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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright © 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below). A copy of the license isincluded in the section entitled "GNU Free Documentation License". <p>(a) The FSF's Front-Cover Text is: <p>A GNU Manual <p>(b) The FSF's Back-Cover Text is: <p>You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.--><meta http-equiv="Content-Style-Type" content="text/css"><style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller }--></style></head><body><div class="node"><p>Node: <a name="Warning%20Options">Warning Options</a>,Next: <a rel="next" accesskey="n" href="Debugging-Options.html#Debugging%20Options">Debugging Options</a>,Previous: <a rel="previous" accesskey="p" href="Language-Independent-Options.html#Language%20Independent%20Options">Language Independent Options</a>,Up: <a rel="up" accesskey="u" href="Invoking-GCC.html#Invoking%20GCC">Invoking GCC</a><hr><br></div><h3 class="section">Options to Request or Suppress Warnings</h3><p>Warnings are diagnostic messages that report constructions whichare not inherently erroneous but which are risky or suggest theremay have been an error. <p>You can request many specific warnings with options beginning <code>-W</code>,for example <code>-Wimplicit</code> to request warnings on implicitdeclarations. Each of these specific warning options also has anegative form beginning <code>-Wno-</code> to turn off warnings;for example, <code>-Wno-implicit</code>. This manual lists only one of thetwo forms, whichever is not the default. <p>The following options control the amount and kinds of warnings producedby GCC; for further, language-specific options also refer to<a href="C---Dialect-Options.html#C++%20Dialect%20Options">C++ Dialect Options</a> and <a href="Objective-C-Dialect-Options.html#Objective-C%20Dialect%20Options">Objective-C Dialect Options</a>. <dl><dt><code>-fsyntax-only</code> <dd>Check the code for syntax errors, but don't do anything beyond that. <br><dt><code>-pedantic</code> <dd>Issue all the warnings demanded by strict ISO C and ISO C++;reject all programs that use forbidden extensions, and some otherprograms that do not follow ISO C and ISO C++. For ISO C, follows theversion of the ISO C standard specified by any <code>-std</code> option used. <p>Valid ISO C and ISO C++ programs should compile properly with or withoutthis option (though a rare few will require <code>-ansi</code> or a<code>-std</code> 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><code>-pedantic</code> does not cause warning messages for use of thealternate keywords whose names begin and end with <code>__</code>. Pedanticwarnings are also disabled in the expression that follows<code>__extension__</code>. However, only system header files should usethese escape routes; application programs should avoid them. See <a href="Alternate-Keywords.html#Alternate%20Keywords">Alternate Keywords</a>. <p>Some users try to use <code>-pedantic</code> to check programs for strict ISOC conformance. They soon find that it does not do quite what they want:it finds some non-ISO practices, but not all--only those for whichISO C <em>requires</em> a diagnostic, and some others for whichdiagnostics have been added. <p>A feature to report any failure to conform to ISO C might be useful insome instances, but would require considerable additional work and wouldbe quite different from <code>-pedantic</code>. We don't have plans tosupport such a feature in the near future. <p>Where the standard specified with <code>-std</code> represents a GNUextended dialect of C, such as <code>gnu89</code> or <code>gnu99</code>, there is acorresponding <dfn>base standard</dfn>, the version of ISO C on which the GNUextended dialect is based. Warnings from <code>-pedantic</code> are givenwhere they are required by the base standard. (It would not make sensefor such warnings to be given only for features not in the specified GNUC dialect, since by definition the GNU dialects of C include allfeatures the compiler supports with the given option, and there would benothing to warn about.) <br><dt><code>-pedantic-errors</code> <dd>Like <code>-pedantic</code>, except that errors are produced rather thanwarnings. <br><dt><code>-w</code> <dd>Inhibit all warning messages. <br><dt><code>-Wno-import</code> <dd>Inhibit warning messages about the use of <code>#import</code>. <br><dt><code>-Wchar-subscripts</code> <dd>Warn if an array subscript has type <code>char</code>. This is a common causeof error, as programmers often forget that this type is signed on somemachines. <br><dt><code>-Wcomment</code> <dd>Warn whenever a comment-start sequence <code>/*</code> appears in a <code>/*</code>comment, or whenever a Backslash-Newline appears in a <code>//</code> comment. <br><dt><code>-Wformat</code> <dd>Check calls to <code>printf</code> and <code>scanf</code>, etc., to make sure thatthe arguments supplied have types appropriate to the format stringspecified, and that the conversions specified in the format string makesense. This includes standard functions, and others specified by formatattributes (see <a href="Function-Attributes.html#Function%20Attributes">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. <p>The formats are checked against the format features supported by GNUlibc version 2.2. These include all ISO C90 and C99 features, as wellas features from the Single Unix Specification and some BSD and GNUextensions. Other library implementations may not support all thesefeatures; GCC does not support warning about features that go beyond aparticular library's limitations. However, if <code>-pedantic</code> is usedwith <code>-Wformat</code>, warnings will be given about format features notin 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%20Dialect%20Options">Options Controlling C Dialect</a>. <p>Since <code>-Wformat</code> also checks for null format arguments forseveral functions, <code>-Wformat</code> also implies <code>-Wnonnull</code>. <p><code>-Wformat</code> is included in <code>-Wall</code>. For more control over someaspects of format checking, the options <code>-Wformat-y2k</code>,<code>-Wno-format-extra-args</code>, <code>-Wno-format-zero-length</code>,<code>-Wformat-nonliteral</code>, <code>-Wformat-security</code>, and<code>-Wformat=2</code> are available, but are not included in <code>-Wall</code>. <br><dt><code>-Wformat-y2k</code> <dd>If <code>-Wformat</code> is specified, also warn about <code>strftime</code>formats which may yield only a two-digit year. <br><dt><code>-Wno-format-extra-args</code> <dd>If <code>-Wformat</code> is specified, do not warn about excess arguments to a<code>printf</code> or <code>scanf</code> format function. The C standard specifiesthat such arguments are ignored. <p>Where the unused arguments lie between used arguments that arespecified with <code>$</code> operand number specifications, normallywarnings are still given, since the implementation could not know whattype to pass to <code>va_arg</code> to skip the unused arguments. However,in the case of <code>scanf</code> formats, this option will suppress thewarning if the unused arguments are all pointers, since the SingleUnix Specification says that such unused arguments are allowed. <br><dt><code>-Wno-format-zero-length</code> <dd>If <code>-Wformat</code> is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed. <br><dt><code>-Wformat-nonliteral</code> <dd>If <code>-Wformat</code> is specified, also warn if the format string is not astring literal and so cannot be checked, unless the format functiontakes its format arguments as a <code>va_list</code>. <br><dt><code>-Wformat-security</code> <dd>If <code>-Wformat</code> is specified, also warn about uses of formatfunctions that represent possible security problems. At present, thiswarns about calls to <code>printf</code> and <code>scanf</code> functions where theformat 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 formatstring came from untrusted input and contains <code>%n</code>. (This iscurrently a subset of what <code>-Wformat-nonliteral</code> warns about, butin future warnings may be added to <code>-Wformat-security</code> that are notincluded in <code>-Wformat-nonliteral</code>.) <br><dt><code>-Wformat=2</code> <dd>Enable <code>-Wformat</code> plus format checks not included in<code>-Wformat</code>. Currently equivalent to <code>-Wformat-Wformat-nonliteral -Wformat-security -Wformat-y2k</code>. <br><dt><code>-Wnonnull</code> <dd>Warn about passing a null pointer for arguments marked asrequiring a non-null value by the <code>nonnull</code> function attribute. <p><code>-Wnonnull</code> is included in <code>-Wall</code> and <code>-Wformat</code>. Itcan be disabled with the <code>-Wno-nonnull</code> option. <br><dt><code>-Winit-self </code>(C, C++, and Objective-C only)<code></code> <dd>Warn about uninitialized variables which are initialized with themselves. Note this option can only be used with the <code>-Wuninitialized</code> option,which in turn only works with <code>-O1</code> and above. <p>For example, GCC will warn about <code>i</code> being uninitialized in thefollowing snippet only when <code>-Winit-self</code> has been specified: <pre class="smallexample"> int f() { int i = i; return i; } </pre> <br><dt><code>-Wimplicit-int</code> <dd>Warn when a declaration does not specify a type. <br><dt><code>-Wimplicit-function-declaration</code> <dd><dt><code>-Werror-implicit-function-declaration</code> <dd>Give a warning (or error) whenever a function is used before beingdeclared. <br><dt><code>-Wimplicit</code> <dd>Same as <code>-Wimplicit-int</code> and <code>-Wimplicit-function-declaration</code>. <br><dt><code>-Wmain</code> <dd>Warn if the type of <code>main</code> is suspicious. <code>main</code> should be afunction with external linkage, returning int, taking either zeroarguments, two, or three arguments of appropriate types. <br><dt><code>-Wmissing-braces</code> <dd>Warn if an aggregate or union initializer is not fully bracketed. Inthe following example, the initializer for <code>a</code> is not fullybracketed, but that for <code>b</code> is fully bracketed. <pre class="smallexample"> int a[2][2] = { 0, 1, 2, 3 }; int b[2][2] = { { 0, 1 }, { 2, 3 } }; </pre> <br><dt><code>-Wparentheses</code> <dd>Warn if parentheses are omitted in certain contexts, suchas when there is an assignment in a context where a truth valueis expected, or when operators are nested whose precedence peopleoften get confused about. <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 ofsuch a case: <pre class="smallexample"> { if (a) if (b) foo (); else bar (); } </pre> <p>In 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 notwhat the programmer expected, as illustrated in the above example byindentation the programmer chose. When there is the potential for thisconfusion, GCC will issue 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> could belong tothe enclosing <code>if</code>. The resulting code would look like this: <pre class="smallexample"> { if (a) { if (b) foo (); else bar (); } } </pre> <br><dt><code>-Wsequence-point</code> <dd>Warn about code that may have undefined semantics because of violationsof sequence point rules in the C standard. <p>The C standard defines the order in which expressions in a C program areevaluated in terms of <dfn>sequence points</dfn>, which represent a partialordering between the execution of parts of the program: those executedbefore the sequence point, and those executed after it. These occurafter the evaluation of a full expression (one which is not part of alarger expression), after the evaluation of the first operand of a<code>&&</code>, <code>||</code>, <code>? :</code> or <code>,</code> (comma) operator, before afunction is called (but after the evaluation of its arguments and theexpression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order ofevaluation of subexpressions of an expression is not specified. Allthese rules describe only a partial order rather than a total order,since, for example, if two functions are called within one expressionwith no sequence point between them, the order in which the functionsare called is not specified. However, the standards committee haveruled that function calls do not overlap. <p>It is not specified when between sequence points modifications to thevalues of objects take effect. Programs whose behavior depends on thishave undefined behavior; the C standard specifies that "Between theprevious and next sequence point an object shall have its stored valuemodified at most once by the evaluation of an expression. Furthermore,⌨️ 快捷键说明
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