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