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  18 <h1>Clang Compiler User's Manual</h1>
  19
  20 <ul>
  21 <li><a href="#intro">Introduction</a>
  22   <ul>
  23   <li><a href="#terminology">Terminology</a></li>
  24   <li><a href="#basicusage">Basic Usage</a></li>
  25   </ul>
  26 </li>
  27 <li><a href="#commandline">Command Line Options</a>
  28   <ul>
  29   <li><a href="#cl_diagnostics">Options to Control Error and Warning
  30       Messages</a></li>
  31   </ul>
  32 </li>
  33 <li><a href="#general_features">Language and Target-Independent Features</a>
  34  <ul>
  35   <li><a href="#diagnostics">Controlling Errors and Warnings</a></li>
  36    <ul>
  37    <li><a href="#diagnostics_display">Controlling How Clang Displays Diagnostics</a></li>
  38    <li><a href="#diagnostics_mappings">Diagnostic Mappings</a></li>
  39    <li><a href="#diagnostics_categories">Diagnostic Categories</a></li>
  40    <li><a href="#diagnostics_commandline">Controlling Diagnostics via Command Line Flags</a></li>
  41    <li><a href="#diagnostics_pragmas">Controlling Diagnostics via Pragmas</a></li>
  42    <li><a href="#analyzer_diagnositics">Controlling Static Analyzer Diagnostics</a></li>
  43    </ul>
  44   <li><a href="#precompiledheaders">Precompiled Headers</a></li>
  45   <li><a href="#codegen">Controlling Code Generation</a></li>
  46   </ul>
  47 </li>
  48 <li><a href="#c">C Language Features</a>
  49   <ul>
  50   <li><a href="#c_ext">Extensions supported by clang</a></li>
  51   <li><a href="#c_modes">Differences between various standard modes</a></li>
  52   <li><a href="#c_unimpl_gcc">GCC extensions not implemented yet</a></li>
  53   <li><a href="#c_unsupp_gcc">Intentionally unsupported GCC extensions</a></li>
  54   <li><a href="#c_ms">Microsoft extensions</a></li>
  55   </ul>
  56 </li>
  57 <li><a href="#target_features">Target-Specific Features and Limitations</a>
  58   <ul>
  59   <li><a href="#target_arch">CPU Architectures Features and Limitations</a>
  60     <ul>
  61     <li><a href="#target_arch_x86">X86</a></li>
  62     <li><a href="#target_arch_arm">ARM</a></li>
  63     <li><a href="#target_arch_other">Other platforms</a></li>
  64     </ul>
  65   </li>
  66   <li><a href="#target_os">Operating System Features and Limitations</a>
  67     <ul>
  68     <li><a href="#target_os_darwin">Darwin (Mac OS/X)</a></li>
  69     <li>Linux, etc.</li>
  70     </ul>
  71
  72   </li>
  73   </ul>
  74 </li>
  75 </ul>
  76
  77
  78 <!-- ======================================================================= -->
  79 <h2 id="intro">Introduction</h2>
  80 <!-- ======================================================================= -->
  81
  82 <p>The Clang Compiler is an open-source compiler for the C family of programming
  83 languages, aiming to be the best in class implementation of these languages.
  84 Clang builds on the LLVM optimizer and code generator, allowing it to provide
  85 high-quality optimization and code generation support for many targets.  For
  86 more general information, please see the <a href="http://clang.llvm.org">Clang
  87 Web Site</a> or the <a href="http://llvm.org">LLVM Web Site</a>.</p>
  88
  89 <p>This document describes important notes about using Clang as a compiler for
  90 an end-user, documenting the supported features, command line options, etc.  If
  91 you are interested in using Clang to build a tool that processes code, please
  92 see <a href="InternalsManual.html">the Clang Internals Manual</a>.  If you are
  93 interested in the <a href="http://clang.llvm.org/StaticAnalysis.html">Clang
  94 Static Analyzer</a>, please see its web page.</p>
  95
  96 <p>Clang is designed to support the C family of programming languages, which
  97 includes <a href="#c">C</a>, <a href="#objc">Objective-C</a>, <a
  98 href="#cxx">C++</a>, and <a href="#objcxx">Objective-C++</a> as well as many
  99 dialects of those.  For language-specific information, please see the
 100 corresponding language specific section:</p>
 101
 102 <ul>
 103 <li><a href="#c">C Language</a>: K&amp;R C, ANSI C89, ISO C90, ISO C94
 104     (C89+AMD1), ISO C99 (+TC1, TC2, TC3). </li>
 105 <li><a href="#objc">Objective-C Language</a>: ObjC 1, ObjC 2, ObjC 2.1, plus
 106     variants depending on base language.</li>
 107 <li><a href="#cxx">C++ Language Features</a></li>
 108 <li><a href="#objcxx">Objective C++ Language</a></li>
 109 </ul>
 110
 111 <p>In addition to these base languages and their dialects, Clang supports a
 112 broad variety of language extensions, which are documented in the corresponding
 113 language section.  These extensions are provided to be compatible with the GCC,
 114 Microsoft, and other popular compilers as well as to improve functionality
 115 through Clang-specific features.  The Clang driver and language features are
 116 intentionally designed to be as compatible with the GNU GCC compiler as
 117 reasonably possible, easing migration from GCC to Clang.  In most cases, code
 118 "just works".</p>
 119
 120 <p>In addition to language specific features, Clang has a variety of features
 121 that depend on what CPU architecture or operating system is being compiled for.
 122 Please see the <a href="#target_features">Target-Specific Features and
 123 Limitations</a> section for more details.</p>
 124
 125 <p>The rest of the introduction introduces some basic <a
 126 href="#terminology">compiler terminology</a> that is used throughout this manual
 127 and contains a basic <a href="#basicusage">introduction to using Clang</a>
 128 as a command line compiler.</p>
 129
 130 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 131 <h3 id="terminology">Terminology</h3>
 132 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 133
 134 <p>Front end, parser, backend, preprocessor, undefined behavior, diagnostic,
 135  optimizer</p>
 136
 137 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 138 <h3 id="basicusage">Basic Usage</h3>
 139 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 140
 141 <p>Intro to how to use a C compiler for newbies.</p>
 142 <p>
 143 compile + link
 144
 145 compile then link
 146
 147 debug info
 148
 149 enabling optimizations
 150
 151 picking a language to use, defaults to C99 by default.  Autosenses based on
 152 extension.
 153
 154 using a makefile
 155 </p>
 156
 157
 158 <!-- ======================================================================= -->
 159 <h2 id="commandline">Command Line Options</h2>
 160 <!-- ======================================================================= -->
 161
 162 <p>
 163 This section is generally an index into other sections.  It does not go into
 164 depth on the ones that are covered by other sections.  However, the first part
 165 introduces the language selection and other high level options like -c, -g, etc.
 166 </p>
 167
 168
 169 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 170 <h3 id="cl_diagnostics">Options to Control Error and Warning Messages</h3>
 171 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 172
 173 <p><b>-Werror</b>: Turn warnings into errors.</p>
 174 <p><b>-Werror=foo</b>: Turn warning "foo" into an error.</p>
 175 <p><b>-Wno-error=foo</b>: Turn warning "foo" into an warning even if -Werror is
 176    specified.</p>
 177 <p><b>-Wfoo</b>: Enable warning foo</p>
 178 <p><b>-Wno-foo</b>: Disable warning foo</p>
 179 <p><b>-w</b>: Disable all warnings.</p>
 180 <p><b>-pedantic</b>: Warn on language extensions.</p>
 181 <p><b>-pedantic-errors</b>: Error on language extensions.</p>
 182 <p><b>-Wsystem-headers</b>: Enable warnings from system headers.</p>
 183
 184 <p><b>-ferror-limit=123</b>: Stop emitting diagnostics after 123 errors have
 185    been produced.  The default is 20, and the error limit can be disabled with
 186    -ferror-limit=0.</p>
 187
 188 <p><b>-ftemplate-backtrace-limit=123</b>: Only emit up to 123 template instantiation notes within the template instantiation backtrace for a single warning or error. The default is 10, and the limit can be disabled with -ftemplate-backtrace-limit=0.</p>
 189
 190 <!-- ================================================= -->
 191 <h4 id="cl_diag_formatting">Formatting of Diagnostics</h4>
 192 <!-- ================================================= -->
 193
 194 <p>Clang aims to produce beautiful diagnostics by default, particularly for new
 195 users that first come to Clang.  However, different people have different
 196 preferences, and sometimes Clang is driven by another program that wants to
 197 parse simple and consistent output, not a person. For these cases, Clang
 198 provides a wide range of options to control the exact output format of the
 199 diagnostics that it generates.</p>
 200
 201 <dl>
 202
 203 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 204 <dt id="opt_fshow-column"><b>-f[no-]show-column</b>: Print column number in
 205 diagnostic.</dt>
 206 <dd>This option, which defaults to on, controls whether or not Clang prints the
 207 column number of a diagnostic.  For example, when this is enabled, Clang will
 208 print something like:</p>
 209
 210 <pre>
 211   test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
 212   #endif bad
 213          ^
 214          //
 215 </pre>
 216
 217 <p>When this is disabled, Clang will print "test.c:28: warning..." with no
 218 column number.</p>
 219 </dd>
 220
 221 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 222 <dt id="opt_fshow-source-location"><b>-f[no-]show-source-location</b>: Print
 223 source file/line/column information in diagnostic.</dt>
 224 <dd>This option, which defaults to on, controls whether or not Clang prints the
 225 filename, line number and column number of a diagnostic.  For example,
 226 when this is enabled, Clang will print something like:</p>
 227
 228 <pre>
 229   test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
 230   #endif bad
 231          ^
 232          //
 233 </pre>
 234
 235 <p>When this is disabled, Clang will not print the "test.c:28:8: " part.</p>
 236 </dd>
 237
 238 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 239 <dt id="opt_fcaret-diagnostics"><b>-f[no-]caret-diagnostics</b>: Print source
 240 line and ranges from source code in diagnostic.</dt>
 241 <dd>This option, which defaults to on, controls whether or not Clang prints the
 242 source line, source ranges, and caret when emitting a diagnostic.  For example,
 243 when this is enabled, Clang will print something like:</p>
 244
 245 <pre>
 246   test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
 247   #endif bad
 248          ^
 249          //
 250 </pre>
 251
 252 <p>When this is disabled, Clang will just print:</p>
 253
 254 <pre>
 255   <b><font color="black">test.c:28:8: <font color="magenta">warning</font>: extra tokens at end of #endif directive [-Wextra-tokens]</font></b>
 256   #endif bad
 257          <font color="green">^</font>
 258          <font color="green">//</font>
 259 </pre>
 260
 261 </dd>
 262
 263 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 264 <dt id="opt_fcolor_diagnostics"><b>-f[no-]color-diagnostics</b>: </dt>
 265 <dd>This option, which defaults to on when a color-capable terminal is
 266   detected, controls whether or not Clang prints diagnostics in color.
 267   When this option is enabled, Clang will use colors to highlight
 268   specific parts of the diagnostic, e.g.,
 269
 270 <pre>
 271 <test.c:2:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
 272 #endif bad
 273        ^
 274        //
 275 </pre>
 276 </dd>'
 277 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 278 <dt id="opt_fdiagnostics-show-option"><b>-f[no-]diagnostics-show-option</b>:
 279 Enable <tt>[-Woption]</tt> information in diagnostic line.</dt>
 280 <dd>This option, which defaults to on,
 281 controls whether or not Clang prints the associated <A
 282 href="#cl_diag_warning_groups">warning group</a> option name when outputting
 283 a warning diagnostic.  For example, in this output:</p>
 284
 285 <pre>
 286   test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
 287   #endif bad
 288          ^
 289          //
 290 </pre>
 291
 292 <p>Passing <b>-fno-diagnostics-show-option</b> will prevent Clang from printing
 293 the [<a href="#opt_Wextra-tokens">-Wextra-tokens</a>] information in the
 294 diagnostic.  This information tells you the flag needed to enable or disable the
 295 diagnostic, either from the command line or through <a
 296 href="#pragma_GCC_diagnostic">#pragma GCC diagnostic</a>.</dd>
 297
 298 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 299 <dt id="opt_fdiagnostics-show-category"><b>-fdiagnostics-show-category=none/id/name</b>:
 300 Enable printing category information in diagnostic line.</dt>
 301 <dd>This option, which defaults to "none",
 302 controls whether or not Clang prints the category associated with a diagnostic
 303 when emitting it.  Each diagnostic may or many not have an associated category,
 304 if it has one, it is listed in the diagnostic categorization field of the
 305 diagnostic line (in the []'s).</p>
 306
 307 <p>For example, a format string warning will produce these three renditions
 308 based on the setting of this option:</p>
 309
 310 <pre>
 311   t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat]
 312   t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat<b>,1</b>]
 313   t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat<b>,Format String</b>]
 314 </pre>
 315
 316 <p>This category can be used by clients that want to group diagnostics by
 317 category, so it should be a high level category.  We want dozens of these, not
 318 hundreds or thousands of them.</p>
 319 </dd>
 320
 321
 322
 323 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 324 <dt id="opt_fdiagnostics-fixit-info"><b>-f[no-]diagnostics-fixit-info</b>:
 325 Enable "FixIt" information in the diagnostics output.</dt>
 326 <dd>This option, which defaults to on, controls whether or not Clang prints the
 327 information on how to fix a specific diagnostic underneath it when it knows.
 328 For example, in this output:</p>
 329
 330 <pre>
 331   test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
 332   #endif bad
 333          ^
 334          //
 335 </pre>
 336
 337 <p>Passing <b>-fno-diagnostics-fixit-info</b> will prevent Clang from printing
 338 the "//" line at the end of the message.  This information is useful for users
 339 who may not understand what is wrong, but can be confusing for machine
 340 parsing.</p>
 341 </dd>
 342
 343 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 344 <dt id="opt_fdiagnostics-print-source-range-info">
 345 <b>-f[no-]diagnostics-print-source-range-info</b>:
 346 Print machine parsable information about source ranges.</dt>
 347 <dd>This option, which defaults to off, controls whether or not Clang prints
 348 information about source ranges in a machine parsable format after the
 349 file/line/column number information.  The information is a simple sequence of
 350 brace enclosed ranges, where each range lists the start and end line/column
 351 locations.  For example, in this output:</p>
 352
 353 <pre>
 354 exprs.c:47:15:{47:8-47:14}{47:17-47:24}: error: invalid operands to binary expression ('int *' and '_Complex float')
 355    P = (P-42) + Gamma*4;
 356        ~~~~~~ ^ ~~~~~~~
 357 </pre>
 358
 359 <p>The {}'s are generated by -fdiagnostics-print-source-range-info.</p>
 360 </dd>
 361
 362 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 363 <dt id="opt_fdiagnostics-parseable-fixits">
 364 <b>-fdiagnostics-parseable-fixits</b>:
 365 Print Fix-Its in a machine parseable form.</dt>
 366 <dd><p>This option makes Clang print available Fix-Its in a machine parseable format at the end of diagnostics. The following example illustrates the format:</p>
 367
 368 <pre>
 369  fix-it:"t.cpp":{7:25-7:29}:"Gamma"
 370 </pre>
 371
 372 <p>The range printed is a half-open range, so in this example the characters at column 25 up to but not including column 29 on line 7 in t.cpp should be replaced with the string "Gamma". Either the range or the replacement string may be empty (representing strict insertions and strict erasures, respectively). Both the file name and the insertion string escape backslash (as "\\"), tabs (as "\t"), newlines (as "\n"), double quotes(as "\"") and non-printable characters (as octal "\xxx").</p>
 373 </dd>
 374
 375 </dl>
 376
 377
 378
 379
 380 <!-- ===================================================== -->
 381 <h4 id="cl_diag_warning_groups">Individual Warning Groups</h4>
 382 <!-- ===================================================== -->
 383
 384 <p>TODO: Generate this from tblgen.  Define one anchor per warning group.</p>
 385
 386
 387 <dl>
 388
 389
 390 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 391 <dt id="opt_Wextra-tokens"><b>-Wextra-tokens</b>: Warn about excess tokens at
 392     the end of a preprocessor directive.</dt>
 393 <dd>This option, which defaults to on, enables warnings about extra tokens at
 394 the end of preprocessor directives.  For example:</p>
 395
 396 <pre>
 397   test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
 398   #endif bad
 399          ^
 400 </pre>
 401
 402 <p>These extra tokens are not strictly conforming, and are usually best handled
 403 by commenting them out.</p>
 404
 405 <p>This option is also enabled by <a href="">-Wfoo</a>, <a href="">-Wbar</a>,
 406  and <a href="">-Wbaz</a>.</p>
 407 </dd>
 408
 409 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 410 <dt id="opt_Wambiguous-member-template"><b>-Wambiguous-member-template</b>:
 411 Warn about unqualified uses of a member template whose name resolves
 412 to another template at the location of the use.</dt>
 413 <dd>This option, which defaults to on, enables a warning in the
 414 following code:</p>
 415
 416 <pre>
 417 template&lt;typename T> struct set{};
 418 template&lt;typename T> struct trait { typedef const T& type; };
 419 struct Value {
 420   template&lt;typename T> void set(typename trait&lt;T>::type value) {}
 421 };
 422 void foo() {
 423   Value v;
 424   v.set&lt;double>(3.2);
 425 }
 426 </pre>
 427
 428 <p>C++ [basic.lookup.classref] requires this to be an error, but,
 429 because it's hard to work around, Clang downgrades it to a warning as
 430 an extension.</p>
 431 </dd>
 432
 433 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 434 <dt id="opt_Wbind-to-temporary-copy"><b>-Wbind-to-temporary-copy</b>: Warn about
 435 an unusable copy constructor when binding a reference to a temporary.</dt>
 436 <dd>This option, which defaults to on, enables warnings about binding a
 437 reference to a temporary when the temporary doesn't have a usable copy
 438 constructor.  For example:</p>
 439
 440 <pre>
 441   struct NonCopyable {
 442     NonCopyable();
 443   private:
 444     NonCopyable(const NonCopyable&);
 445   };
 446   void foo(const NonCopyable&);
 447   void bar() {
 448     foo(NonCopyable());  // Disallowed in C++98; allowed in C++0x.
 449   }
 450 </pre>
 451 <pre>
 452   struct NonCopyable2 {
 453     NonCopyable2();
 454     NonCopyable2(NonCopyable2&);
 455   };
 456   void foo(const NonCopyable2&);
 457   void bar() {
 458     foo(NonCopyable2());  // Disallowed in C++98; allowed in C++0x.
 459   }
 460 </pre>
 461
 462 <p>Note that if <tt>NonCopyable2::NonCopyable2()</tt> has a default
 463 argument whose instantiation produces a compile error, that error will
 464 still be a hard error in C++98 mode even if this warning is turned
 465 off.</p>
 466
 467 </dd>
 468
 469 </dl>
 470
 471 <!-- ======================================================================= -->
 472 <h2 id="general_features">Language and Target-Independent Features</h2>
 473 <!-- ======================================================================= -->
 474
 475
 476 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 477 <h3 id="diagnostics">Controlling Errors and Warnings</h3>
 478 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 479
 480 <p>Clang provides a number of ways to control which code constructs cause it to
 481 emit errors and warning messages, and how they are displayed to the console.</p>
 482
 483 <h4 id="diagnostics_display">Controlling How Clang Displays Diagnostics</h4>
 484
 485 <p>When Clang emits a diagnostic, it includes rich information in the output,
 486 and gives you fine-grain control over which information is printed.  Clang has
 487 the ability to print this information, and these are the options that control
 488 it:</p>
 489
 490 <p>
 491 <ol>
 492 <li>A file/line/column indicator that shows exactly where the diagnostic occurs
 493     in your code [<a href="#opt_fshow-column">-fshow-column</a>, <a
 494     href="#opt_fshow-source-location">-fshow-source-location</a>].</li>
 495 <li>A categorization of the diagnostic as a note, warning, error, or fatal
 496     error.</li>
 497 <li>A text string that describes what the problem is.</li>
 498 <li>An option that indicates how to control the diagnostic (for diagnostics that
 499     support it) [<a
 500    href="#opt_fdiagnostics-show-option">-fdiagnostics-show-option</a>].</li>
 501 <li>A <a href="#diagnostics_categories">high-level category</a> for the
 502     diagnostic for clients that want to group diagnostics by class (for
 503     diagnostics that support it) [<a
 504    href="#opt_fdiagnostics-show-category">-fdiagnostics-show-category</a>].</li>
 505 <li>The line of source code that the issue occurs on, along with a caret and
 506     ranges that indicate the important locations [<a
 507     href="opt_fcaret-diagnostics">-fcaret-diagnostics</a>].</li>
 508 <li>"FixIt" information, which is a concise explanation of how to fix the
 509     problem (when Clang is certain it knows) [<a
 510     href="opt_fdiagnostics-fixit-info">-fdiagnostics-fixit-info</a>].</li>
 511 <li>A machine-parsable representation of the ranges involved (off by
 512     default) [<a
 513       href="opt_fdiagnostics-print-source-range-info">-fdiagnostics-print-source-range-info</a>].</li>
 514 </ol></p>
 515
 516 <p>For more information please see <a href="#cl_diag_formatting">Formatting of
 517 Diagnostics</a>.</p>
 518
 519
 520 <h4 id="diagnostics_mappings">Diagnostic Mappings</h4>
 521
 522 <p>All diagnostics are mapped into one of these 5 classes:</p>
 523
 524 <p>
 525 <ul>
 526 <li>Ignored</li>
 527 <li>Note</li>
 528 <li>Warning</li>
 529 <li>Error</li>
 530 <li>Fatal</li>
 531 </ul></p>
 532
 533 <h4 id="diagnostics_categories">Diagnostic Categories</h4>
 534
 535 <p>Though not shown by default, diagnostics may each be associated with a
 536    high-level category.  This category is intended to make it possible to triage
 537    builds that produce a large number of errors or warnings in a grouped way.
 538 </p>
 539
 540 <p>Categories are not shown by default, but they can be turned on with the
 541 <a href="#opt_fdiagnostics-show-category">-fdiagnostics-show-category</a> option.
 542 When set to "<tt>name</tt>", the category is printed textually in the diagnostic
 543 output.  When it is set to "<tt>id</tt>", a category number is printed.  The
 544 mapping of category names to category id's can be obtained by running '<tt>clang
 545   --print-diagnostic-categories</tt>'.
 546 </p>
 547
 548 <h4 id="diagnostics_commandline">Controlling Diagnostics via Command Line
 549  Flags</h4>
 550
 551 <p>-W flags, -pedantic, etc</p>
 552
 553 <h4 id="diagnostics_pragmas">Controlling Diagnostics via Pragmas</h4>
 554
 555 <p>Clang can also control what diagnostics are enabled through the use of
 556 pragmas in the source code. This is useful for turning off specific warnings
 557 in a section of source code. Clang supports GCC's pragma for compatibility
 558 with existing source code, as well as several extensions. </p>
 559
 560 <p>The pragma may control any warning that can be used from the command line.
 561 Warnings may be set to ignored, warning, error, or fatal. The following
 562 example code will tell Clang or GCC to ignore the -Wall warnings:</p>
 563
 564 <pre>
 565 #pragma GCC diagnostic ignored "-Wall"
 566 </pre>
 567
 568 <p>In addition to all of the functionality of provided by GCC's pragma, Clang
 569 also allows you to push and pop the current warning state.  This is particularly
 570 useful when writing a header file that will be compiled by other people, because
 571 you don't know what warning flags they build with.</p>
 572
 573 <p>In the below example
 574 -Wmultichar is ignored for only a single line of code, after which the
 575 diagnostics return to whatever state had previously existed.</p>
 576
 577 <pre>
 578 #pragma clang diagnostic push
 579 #pragma clang diagnostic ignored "-Wmultichar"
 580
 581 char b = 'df'; // no warning.
 582
 583 #pragma clang diagnostic pop
 584 </pre>
 585
 586 <p>The push and pop pragmas will save and restore the full diagnostic state of
 587 the compiler, regardless of how it was set. That means that it is possible to
 588 use push and pop around GCC compatible diagnostics and Clang will push and pop
 589 them appropriately, while GCC will ignore the pushes and pops as unknown
 590 pragmas. It should be noted that while Clang supports the GCC pragma, Clang and
 591 GCC do not support the exact same set of warnings, so even when using GCC
 592 compatible #pragmas there is no guarantee that they will have identical behaviour
 593 on both compilers. </p>
 594
 595 <h4 id="analyzer_diagnositics">Controlling Static Analyzer Diagnostics</h4>
 596
 597 <p>While not strictly part of the compiler, the diagnostics from Clang's <a
 598 href="http://clang-analyzer.llvm.org">static analyzer</a> can also be influenced
 599 by the user via changes to the source code.  This can be done in two ways:
 600
 601 <ul>
 602
 603 <li id="analyzer_annotations"><b>Annotations</b>: The static analyzer recognizes various GCC-style
 604 attributes (e.g., <tt>__attribute__((nonnull)))</tt>) that can either suppress
 605 static analyzer warnings or teach the analyzer about code invariants which
 606 enable it to find more bugs. While many of these attributes are standard GCC
 607 attributes, additional ones have added to Clang to specifically support the
 608 static analyzer. Detailed information on these annotations can be found in the
 609 <a href="http://clang-analyzer.llvm.org/annotations.html">analyzer's
 610 documentation</a>.</li>
 611
 612 <li><b><tt>__clang_analyzer__</tt></b>: When the static analyzer is using Clang
 613 to parse source files, it implicitly defines the preprocessor macro
 614 <tt>__clang_analyzer__</tt>. While discouraged, code can use this macro to
 615 selectively exclude code the analyzer examines.  Here is an example:
 616
 617 <pre>
 618 #ifndef __clang_analyzer__
 619 // Code not to be analyzed
 620 #endif
 621 </pre>
 622
 623 In general, this usage is discouraged. Instead, we prefer that users file bugs
 624 against the analyzer when it flags false positives. There is also active
 625 discussion of allowing users in the future to selectively silence specific
 626 analyzer warnings (some which can already be done using <a
 627 href="analyzer_annotations">annotations</a>).</li>
 628
 629 </ul>
 630
 631 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 632 <h3 id="precompiledheaders">Precompiled Headers</h3>
 633 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 634
 635 <p><a href="http://en.wikipedia.org/wiki/Precompiled_header">Precompiled
 636 headers</a> are a general approach employed by many compilers to reduce
 637 compilation time. The underlying motivation of the approach is that it is
 638 common for the same (and often large) header files to be included by
 639 multiple source files. Consequently, compile times can often be greatly improved
 640 by caching some of the (redundant) work done by a compiler to process headers.
 641 Precompiled header files, which represent one of many ways to implement
 642 this optimization, are literally files that represent an on-disk cache that
 643 contains the vital information necessary to reduce some of the work
 644 needed to process a corresponding header file. While details of precompiled
 645 headers vary between compilers, precompiled headers have been shown to be a
 646 highly effective at speeding up program compilation on systems with very large
 647 system headers (e.g., Mac OS/X).</p>
 648
 649 <h4>Generating a PCH File</h4>
 650
 651 <p>To generate a PCH file using Clang, one invokes Clang with
 652 the <b><tt>-x <i>&lt;language&gt;</i>-header</tt></b> option. This mirrors the
 653 interface in GCC for generating PCH files:</p>
 654
 655 <pre>
 656   $ gcc -x c-header test.h -o test.h.gch
 657   $ clang -x c-header test.h -o test.h.pch
 658 </pre>
 659
 660 <h4>Using a PCH File</h4>
 661
 662 <p>A PCH file can then be used as a prefix header when a
 663 <b><tt>-include</tt></b> option is passed to <tt>clang</tt>:</p>
 664
 665 <pre>
 666   $ clang -include test.h test.c -o test
 667 </pre>
 668
 669 <p>The <tt>clang</tt> driver will first check if a PCH file for <tt>test.h</tt>
 670 is available; if so, the contents of <tt>test.h</tt> (and the files it includes)
 671 will be processed from the PCH file. Otherwise, Clang falls back to
 672 directly processing the content of <tt>test.h</tt>. This mirrors the behavior of
 673 GCC.</p>
 674
 675 <p><b>NOTE:</b> Clang does <em>not</em> automatically use PCH files
 676 for headers that are directly included within a source file. For example:</p>
 677
 678 <pre>
 679   $ clang -x c-header test.h -o test.h.pch
 680   $ cat test.c
 681   #include "test.h"
 682   $ clang test.c -o test
 683 </pre>
 684
 685 <p>In this example, <tt>clang</tt> will not automatically use the PCH file for
 686 <tt>test.h</tt> since <tt>test.h</tt> was included directly in the source file
 687 and not specified on the command line using <tt>-include</tt>.</p>
 688
 689 <h4>Relocatable PCH Files</h4>
 690 <p>It is sometimes necessary to build a precompiled header from headers that
 691 are not yet in their final, installed locations. For example, one might build a
 692 precompiled header within the build tree that is then meant to be installed
 693 alongside the headers. Clang permits the creation of "relocatable" precompiled
 694 headers, which are built with a given path (into the build directory) and can
 695 later be used from an installed location.</p>
 696
 697 <p>To build a relocatable precompiled header, place your headers into a
 698 subdirectory whose structure mimics the installed location. For example, if you
 699 want to build a precompiled header for the header <code>mylib.h</code> that
 700 will be installed into <code>/usr/include</code>, create a subdirectory
 701 <code>build/usr/include</code> and place the header <code>mylib.h</code> into
 702 that subdirectory. If <code>mylib.h</code> depends on other headers, then
 703 they can be stored within <code>build/usr/include</code> in a way that mimics
 704 the installed location.</p>
 705
 706 <p>Building a relocatable precompiled header requires two additional arguments.
 707 First, pass the <code>--relocatable-pch</code> flag to indicate that the
 708 resulting PCH file should be relocatable. Second, pass
 709 <code>-isysroot /path/to/build</code>, which makes all includes for your
 710 library relative to the build directory. For example:</p>
 711
 712 <pre>
 713   # clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch
 714 </pre>
 715
 716 <p>When loading the relocatable PCH file, the various headers used in the PCH
 717 file are found from the system header root. For example, <code>mylib.h</code>
 718 can be found in <code>/usr/include/mylib.h</code>. If the headers are installed
 719 in some other system root, the <code>-isysroot</code> option can be used provide
 720 a different system root from which the headers will be based. For example,
 721 <code>-isysroot /Developer/SDKs/MacOSX10.4u.sdk</code> will look for
 722 <code>mylib.h</code> in
 723 <code>/Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h</code>.</p>
 724
 725 <p>Relocatable precompiled headers are intended to be used in a limited number
 726 of cases where the compilation environment is tightly controlled and the
 727 precompiled header cannot be generated after headers have been installed.
 728 Relocatable precompiled headers also have some performance impact, because
 729 the difference in location between the header locations at PCH build time vs.
 730 at the time of PCH use requires one of the PCH optimizations,
 731 <code>stat()</code> caching, to be disabled. However, this change is only
 732 likely to affect PCH files that reference a large number of headers.</p>
 733
 734 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 735 <h3 id="codegen">Controlling Code Generation</h3>
 736 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 737
 738 <p>Clang provides a number of ways to control code generation.  The options are listed below.</p>
 739
 740 <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
 741 <dt id="opt_fcatch-undefined-behavior"><b>-fcatch-undefined-behavior</b>: Turn
 742 on runtime code generation to check for undefined behavior.</dt>
 743
 744 <dd>This option, which defaults to off, controls whether or not Clang
 745 adds runtime checks for undefined runtime behavior.  If the check fails,
 746 <tt>__builtin_trap()</tt> is used to indicate failure.
 747 The checks are:
 748 <p>
 749 <li>Subscripting where the static type of one operand is variable
 750     which is decayed from an array type and the other operand is
 751     greater than the size of the array or less than zero.</li>
 752 <li>Shift operators where the amount shifted is greater or equal to the
 753     promoted bit-width of the left-hand-side or less than zero.</li>
 754 <li>If control flow reaches __builtin_unreachable.
 755 <li>When llvm implements more __builtin_object_size support, reads and
 756     writes for objects that __builtin_object_size indicates we aren't
 757     accessing valid memory.  Bit-fields and vectors are not yet checked.
 758 </p>
 759 </dd>
 760
 761 <dt id="opt_fno-assume-sane-operator-new"><b>-fno-assume-sane-operator-new</b>:
 762 Don't assume that the C++'s new operator is sane.</dt>
 763 <dd>This option tells the compiler to do not assume that C++'s global new
 764 operator will always return a pointer that do not
 765 alias any other pointer when the function returns.</dd>
 766
 767 <!-- ======================================================================= -->
 768 <h2 id="c">C Language Features</h2>
 769 <!-- ======================================================================= -->
 770
 771 <p>The support for standard C in clang is feature-complete except for the C99
 772 floating-point pragmas.</p>
 773
 774 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 775 <h3 id="c_ext">Extensions supported by clang</h3>
 776 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 777
 778 <p>See <a href="LanguageExtensions.html">clang language extensions</a>.</p>
 779
 780 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 781 <h3 id="c_modes">Differences between various standard modes</h3>
 782 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 783
 784 <p>clang supports the -std option, which changes what language mode clang uses.
 785 The supported modes for C are c89, gnu89, c94, c99, gnu99 and various aliases
 786 for those modes.  If no -std option is specified, clang defaults to gnu99 mode.
 787 </p>
 788
 789 <p>Differences between all c* and gnu* modes:</p>
 790 <ul>
 791 <li>c* modes define "__STRICT_ANSI__".</li>
 792 <li>Target-specific defines not prefixed by underscores, like "linux", are
 793 defined in gnu* modes.</li>
 794 <li>Trigraphs default to being off in gnu* modes; they can be enabled by the
 795 -trigraphs option.</li>
 796 <li>The parser recognizes "asm" and "typeof" as keywords in gnu* modes; the
 797 variants "__asm__" and "__typeof__" are recognized in all modes.</li>
 798 <li>The Apple "blocks" extension is recognized by default in gnu* modes
 799 on some platforms; it can be enabled in any mode with the "-fblocks"
 800 option.</li>
 801 </ul>
 802
 803 <p>Differences between *89 and *99 modes:</p>
 804 <ul>
 805 <li>The *99 modes default to implementing "inline" as specified in C99, while
 806 the *89 modes implement the GNU version.  This can be overridden for individual
 807 functions with the __gnu_inline__ attribute.</li>
 808 <li>Digraphs are not recognized in c89 mode.</li>
 809 <li>The scope of names defined inside a "for", "if", "switch", "while", or "do"
 810 statement is different. (example: "if ((struct x {int x;}*)0) {}".)</li>
 811 <li>__STDC_VERSION__ is not defined in *89 modes.</li>
 812 <li>"inline" is not recognized as a keyword in c89 mode.</li>
 813 <li>"restrict" is not recognized as a keyword in *89 modes.</li>
 814 <li>Commas are allowed in integer constant expressions in *99 modes.</li>
 815 <li>Arrays which are not lvalues are not implicitly promoted to pointers in
 816 *89 modes.</li>
 817 <li>Some warnings are different.</li>
 818 </ul>
 819
 820 <p>c94 mode is identical to c89 mode except that digraphs are enabled in
 821 c94 mode (FIXME: And __STDC_VERSION__ should be defined!).</p>
 822
 823 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 824 <h3 id="c_unimpl_gcc">GCC extensions not implemented yet</h3>
 825 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 826
 827 <p>clang tries to be compatible with gcc as much as possible, but some gcc
 828 extensions are not implemented yet:</p>
 829
 830 <ul>
 831 <li>clang does not support __label__
 832 (<a href="http://llvm.org/bugs/show_bug.cgi?id=3429">bug 3429</a>). This is
 833 a relatively small feature, so it is likely to be implemented relatively
 834 soon.</li>
 835
 836 <li>clang does not support #pragma weak
 837 (<a href="http://llvm.org/bugs/show_bug.cgi?id=3679">bug 3679</a>). Due to
 838 the uses described in the bug, this is likely to be implemented at some
 839 point, at least partially.</li>
 840
 841 <li>clang does not support code generation for local variables pinned to
 842 registers (<a href="http://llvm.org/bugs/show_bug.cgi?id=3933">bug 3933</a>).
 843 This is a relatively small feature, so it is likely to be implemented
 844 relatively soon.</li>
 845
 846 <li>clang does not support decimal floating point types (_Decimal32 and
 847 friends) or fixed-point types (_Fract and friends); nobody has expressed
 848 interest in these features yet, so it's hard to say when they will be
 849 implemented.</li>
 850
 851 <li>clang does not support nested functions; this is a complex feature which
 852 is infrequently used, so it is unlikely to be implemented anytime soon.</li>
 853
 854 <li>clang does not support global register variables, this is unlikely
 855 to be implemented soon because it requires additional LLVM backend support.
 856 </li>
 857
 858 <li>clang does not support static initialization of flexible array
 859 members. This appears to be a rarely used extension, but could be
 860 implemented pending user demand.</li>
 861
 862 <li>clang does not support __builtin_va_arg_pack/__builtin_va_arg_pack_len.
 863 This is used rarely, but in some potentially interesting places, like the
 864 glibc headers, so it may be implemented pending user demand.  Note that
 865 because clang pretends to be like GCC 4.2, and this extension was introduced
 866 in 4.3, the glibc headers will not try to use this extension with clang at
 867 the moment.</li>
 868
 869 <li>clang does not support the gcc extension for forward-declaring function
 870 parameters; this has not showed up in any real-world code yet, though, so it
 871 might never be implemented.</li>
 872
 873 </ul>
 874
 875 <p>This is not a complete list; if you find an unsupported extension
 876 missing from this list, please send an e-mail to cfe-dev.  This list
 877 currently excludes C++; see <a href="#cxx">C++ Language Features</a>.
 878 Also, this list does not include bugs in mostly-implemented features; please
 879 see the <a href="http://llvm.org/bugs/buglist.cgi?quicksearch=product%3Aclang+component%3A-New%2BBugs%2CAST%2CBasic%2CDriver%2CHeaders%2CLLVM%2BCodeGen%2Cparser%2Cpreprocessor%2CSemantic%2BAnalyzer">
 880 bug tracker</a> for known existing bugs (FIXME: Is there a section for
 881 bug-reporting guidelines somewhere?).</p>
 882
 883 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 884 <h3 id="c_unsupp_gcc">Intentionally unsupported GCC extensions</h3>
 885 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 886
 887 <ul>
 888
 889 <li>clang does not support the gcc extension that allows variable-length arrays
 890 in structures.  This is for a few of reasons: one, it is tricky
 891 to implement, two, the extension is completely undocumented, and three, the
 892 extension appears to be rarely used.  Note that clang <em>does</em> support
 893 flexible array members (arrays with a zero or unspecified size at the end of
 894 a structure).</li>
 895
 896 <li>clang does not support duplicate definitions of a function where one is
 897 inline. This complicates clients of the AST which normally can expect there is
 898 at most one definition for each function. Source code using this feature should
 899 be changed to define the inline and out-of-line definitions in separate
 900 translation units.</li>
 901
 902 <li>clang does not have an equivalent to gcc's "fold"; this means that
 903 clang doesn't accept some constructs gcc might accept in contexts where a
 904 constant expression is required, like "x-x" where x is a variable, or calls
 905 to C library functions like strlen.</li>
 906
 907 <li>clang does not support multiple alternative constraints in inline asm; this
 908 is an extremely obscure feature which would be complicated to implement
 909 correctly.</li>
 910
 911 <li>clang does not support __builtin_apply and friends; this extension is
 912 extremely obscure and difficult to implement reliably.</li>
 913
 914 </ul>
 915
 916 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 917 <h3 id="c_ms">Microsoft extensions</h3>
 918 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 919
 920 <p>clang has some experimental support for extensions from
 921 Microsoft Visual C++; to enable it, use the -fms-extensions command-line
 922 option.  This is the default for Windows targets.  Note that the
 923 support is incomplete; enabling Microsoft extensions will silently drop
 924 certain constructs (including __declspec and Microsoft-style asm statements).
 925 </p>
 926
 927 <li>clang allows setting _MCS_VER with -fmsc-version=. It defaults to 1300 which
 928 is the same as Visual C/C++ 2003. Any number is supported and can greatly affect
 929 what Windows SDK and c++stdlib headers clang can compile. This option will be
 930 removed when clang supports the full set of MS extensions required for these
 931 headers.</li>
 932
 933 <li>clang does not support the Microsoft extension where anonymous
 934 record members can be declared using user defined typedefs.</li>
 935
 936 <li>clang supports the Microsoft "#pragma pack" feature for
 937 controlling record layout. GCC also contains support for this feature,
 938 however where MSVC and GCC are incompatible clang follows the MSVC
 939 definition.</li>
 940
 941 <!-- ======================================================================= -->
 942 <h2 id="target_features">Target-Specific Features and Limitations</h2>
 943 <!-- ======================================================================= -->
 944
 945
 946 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 947 <h3 id="target_arch">CPU Architectures Features and Limitations</h3>
 948 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 949
 950 <!-- ======================== -->
 951 <h4 id="target_arch_x86">X86</h4>
 952 <!-- ======================== -->
 953
 954 <p>The support for X86 (both 32-bit and 64-bit) is considered stable on Darwin
 955 (Mac OS/X), Linux, FreeBSD, and Dragonfly BSD: it has been tested to correctly
 956 compile many large C, C++, Objective-C, and Objective-C++ codebases.</p>
 957
 958 <!-- ======================== -->
 959 <h4 id="target_arch_arm">ARM</h4>
 960 <!-- ======================== -->
 961
 962 <p>The support for ARM (specifically ARMv6 and ARMv7) is considered stable on
 963 Darwin (iOS): it has been tested to correctly compile many large C, C++,
 964 Objective-C, and Objective-C++ codebases.</p>
 965
 966 <p>It is not quite feature complete, the following areas still need work:</p>
 967 <ul>
 968   <li>Vector support (NEON in particular) is not yet well tested and may have
 969     ABI errors.</li>
 970   <li>Clang only supports a limited number of ARM architectures. It does not yet
 971     fully support ARMv5, for example.</li>
 972 </ul>
 973
 974 <!-- ======================== -->
 975 <h4 id="target_arch_other">Other platforms</h4>
 976 <!-- ======================== -->
 977 clang currently contains some support for PPC and Sparc; however, significant
 978 pieces of code generation are still missing, and they haven't undergone
 979 significant testing.
 980
 981 <p>clang contains limited support for the MSP430 embedded processor, but both
 982 the clang support and the LLVM backend support are highly experimental.
 983
 984 <p>Other platforms are completely unsupported at the moment.  Adding the
 985 minimal support needed for parsing and semantic analysis on a new platform
 986 is quite easy; see lib/Basic/Targets.cpp in the clang source tree. This level
 987 of support is also sufficient for conversion to LLVM IR for simple programs.
 988 Proper support for conversion to LLVM IR requires adding code to
 989 lib/CodeGen/CGCall.cpp at the moment; this is likely to change soon, though.
 990 Generating assembly requires a suitable LLVM backend.
 991
 992 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 993 <h3 id="target_os">Operating System Features and Limitations</h3>
 994 <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
 995
 996 <!-- ======================================= -->
 997 <h4 id="target_os_darwin">Darwin (Mac OS/X)</h4>
 998 <!-- ======================================= -->
 999
1000 <p>No __thread support, 64-bit ObjC support requires SL tools.</p>
1001
1002 </div>
1003 </body>
1004 </html>