long</code><dd><a name="index-Wlong_002dlong-514"></a><a name="index-Wno_002dlong_002dlong-515"></a>Warn if &lsquo;<samp><span class="samp">long long</span></samp>&rsquo; type is used. This is enabled by either
<samp><span class="option">-Wpedantic</span></samp> or <samp><span class="option">-Wtraditional</span></samp> in ISO C90 and C++98
modes. To inhibit the warning messages, use <samp><span class="option">-Wno-long-long</span></samp>.
<br><dt><code>-Wvariadic-macros</code><dd><a name="index-Wvariadic_002dmacros-516"></a><a name="index-Wno_002dvariadic_002dmacros-517"></a>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 <samp><span class="option">-Wno-variadic-macros</span></samp>.
<br><dt><code>-Wvarargs</code><dd><a name="index-Wvarargs-518"></a><a name="index-Wno_002dvarargs-519"></a>Warn upon questionable usage of the macros used to handle variable
arguments like &lsquo;<samp><span class="samp">va_start</span></samp>&rsquo;. This is default. To inhibit the
warning messages, use <samp><span class="option">-Wno-varargs</span></samp>.
<br><dt><code>-Wvector-operation-performance</code><dd><a name="index-Wvector_002doperation_002dperformance-520"></a><a name="index-Wno_002dvector_002doperation_002dperformance-521"></a>Warn if vector operation is not implemented via SIMD capabilities of the
architecture. Mainly useful for the performance tuning.
Vector operation can be implemented <code>piecewise</code>, which means that the
scalar operation is performed on every vector element;
<code>in parallel</code>, which means that the vector operation is implemented
using scalars of wider type, which normally is more performance efficient;
and <code>as a single scalar</code>, which means that vector fits into a
scalar type.
<br><dt><code>-Wno-virtual-move-assign</code><dd><a name="index-Wvirtual_002dmove_002dassign-522"></a><a name="index-Wno_002dvirtual_002dmove_002dassign-523"></a>Suppress warnings about inheriting from a virtual base with a
non-trivial C++11 move assignment operator. This is dangerous because
if the virtual base is reachable along more than one path, it will be
moved multiple times, which can mean both objects end up in the
moved-from state. If the move assignment operator is written to avoid
moving from a moved-from object, this warning can be disabled.
<br><dt><code>-Wvla</code><dd><a name="index-Wvla-524"></a><a name="index-Wno_002dvla-525"></a>Warn if variable length array is used in the code.
<samp><span class="option">-Wno-vla</span></samp> prevents the <samp><span class="option">-Wpedantic</span></samp> warning of
the variable length array.
<br><dt><code>-Wvolatile-register-var</code><dd><a name="index-Wvolatile_002dregister_002dvar-526"></a><a name="index-Wno_002dvolatile_002dregister_002dvar-527"></a>Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate reads
and/or writes to register variables. This warning is enabled by
<samp><span class="option">-Wall</span></samp>.
<br><dt><code>-Wdisabled-optimization</code><dd><a name="index-Wdisabled_002doptimization-528"></a><a name="index-Wno_002ddisabled_002doptimization-529"></a>Warn if a requested optimization pass is disabled. This warning does
not generally indicate that there is anything wrong with your code; it
merely indicates that GCC's optimizers are unable to handle the code
effectively. Often, the problem is that your code is too big or too
complex; GCC refuses to optimize programs when the optimization
itself is likely to take inordinate amounts of time.
<br><dt><code>-Wpointer-sign </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wpointer_002dsign-530"></a><a name="index-Wno_002dpointer_002dsign-531"></a>Warn for pointer argument passing or assignment with different signedness.
This option is only supported for C and Objective-C. It is implied by
<samp><span class="option">-Wall</span></samp> and by <samp><span class="option">-Wpedantic</span></samp>, which can be disabled with
<samp><span class="option">-Wno-pointer-sign</span></samp>.
<br><dt><code>-Wstack-protector</code><dd><a name="index-Wstack_002dprotector-532"></a><a name="index-Wno_002dstack_002dprotector-533"></a>This option is only active when <samp><span class="option">-fstack-protector</span></samp> is active. It
warns about functions that are not protected against stack smashing.
<br><dt><code>-Wno-mudflap</code><dd><a name="index-Wno_002dmudflap-534"></a>Suppress warnings about constructs that cannot be instrumented by
<samp><span class="option">-fmudflap</span></samp>.
<br><dt><code>-Woverlength-strings</code><dd><a name="index-Woverlength_002dstrings-535"></a><a name="index-Wno_002doverlength_002dstrings-536"></a>Warn about string constants that are longer than the &ldquo;minimum
maximum&rdquo; length specified in the C standard. Modern compilers
generally allow string constants that are much longer than the
standard's minimum limit, but very portable programs should avoid
using longer strings.
<p>The limit applies <em>after</em> string constant concatenation, and does
not count the trailing NUL. In C90, the limit was 509 characters; in
C99, it was raised to 4095. C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++.
<p>This option is implied by <samp><span class="option">-Wpedantic</span></samp>, and can be disabled with
<samp><span class="option">-Wno-overlength-strings</span></samp>.
<br><dt><code>-Wunsuffixed-float-constants </code><span class="roman">(C and Objective-C only)</span><dd><a name="index-Wunsuffixed_002dfloat_002dconstants-537"></a>
Issue a warning for any floating constant that does not have
a suffix. When used together with <samp><span class="option">-Wsystem-headers</span></samp> it
warns about such constants in system header files. This can be useful
when preparing code to use with the <code>FLOAT_CONST_DECIMAL64</code> pragma
from the decimal floating-point extension to C99.
</dl>
</body></html>