| Shih-wei Liao | f8fd82b | 2010-02-10 11:10:31 -0800 | [diff] [blame^] | 1 | <html> |
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| 3 | <title>Clang Compiler User's Manual</title> |
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| 13 | |
| 14 | <!--#include virtual="../menu.html.incl"--> |
| 15 | |
| 16 | <div id="content"> |
| 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_commandline">Controlling Diagnostics via Command Line Flags</a></li> |
| 40 | <li><a href="#diagnostics_pragmas">Controlling Diagnostics via Pragmas</a></li> |
| 41 | </ul> |
| 42 | <li><a href="#precompiledheaders">Precompiled Headers</a></li> |
| 43 | <li><a href="#codegen">Controlling Code Generation</a></li> |
| 44 | </ul> |
| 45 | </li> |
| 46 | <li><a href="#c">C Language Features</a> |
| 47 | <ul> |
| 48 | <li><a href="#c_ext">Extensions supported by clang</a></li> |
| 49 | <li><a href="#c_modes">Differences between various standard modes</a></li> |
| 50 | <li><a href="#c_unimpl_gcc">GCC extensions not implemented yet</a></li> |
| 51 | <li><a href="#c_unsupp_gcc">Intentionally unsupported GCC extensions</a></li> |
| 52 | <li><a href="#c_ms">Microsoft extensions</a></li> |
| 53 | </ul> |
| 54 | </li> |
| 55 | <li><a href="#objc">Objective-C Language Features</a> |
| 56 | <ul> |
| 57 | <li><a href="#objc_incompatibilities">Intentional Incompatibilities with |
| 58 | GCC</a></li> |
| 59 | </ul> |
| 60 | </li> |
| 61 | <li><a href="#cxx">C++ Language Features</a> |
| 62 | </li> |
| 63 | <li><a href="#objcxx">Objective C++ Language Features</a> |
| 64 | </li> |
| 65 | <li><a href="#target_features">Target-Specific Features and Limitations</a> |
| 66 | <ul> |
| 67 | <li><a href="#target_arch">CPU Architectures Features and Limitations</a> |
| 68 | <ul> |
| 69 | <li><a href="#target_arch_x86">X86</a></li> |
| 70 | <li><a href="#target_arch_arm">ARM</a></li> |
| 71 | <li><a href="#target_arch_other">Other platforms</a></li> |
| 72 | </ul> |
| 73 | </li> |
| 74 | <li><a href="#target_os">Operating System Features and Limitations</a> |
| 75 | <ul> |
| 76 | <li><a href="#target_os_darwin">Darwin (Mac OS/X)</a></li> |
| 77 | <li>Linux, etc.</li> |
| 78 | </ul> |
| 79 | |
| 80 | </li> |
| 81 | </ul> |
| 82 | </li> |
| 83 | </ul> |
| 84 | |
| 85 | |
| 86 | <!-- ======================================================================= --> |
| 87 | <h2 id="intro">Introduction</h2> |
| 88 | <!-- ======================================================================= --> |
| 89 | |
| 90 | <p>The Clang Compiler is an open-source compiler for the C family of programming |
| 91 | languages, aiming to be the best in class implementation of these languages. |
| 92 | Clang builds on the LLVM optimizer and code generator, allowing it to provide |
| 93 | high-quality optimization and code generation support for many targets. For |
| 94 | more general information, please see the <a href="http://clang.llvm.org">Clang |
| 95 | Web Site</a> or the <a href="http://llvm.org">LLVM Web Site</a>.</p> |
| 96 | |
| 97 | <p>This document describes important notes about using Clang as a compiler for |
| 98 | an end-user, documenting the supported features, command line options, etc. If |
| 99 | you are interested in using Clang to build a tool that processes code, please |
| 100 | see <a href="InternalsManual.html">the Clang Internals Manual</a>. If you are |
| 101 | interested in the <a href="http://clang.llvm.org/StaticAnalysis.html">Clang |
| 102 | Static Analyzer</a>, please see its web page.</p> |
| 103 | |
| 104 | <p>Clang is designed to support the C family of programming languages, which |
| 105 | includes <a href="#c">C</a>, <a href="#objc">Objective-C</a>, <a |
| 106 | href="#cxx">C++</a>, and <a href="#objcxx">Objective-C++</a> as well as many |
| 107 | dialects of those. For language-specific information, please see the |
| 108 | corresponding language specific section:</p> |
| 109 | |
| 110 | <ul> |
| 111 | <li><a href="#c">C Language</a>: K&R C, ANSI C89, ISO C90, ISO C94 |
| 112 | (C89+AMD1), ISO C99 (+TC1, TC2, TC3). </li> |
| 113 | <li><a href="#objc">Objective-C Language</a>: ObjC 1, ObjC 2, ObjC 2.1, plus |
| 114 | variants depending on base language.</li> |
| 115 | <li><a href="#cxx">C++ Language Features</a></li> |
| 116 | <li><a href="#objcxx">Objective C++ Language</a></li> |
| 117 | </ul> |
| 118 | |
| 119 | <p>In addition to these base languages and their dialects, Clang supports a |
| 120 | broad variety of language extensions, which are documented in the corresponding |
| 121 | language section. These extensions are provided to be compatible with the GCC, |
| 122 | Microsoft, and other popular compilers as well as to improve functionality |
| 123 | through Clang-specific features. The Clang driver and language features are |
| 124 | intentionally designed to be as compatible with the GNU GCC compiler as |
| 125 | reasonably possible, easing migration from GCC to Clang. In most cases, code |
| 126 | "just works".</p> |
| 127 | |
| 128 | <p>In addition to language specific features, Clang has a variety of features |
| 129 | that depend on what CPU architecture or operating system is being compiled for. |
| 130 | Please see the <a href="#target_features">Target-Specific Features and |
| 131 | Limitations</a> section for more details.</p> |
| 132 | |
| 133 | <p>The rest of the introduction introduces some basic <a |
| 134 | href="#terminology">compiler terminology</a> that is used throughout this manual |
| 135 | and contains a basic <a href="#basicusage">introduction to using Clang</a> |
| 136 | as a command line compiler.</p> |
| 137 | |
| 138 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 139 | <h3 id="terminology">Terminology</h3> |
| 140 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 141 | |
| 142 | <p>Front end, parser, backend, preprocessor, undefined behavior, diagnostic, |
| 143 | optimizer</p> |
| 144 | |
| 145 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 146 | <h3 id="basicusage">Basic Usage</h3> |
| 147 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 148 | |
| 149 | <p>Intro to how to use a C compiler for newbies.</p> |
| 150 | <p> |
| 151 | compile + link |
| 152 | |
| 153 | compile then link |
| 154 | |
| 155 | debug info |
| 156 | |
| 157 | enabling optimizations |
| 158 | |
| 159 | picking a language to use, defaults to C99 by default. Autosenses based on |
| 160 | extension. |
| 161 | |
| 162 | using a makefile |
| 163 | </p> |
| 164 | |
| 165 | |
| 166 | <!-- ======================================================================= --> |
| 167 | <h2 id="commandline">Command Line Options</h2> |
| 168 | <!-- ======================================================================= --> |
| 169 | |
| 170 | <p> |
| 171 | This section is generally an index into other sections. It does not go into |
| 172 | depth on the ones that are covered by other sections. However, the first part |
| 173 | introduces the language selection and other high level options like -c, -g, etc. |
| 174 | </p> |
| 175 | |
| 176 | |
| 177 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 178 | <h3 id="cl_diagnostics">Options to Control Error and Warning Messages</h3> |
| 179 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 180 | |
| 181 | <p><b>-Werror</b>: Turn warnings into errors.</p> |
| 182 | <p><b>-Werror=foo</b>: Turn warning "foo" into an error.</p> |
| 183 | <p><b>-Wno-error=foo</b>: Turn warning "foo" into an warning even if -Werror is |
| 184 | specified.</p> |
| 185 | <p><b>-Wfoo</b>: Enable warning foo</p> |
| 186 | <p><b>-Wno-foo</b>: Disable warning foo</p> |
| 187 | <p><b>-w</b>: Disable all warnings.</p> |
| 188 | <p><b>-pedantic</b>: Warn on language extensions.</p> |
| 189 | <p><b>-pedantic-errors</b>: Error on language extensions.</p> |
| 190 | <p><b>-Wsystem-headers</b>: Enable warnings from system headers.</p> |
| 191 | |
| 192 | <!-- ================================================= --> |
| 193 | <h4 id="cl_diag_formatting">Formatting of Diagnostics</h4> |
| 194 | <!-- ================================================= --> |
| 195 | |
| 196 | <p>Clang aims to produce beautiful diagnostics by default, particularly for new |
| 197 | users that first come to Clang. However, different people have different |
| 198 | preferences, and sometimes Clang is driven by another program that wants to |
| 199 | parse simple and consistent output, not a person. For these cases, Clang |
| 200 | provides a wide range of options to control the exact output format of the |
| 201 | diagnostics that it generates.</p> |
| 202 | |
| 203 | <dl> |
| 204 | |
| 205 | <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| 206 | <dt id="opt_fshow-column"><b>-f[no-]show-column</b>: Print column number in |
| 207 | diagnostic.</dt> |
| 208 | <dd>This option, which defaults to on, controls whether or not Clang prints the |
| 209 | column number of a diagnostic. For example, when this is enabled, Clang will |
| 210 | print something like:</p> |
| 211 | |
| 212 | <pre> |
| 213 | test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| 214 | #endif bad |
| 215 | ^ |
| 216 | // |
| 217 | </pre> |
| 218 | |
| 219 | <p>When this is disabled, Clang will print "test.c:28: warning..." with no |
| 220 | column number.</p> |
| 221 | </dd> |
| 222 | |
| 223 | <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| 224 | <dt id="opt_fshow-source-location"><b>-f[no-]show-source-location</b>: Print |
| 225 | source file/line/column information in diagnostic.</dt> |
| 226 | <dd>This option, which defaults to on, controls whether or not Clang prints the |
| 227 | filename, line number and column number of a diagnostic. For example, |
| 228 | when this is enabled, Clang will print something like:</p> |
| 229 | |
| 230 | <pre> |
| 231 | test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| 232 | #endif bad |
| 233 | ^ |
| 234 | // |
| 235 | </pre> |
| 236 | |
| 237 | <p>When this is disabled, Clang will not print the "test.c:28:8: " part.</p> |
| 238 | </dd> |
| 239 | |
| 240 | <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| 241 | <dt id="opt_fcaret-diagnostics"><b>-f[no-]caret-diagnostics</b>: Print source |
| 242 | line and ranges from source code in diagnostic.</dt> |
| 243 | <dd>This option, which defaults to on, controls whether or not Clang prints the |
| 244 | source line, source ranges, and caret when emitting a diagnostic. For example, |
| 245 | when this is enabled, Clang will print something like:</p> |
| 246 | |
| 247 | <pre> |
| 248 | test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| 249 | #endif bad |
| 250 | ^ |
| 251 | // |
| 252 | </pre> |
| 253 | |
| 254 | <p>When this is disabled, Clang will just print:</p> |
| 255 | |
| 256 | <pre> |
| 257 | test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| 258 | </pre> |
| 259 | |
| 260 | </dd> |
| 261 | |
| 262 | <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| 263 | <dt id="opt_fdiagnostics-show-option"><b>-f[no-]diagnostics-show-option</b>: |
| 264 | Enable <tt>[-Woption]</tt> information in diagnostic line.</dt> |
| 265 | <dd>This option, which defaults to on, |
| 266 | controls whether or not Clang prints the associated <A |
| 267 | href="#cl_diag_warning_groups">warning group</a> option name when outputting |
| 268 | a warning diagnostic. For example, in this output:</p> |
| 269 | |
| 270 | <pre> |
| 271 | test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| 272 | #endif bad |
| 273 | ^ |
| 274 | // |
| 275 | </pre> |
| 276 | |
| 277 | <p>Passing <b>-fno-diagnostics-show-option</b> will prevent Clang from printing |
| 278 | the [<a href="#opt_Wextra-tokens">-Wextra-tokens</a>] information in the |
| 279 | diagnostic. This information tells you the flag needed to enable or disable the |
| 280 | diagnostic, either from the command line or through <a |
| 281 | href="#pragma_GCC_diagnostic">#pragma GCC diagnostic</a>.</dd> |
| 282 | |
| 283 | |
| 284 | <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| 285 | <dt id="opt_fdiagnostics-fixit-info"><b>-f[no-]diagnostics-fixit-info</b>: |
| 286 | Enable "FixIt" information in the diagnostics output.</dt> |
| 287 | <dd>This option, which defaults to on, controls whether or not Clang prints the |
| 288 | information on how to fix a specific diagnostic underneath it when it knows. |
| 289 | For example, in this output:</p> |
| 290 | |
| 291 | <pre> |
| 292 | test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| 293 | #endif bad |
| 294 | ^ |
| 295 | // |
| 296 | </pre> |
| 297 | |
| 298 | <p>Passing <b>-fno-diagnostics-fixit-info</b> will prevent Clang from printing |
| 299 | the "//" line at the end of the message. This information is useful for users |
| 300 | who may not understand what is wrong, but can be confusing for machine |
| 301 | parsing.</p> |
| 302 | </dd> |
| 303 | |
| 304 | <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| 305 | <dt id="opt_fdiagnostics-print-source-range-info"> |
| 306 | <b>-f[no-]diagnostics-print-source-range-info</b>: |
| 307 | Print machine parsable information about source ranges.</dt> |
| 308 | <dd>This option, which defaults to off, controls whether or not Clang prints |
| 309 | information about source ranges in a machine parsable format after the |
| 310 | file/line/column number information. The information is a simple sequence of |
| 311 | brace enclosed ranges, where each range lists the start and end line/column |
| 312 | locations. For example, in this output:</p> |
| 313 | |
| 314 | <pre> |
| 315 | exprs.c:47:15:{47:8-47:14}{47:17-47:24}: error: invalid operands to binary expression ('int *' and '_Complex float') |
| 316 | P = (P-42) + Gamma*4; |
| 317 | ~~~~~~ ^ ~~~~~~~ |
| 318 | </pre> |
| 319 | |
| 320 | <p>The {}'s are generated by -fdiagnostics-print-source-range-info.</p> |
| 321 | </dd> |
| 322 | |
| 323 | |
| 324 | </dl> |
| 325 | |
| 326 | |
| 327 | |
| 328 | |
| 329 | <!-- ===================================================== --> |
| 330 | <h4 id="cl_diag_warning_groups">Individual Warning Groups</h4> |
| 331 | <!-- ===================================================== --> |
| 332 | |
| 333 | <p>TODO: Generate this from tblgen. Define one anchor per warning group.</p> |
| 334 | |
| 335 | |
| 336 | <dl> |
| 337 | |
| 338 | |
| 339 | <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| 340 | <dt id="opt_Wextra-tokens"><b>-Wextra-tokens</b>: Warn about excess tokens at |
| 341 | the end of a preprocessor directive.</dt> |
| 342 | <dd>This option, which defaults to on, enables warnings about extra tokens at |
| 343 | the end of preprocessor directives. For example:</p> |
| 344 | |
| 345 | <pre> |
| 346 | test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] |
| 347 | #endif bad |
| 348 | ^ |
| 349 | </pre> |
| 350 | |
| 351 | <p>These extra tokens are not strictly conforming, and are usually best handled |
| 352 | by commenting them out.</p> |
| 353 | |
| 354 | <p>This option is also enabled by <a href="">-Wfoo</a>, <a href="">-Wbar</a>, |
| 355 | and <a href="">-Wbaz</a>.</p> |
| 356 | </dd> |
| 357 | |
| 358 | </dl> |
| 359 | |
| 360 | <!-- ======================================================================= --> |
| 361 | <h2 id="general_features">Language and Target-Independent Features</h2> |
| 362 | <!-- ======================================================================= --> |
| 363 | |
| 364 | |
| 365 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 366 | <h3 id="diagnostics">Controlling Errors and Warnings</h3> |
| 367 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 368 | |
| 369 | <p>Clang provides a number of ways to control which code constructs cause it to |
| 370 | emit errors and warning messages, and how they are displayed to the console.</p> |
| 371 | |
| 372 | <h4 id="diagnostics_display">Controlling How Clang Displays Diagnostics</h4> |
| 373 | |
| 374 | <p>When Clang emits a diagnostic, it includes rich information in the output, |
| 375 | and gives you fine-grain control over which information is printed. Clang has |
| 376 | the ability to print this information, and these are the options that control |
| 377 | it:</p> |
| 378 | |
| 379 | <p> |
| 380 | <ol> |
| 381 | <li>A file/line/column indicator that shows exactly where the diagnostic occurs |
| 382 | in your code [<a href="#opt_fshow-column">-fshow-column</a>, <a |
| 383 | href="#opt_fshow-source-location">-fshow-source-location</a>].</li> |
| 384 | <li>A categorization of the diagnostic as a note, warning, error, or fatal |
| 385 | error.</li> |
| 386 | <li>A text string that describes what the problem is.</li> |
| 387 | <li>An option that indicates how to control the diagnostic (for diagnostics that |
| 388 | support it) [<a |
| 389 | href="#opt_fdiagnostics-show-option">-fdiagnostics-show-option</a>].</li> |
| 390 | <li>The line of source code that the issue occurs on, along with a caret and |
| 391 | ranges that indicate the important locations [<a |
| 392 | href="opt_fcaret-diagnostics">-fcaret-diagnostics</a>].</li> |
| 393 | <li>"FixIt" information, which is a concise explanation of how to fix the |
| 394 | problem (when Clang is certain it knows) [<a |
| 395 | href="opt_fdiagnostics-fixit-info">-fdiagnostics-fixit-info</a>].</li> |
| 396 | <li>A machine-parsable representation of the ranges involved (off by |
| 397 | default) [<a |
| 398 | href="opt_fdiagnostics-print-source-range-info">-fdiagnostics-print-source-range-info</a>].</li> |
| 399 | </ol></p> |
| 400 | |
| 401 | <p>For more information please see <a href="#cl_diag_formatting">Formatting of |
| 402 | Diagnostics</a>.</p> |
| 403 | |
| 404 | <h4 id="diagnostics_mappings">Diagnostic Mappings</h4> |
| 405 | |
| 406 | <p>All diagnostics are mapped into one of these 5 classes:</p> |
| 407 | |
| 408 | <p> |
| 409 | <ul> |
| 410 | <li>Ignored</li> |
| 411 | <li>Note</li> |
| 412 | <li>Warning</li> |
| 413 | <li>Error</li> |
| 414 | <li>Fatal</li> |
| 415 | </ul></p> |
| 416 | |
| 417 | <h4 id="diagnostics_commandline">Controlling Diagnostics via Command Line Flags</h4> |
| 418 | |
| 419 | <p>-W flags, -pedantic, etc</p> |
| 420 | |
| 421 | <h4 id="diagnostics_pragmas">Controlling Diagnostics via Pragmas</h4> |
| 422 | |
| 423 | <p>Clang can also control what diagnostics are enabled through the use of |
| 424 | pragmas in the source code. This is useful for turning off specific warnings |
| 425 | in a section of source code. Clang supports GCC's pragma for compatibility |
| 426 | with existing source code, as well as several extensions. </p> |
| 427 | |
| 428 | <p>The pragma may control any warning that can be used from the command line. |
| 429 | Warnings may be set to ignored, warning, error, or fatal. The following |
| 430 | example code will tell Clang or GCC to ignore the -Wall warnings:</p> |
| 431 | |
| 432 | <pre> |
| 433 | #pragma GCC diagnostic ignored "-Wall" |
| 434 | </pre> |
| 435 | |
| 436 | <p>In addition to all of the functionality of provided by GCC's pragma, Clang |
| 437 | also allows you to push and pop the current warning state. This is particularly |
| 438 | useful when writing a header file that will be compiled by other people, because |
| 439 | you don't know what warning flags they build with.</p> |
| 440 | |
| 441 | <p>In the below example |
| 442 | -Wmultichar is ignored for only a single line of code, after which the |
| 443 | diagnostics return to whatever state had previously existed.</p> |
| 444 | |
| 445 | <pre> |
| 446 | #pragma clang diagnostic push |
| 447 | #pragma clang diagnostic ignored "-Wmultichar" |
| 448 | |
| 449 | char b = 'df'; // no warning. |
| 450 | |
| 451 | #pragma clang diagnostic pop |
| 452 | </pre> |
| 453 | |
| 454 | <p>The push and pop pragmas will save and restore the full diagnostic state of |
| 455 | the compiler, regardless of how it was set. That means that it is possible to |
| 456 | use push and pop around GCC compatible diagnostics and Clang will push and pop |
| 457 | them appropriately, while GCC will ignore the pushes and pops as unknown |
| 458 | pragmas. It should be noted that while Clang supports the GCC pragma, Clang and |
| 459 | GCC do not support the exact same set of warnings, so even when using GCC |
| 460 | compatible #pragmas there is no guarantee that they will have identical behaviour |
| 461 | on both compilers. </p> |
| 462 | |
| 463 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 464 | <h3 id="precompiledheaders">Precompiled Headers</h3> |
| 465 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 466 | |
| 467 | <p><a href="http://en.wikipedia.org/wiki/Precompiled_header">Precompiled |
| 468 | headers</a> are a general approach employed by many compilers to reduce |
| 469 | compilation time. The underlying motivation of the approach is that it is |
| 470 | common for the same (and often large) header files to be included by |
| 471 | multiple source files. Consequently, compile times can often be greatly improved |
| 472 | by caching some of the (redundant) work done by a compiler to process headers. |
| 473 | Precompiled header files, which represent one of many ways to implement |
| 474 | this optimization, are literally files that represent an on-disk cache that |
| 475 | contains the vital information necessary to reduce some of the work |
| 476 | needed to process a corresponding header file. While details of precompiled |
| 477 | headers vary between compilers, precompiled headers have been shown to be a |
| 478 | highly effective at speeding up program compilation on systems with very large |
| 479 | system headers (e.g., Mac OS/X).</p> |
| 480 | |
| 481 | <h4>Generating a PCH File</h4> |
| 482 | |
| 483 | <p>To generate a PCH file using Clang, one invokes Clang with |
| 484 | the <b><tt>-x <i><language></i>-header</tt></b> option. This mirrors the |
| 485 | interface in GCC for generating PCH files:</p> |
| 486 | |
| 487 | <pre> |
| 488 | $ gcc -x c-header test.h -o test.h.gch |
| 489 | $ clang -x c-header test.h -o test.h.pch |
| 490 | </pre> |
| 491 | |
| 492 | <h4>Using a PCH File</h4> |
| 493 | |
| 494 | <p>A PCH file can then be used as a prefix header when a |
| 495 | <b><tt>-include</tt></b> option is passed to <tt>clang</tt>:</p> |
| 496 | |
| 497 | <pre> |
| 498 | $ clang -include test.h test.c -o test |
| 499 | </pre> |
| 500 | |
| 501 | <p>The <tt>clang</tt> driver will first check if a PCH file for <tt>test.h</tt> |
| 502 | is available; if so, the contents of <tt>test.h</tt> (and the files it includes) |
| 503 | will be processed from the PCH file. Otherwise, Clang falls back to |
| 504 | directly processing the content of <tt>test.h</tt>. This mirrors the behavior of |
| 505 | GCC.</p> |
| 506 | |
| 507 | <p><b>NOTE:</b> Clang does <em>not</em> automatically use PCH files |
| 508 | for headers that are directly included within a source file. For example:</p> |
| 509 | |
| 510 | <pre> |
| 511 | $ clang -x c-header test.h -o test.h.pch |
| 512 | $ cat test.c |
| 513 | #include "test.h" |
| 514 | $ clang test.c -o test |
| 515 | </pre> |
| 516 | |
| 517 | <p>In this example, <tt>clang</tt> will not automatically use the PCH file for |
| 518 | <tt>test.h</tt> since <tt>test.h</tt> was included directly in the source file |
| 519 | and not specified on the command line using <tt>-include</tt>.</p> |
| 520 | |
| 521 | <h4>Relocatable PCH Files</h4> |
| 522 | <p>It is sometimes necessary to build a precompiled header from headers that |
| 523 | are not yet in their final, installed locations. For example, one might build a |
| 524 | precompiled header within the build tree that is then meant to be installed |
| 525 | alongside the headers. Clang permits the creation of "relocatable" precompiled |
| 526 | headers, which are built with a given path (into the build directory) and can |
| 527 | later be used from an installed location.</p> |
| 528 | |
| 529 | <p>To build a relocatable precompiled header, place your headers into a |
| 530 | subdirectory whose structure mimics the installed location. For example, if you |
| 531 | want to build a precompiled header for the header <code>mylib.h</code> that |
| 532 | will be installed into <code>/usr/include</code>, create a subdirectory |
| 533 | <code>build/usr/include</code> and place the header <code>mylib.h</code> into |
| 534 | that subdirectory. If <code>mylib.h</code> depends on other headers, then |
| 535 | they can be stored within <code>build/usr/include</code> in a way that mimics |
| 536 | the installed location.</p> |
| 537 | |
| 538 | <p>Building a relocatable precompiled header requires two additional arguments. |
| 539 | First, pass the <code>--relocatable-pch</code> flag to indicate that the |
| 540 | resulting PCH file should be relocatable. Second, pass |
| 541 | <code>-isysroot /path/to/build</code>, which makes all includes for your |
| 542 | library relative to the build directory. For example:</p> |
| 543 | |
| 544 | <pre> |
| 545 | # clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch |
| 546 | </pre> |
| 547 | |
| 548 | <p>When loading the relocatable PCH file, the various headers used in the PCH |
| 549 | file are found from the system header root. For example, <code>mylib.h</code> |
| 550 | can be found in <code>/usr/include/mylib.h</code>. If the headers are installed |
| 551 | in some other system root, the <code>-isysroot</code> option can be used provide |
| 552 | a different system root from which the headers will be based. For example, |
| 553 | <code>-isysroot /Developer/SDKs/MacOSX10.4u.sdk</code> will look for |
| 554 | <code>mylib.h</code> in |
| 555 | <code>/Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h</code>.</p> |
| 556 | |
| 557 | <p>Relocatable precompiled headers are intended to be used in a limited number |
| 558 | of cases where the compilation environment is tightly controlled and the |
| 559 | precompiled header cannot be generated after headers have been installed. |
| 560 | Relocatable precompiled headers also have some performance impact, because |
| 561 | the difference in location between the header locations at PCH build time vs. |
| 562 | at the time of PCH use requires one of the PCH optimizations, |
| 563 | <code>stat()</code> caching, to be disabled. However, this change is only |
| 564 | likely to affect PCH files that reference a large number of headers.</p> |
| 565 | |
| 566 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 567 | <h3 id="codegen">Controlling Code Generation</h3> |
| 568 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 569 | |
| 570 | <p>Clang provides a number of ways to control code generation. The options are listed below.</p> |
| 571 | |
| 572 | <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> |
| 573 | <dt id="opt_fcatch-undefined-behavior"><b>-fcatch-undefined-behavior</b>: Turn |
| 574 | on runtime code generation to check for undefined behavior.</dt> |
| 575 | |
| 576 | <dd>This option, which defaults to off, controls whether or not Clang |
| 577 | adds runtime checks for undefined runtime behavior. If the check fails, |
| 578 | <tt>__builtin_trap()</tt> is used to indicate failure. |
| 579 | The checks are: |
| 580 | <p> |
| 581 | <li>Subscripting where the static type of one operand is variable |
| 582 | which is decayed from an array type and the other operand is |
| 583 | greater than the size of the array or less than zero.</li> |
| 584 | <li>Shift operators where the amount shifted is greater or equal to the |
| 585 | promoted bit-width of the left-hand-side or less than zero.</li> |
| 586 | <li>If control flow reaches __builtin_unreachable. |
| 587 | <li>When llvm implements more __builtin_object_size support, reads and |
| 588 | writes for objects that __builtin_object_size indicates we aren't |
| 589 | accessing valid memory. Bit-fields and vectors are not yet checked. |
| 590 | </p> |
| 591 | </dd> |
| 592 | |
| 593 | <dt id="opt_fno-assume-sane-operator-new"><b>-fno-assume-sane-operator-new</b>: |
| 594 | Don't assume that the C++'s new operator is sane.</dt> |
| 595 | <dd>This option tells the compiler to do not assume that C++'s global new |
| 596 | operator will always return a pointer that do not |
| 597 | alias any other pointer when the function returns.</dd> |
| 598 | |
| 599 | <!-- ======================================================================= --> |
| 600 | <h2 id="c">C Language Features</h2> |
| 601 | <!-- ======================================================================= --> |
| 602 | |
| 603 | <p>The support for standard C in clang is feature-complete except for the C99 |
| 604 | floating-point pragmas.</p> |
| 605 | |
| 606 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 607 | <h3 id="c_ext">Extensions supported by clang</h3> |
| 608 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 609 | |
| 610 | <p>See <a href="LanguageExtensions.html">clang language extensions</a>.</p> |
| 611 | |
| 612 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 613 | <h3 id="c_modes">Differences between various standard modes</h3> |
| 614 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 615 | |
| 616 | <p>clang supports the -std option, which changes what language mode clang uses. |
| 617 | The supported modes for C are c89, gnu89, c94, c99, gnu99 and various aliases |
| 618 | for those modes. If no -std option is specified, clang defaults to gnu99 mode. |
| 619 | </p> |
| 620 | |
| 621 | <p>Differences between all c* and gnu* modes:</p> |
| 622 | <ul> |
| 623 | <li>c* modes define "__STRICT_ANSI__".</li> |
| 624 | <li>Target-specific defines not prefixed by underscores, like "linux", are |
| 625 | defined in gnu* modes.</li> |
| 626 | <li>Trigraphs default to being off in gnu* modes; they can be enabled by the |
| 627 | -trigraphs option.</li> |
| 628 | <li>The parser recognizes "asm" and "typeof" as keywords in gnu* modes; the |
| 629 | variants "__asm__" and "__typeof__" are recognized in all modes.</li> |
| 630 | <li>The Apple "blocks" extension is recognized by default in gnu* modes |
| 631 | on some platforms; it can be enabled in any mode with the "-fblocks" |
| 632 | option.</li> |
| 633 | </ul> |
| 634 | |
| 635 | <p>Differences between *89 and *99 modes:</p> |
| 636 | <ul> |
| 637 | <li>The *99 modes default to implementing "inline" as specified in C99, while |
| 638 | the *89 modes implement the GNU version. This can be overridden for individual |
| 639 | functions with the __gnu_inline__ attribute.</li> |
| 640 | <li>Digraphs are not recognized in c89 mode.</li> |
| 641 | <li>The scope of names defined inside a "for", "if", "switch", "while", or "do" |
| 642 | statement is different. (example: "if ((struct x {int x;}*)0) {}".)</li> |
| 643 | <li>__STDC_VERSION__ is not defined in *89 modes.</li> |
| 644 | <li>"inline" is not recognized as a keyword in c89 mode.</li> |
| 645 | <li>"restrict" is not recognized as a keyword in *89 modes.</li> |
| 646 | <li>Commas are allowed in integer constant expressions in *99 modes.</li> |
| 647 | <li>Arrays which are not lvalues are not implicitly promoted to pointers in |
| 648 | *89 modes.</li> |
| 649 | <li>Some warnings are different.</li> |
| 650 | </ul> |
| 651 | |
| 652 | <p>c94 mode is identical to c89 mode except that digraphs are enabled in |
| 653 | c94 mode (FIXME: And __STDC_VERSION__ should be defined!).</p> |
| 654 | |
| 655 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 656 | <h3 id="c_unimpl_gcc">GCC extensions not implemented yet</h3> |
| 657 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 658 | |
| 659 | <p>clang tries to be compatible with gcc as much as possible, but some gcc |
| 660 | extensions are not implemented yet:</p> |
| 661 | |
| 662 | <ul> |
| 663 | <li>clang does not support __label__ |
| 664 | (<a href="http://llvm.org/bugs/show_bug.cgi?id=3429">bug 3429</a>). This is |
| 665 | a relatively small feature, so it is likely to be implemented relatively |
| 666 | soon.</li> |
| 667 | |
| 668 | <li>clang does not support attributes on function pointers |
| 669 | (<a href="http://llvm.org/bugs/show_bug.cgi?id=2461">bug 2461</a>). This is |
| 670 | a relatively important feature, so it is likely to be implemented relatively |
| 671 | soon.</li> |
| 672 | |
| 673 | <li>clang does not support #pragma weak |
| 674 | (<a href="http://llvm.org/bugs/show_bug.cgi?id=3679">bug 3679</a>). Due to |
| 675 | the uses described in the bug, this is likely to be implemented at some |
| 676 | point, at least partially.</li> |
| 677 | |
| 678 | <li>clang does not support #pragma align |
| 679 | (<a href="http://llvm.org/bugs/show_bug.cgi?id=3811">bug 3811</a>). This is a |
| 680 | relatively small feature, so it is likely to be implemented relatively |
| 681 | soon.</li> |
| 682 | |
| 683 | <li>clang does not support code generation for local variables pinned to |
| 684 | registers (<a href="http://llvm.org/bugs/show_bug.cgi?id=3933">bug 3933</a>). |
| 685 | This is a relatively small feature, so it is likely to be implemented |
| 686 | relatively soon.</li> |
| 687 | |
| 688 | <li>clang does not support decimal floating point types (_Decimal32 and |
| 689 | friends) or fixed-point types (_Fract and friends); nobody has expressed |
| 690 | interest in these features yet, so it's hard to say when they will be |
| 691 | implemented.</li> |
| 692 | |
| 693 | <li>clang does not support nested functions; this is a complex feature which |
| 694 | is infrequently used, so it is unlikely to be implemented anytime soon.</li> |
| 695 | |
| 696 | <li>clang does not support global register variables, this is unlikely |
| 697 | to be implemented soon because it requires additional LLVM backend support. |
| 698 | </li> |
| 699 | |
| 700 | <li>clang does not support static initialization of flexible array |
| 701 | members. This appears to be a rarely used extension, but could be |
| 702 | implemented pending user demand.</li> |
| 703 | |
| 704 | <li>clang does not support __builtin_va_arg_pack/__builtin_va_arg_pack_len. |
| 705 | This is used rarely, but in some potentially interesting places, like the |
| 706 | glibc headers, so it may be implemented pending user demand. Note that |
| 707 | because clang pretends to be like GCC 4.2, and this extension was introduced |
| 708 | in 4.3, the glibc headers will not try to use this extension with clang at |
| 709 | the moment.</li> |
| 710 | |
| 711 | <li>clang does not support the gcc extension for forward-declaring function |
| 712 | parameters; this has not showed up in any real-world code yet, though, so it |
| 713 | might never be implemented.</li> |
| 714 | |
| 715 | </ul> |
| 716 | |
| 717 | <p>This is not a complete list; if you find an unsupported extension |
| 718 | missing from this list, please send an e-mail to cfe-dev. This list |
| 719 | currently excludes C++; see <a href="#cxx">C++ Language Features</a>. |
| 720 | Also, this list does not include bugs in mostly-implemented features; please |
| 721 | 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"> |
| 722 | bug tracker</a> for known existing bugs (FIXME: Is there a section for |
| 723 | bug-reporting guidelines somewhere?).</p> |
| 724 | |
| 725 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 726 | <h3 id="c_unsupp_gcc">Intentionally unsupported GCC extensions</h3> |
| 727 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 728 | |
| 729 | <ul> |
| 730 | |
| 731 | <li>clang does not support the gcc extension that allows variable-length arrays |
| 732 | in structures. This is for a few of reasons: one, it is tricky |
| 733 | to implement, two, the extension is completely undocumented, and three, the |
| 734 | extension appears to be rarely used.</li> |
| 735 | |
| 736 | <li>clang does not support duplicate definitions of a function where one is |
| 737 | inline. This complicates clients of the AST which normally can expect there is |
| 738 | at most one definition for each function. Source code using this feature should |
| 739 | be changed to define the inline and out-of-line definitions in separate |
| 740 | translation units.</li> |
| 741 | |
| 742 | <li>clang does not have an equivalent to gcc's "fold"; this means that |
| 743 | clang doesn't accept some constructs gcc might accept in contexts where a |
| 744 | constant expression is required, like "x-x" where x is a variable, or calls |
| 745 | to C library functions like strlen.</li> |
| 746 | |
| 747 | <li>clang does not support multiple alternative constraints in inline asm; this |
| 748 | is an extremely obscure feature which would be complicated to implement |
| 749 | correctly.</li> |
| 750 | |
| 751 | <li>clang does not support __builtin_apply and friends; this extension is |
| 752 | extremely obscure and difficult to implement reliably.</li> |
| 753 | |
| 754 | </ul> |
| 755 | |
| 756 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 757 | <h3 id="c_ms">Microsoft extensions</h3> |
| 758 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 759 | |
| 760 | <p>clang has some experimental support for extensions from |
| 761 | Microsoft Visual C++; to enable it, use the -fms-extensions command-line |
| 762 | option. This is the default for Windows targets. Note that the |
| 763 | support is incomplete; enabling Microsoft extensions will silently drop |
| 764 | certain constructs (including __declspec and Microsoft-style asm statements). |
| 765 | </p> |
| 766 | |
| 767 | <li>clang does not support the Microsoft extension where anonymous |
| 768 | record members can be declared using user defined typedefs.</li> |
| 769 | |
| 770 | <li>clang supports the Microsoft "#pragma pack" feature for |
| 771 | controlling record layout. GCC also contains support for this feature, |
| 772 | however where MSVC and GCC are incompatible clang follows the MSVC |
| 773 | definition.</li> |
| 774 | |
| 775 | <!-- ======================================================================= --> |
| 776 | <h2 id="objc">Objective-C Language Features</h2> |
| 777 | <!-- ======================================================================= --> |
| 778 | |
| 779 | |
| 780 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 781 | <h3 id="objc_incompatibilities">Intentional Incompatibilities with GCC</h3> |
| 782 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 783 | |
| 784 | <p>No cast of super, no lvalue casts.</p> |
| 785 | |
| 786 | |
| 787 | |
| 788 | <!-- ======================================================================= --> |
| 789 | <h2 id="cxx">C++ Language Features</h2> |
| 790 | <!-- ======================================================================= --> |
| 791 | |
| 792 | <p>At this point, Clang C++ is not production-quality and is not recommended for use beyond experimentation. However, Clang C++ support |
| 793 | is under active development and is progressing rapidly. Please see the <a |
| 794 | href="http://clang.llvm.org/cxx_status.html">C++ Status</a> page for details or |
| 795 | ask on the mailing list about how you can help.</p> |
| 796 | |
| 797 | <p>Note that released Clang compilers will refuse to even try to use clang to compile C++ code unless you pass the <tt>-ccc-clang-cxx</tt> option to the driver. To turn on Clang's C++ support, please pass that flag. Clang compilers built from the Subversion trunk enable C++ support by default, and do not require the <tt>-ccc-clang-cxx</tt> flag.</p> |
| 798 | |
| 799 | <!-- ======================================================================= --> |
| 800 | <h2 id="objcxx">Objective C++ Language Features</h2> |
| 801 | <!-- ======================================================================= --> |
| 802 | |
| 803 | <p>At this point, Clang C++ support is not generally useful (and therefore, |
| 804 | neither is Objective-C++). Please see the <a href="#cxx">C++ section</a> for |
| 805 | more information.</p> |
| 806 | |
| 807 | <!-- ======================================================================= --> |
| 808 | <h2 id="target_features">Target-Specific Features and Limitations</h2> |
| 809 | <!-- ======================================================================= --> |
| 810 | |
| 811 | |
| 812 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 813 | <h3 id="target_arch">CPU Architectures Features and Limitations</h3> |
| 814 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 815 | |
| 816 | <!-- ======================== --> |
| 817 | <h4 id="target_arch_x86">X86</h4> |
| 818 | <!-- ======================== --> |
| 819 | <p>The support for X86 (both 32-bit and 64-bit) is considered stable |
| 820 | on Darwin (Mac OS/X), Linux, FreeBSD, and Dragonfly BSD: it has been tested to |
| 821 | correctly compile large C and Objective-C codebases. (FIXME: Anything specific |
| 822 | we want to say here? Possibly mention some LLVM x86 limitations?) |
| 823 | |
| 824 | <!-- ======================== --> |
| 825 | <h4 id="target_arch_arm">ARM</h4> |
| 826 | <!-- ======================== --> |
| 827 | ARM support is mostly feature-complete, but still experimental; it hasn't |
| 828 | undergone significant testing. |
| 829 | |
| 830 | <!-- ======================== --> |
| 831 | <h4 id="target_arch_other">Other platforms</h4> |
| 832 | <!-- ======================== --> |
| 833 | clang currently contains some support for PPC and Sparc; however, significant |
| 834 | pieces of code generation are still missing, and they haven't undergone |
| 835 | significant testing. |
| 836 | |
| 837 | <p>clang contains some support for the embedded PIC16 processor |
| 838 | (FIXME: I haven't been keeping track of this; what should this say?). |
| 839 | |
| 840 | <p>clang contains limited support for the MSP430 embedded processor, but both |
| 841 | the clang support and the LLVM backend support are highly experimental. |
| 842 | |
| 843 | <p>Other platforms are completely unsupported at the moment. Adding the |
| 844 | minimal support needed for parsing and semantic analysis on a new platform |
| 845 | is quite easy; see lib/Basic/Targets.cpp in the clang source tree. This level |
| 846 | of support is also sufficient for conversion to LLVM IR for simple programs. |
| 847 | Proper support for conversion to LLVM IR requires adding code to |
| 848 | lib/CodeGen/CGCall.cpp at the moment; this is likely to change soon, though. |
| 849 | Generating assembly requires a suitable LLVM backend. |
| 850 | |
| 851 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 852 | <h3 id="target_os">Operating System Features and Limitations</h3> |
| 853 | <!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> |
| 854 | |
| 855 | <!-- ======================================= --> |
| 856 | <h4 id="target_os_darwin">Darwin (Mac OS/X)</h4> |
| 857 | <!-- ======================================= --> |
| 858 | |
| 859 | <p>No __thread support, 64-bit ObjC support requires SL tools.</p> |
| 860 | |
| 861 | </div> |
| 862 | </body> |
| 863 | </html> |