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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>
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]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>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]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>
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]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>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]42 <li><a href="#precompiledheaders">Precompiled Headers</a></li>
Mike Stump53664ca2009-12-14 23:53:10 +0000[diff] [blame]43 <li><a href="#codegen">Controlling Code Generation</a></li>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]44 </ul>
45</li>
46<li><a href="#c">C Language Features</a>
47 <ul>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]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>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]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>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]62</li>
63<li><a href="#objcxx">Objective C++ Language Features</a>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]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>
Eli Friedman3b658d32009-06-08 05:12:39 +0000[diff] [blame]70 <li><a href="#target_arch_arm">ARM</a></li>
71 <li><a href="#target_arch_other">Other platforms</a></li>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]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
91languages, aiming to be the best in class implementation of these languages.
92Clang builds on the LLVM optimizer and code generator, allowing it to provide
93high-quality optimization and code generation support for many targets. For
94more general information, please see the <a href="http://clang.llvm.org">Clang
95Web 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
98an end-user, documenting the supported features, command line options, etc. If
99you are interested in using Clang to build a tool that processes code, please
100see <a href="InternalsManual.html">the Clang Internals Manual</a>. If you are
101interested in the <a href="http://clang.llvm.org/StaticAnalysis.html">Clang
102Static Analyzer</a>, please see its web page.</p>
103
104<p>Clang is designed to support the C family of programming languages, which
105includes <a href="#c">C</a>, <a href="#objc">Objective-C</a>, <a
106href="#cxx">C++</a>, and <a href="#objcxx">Objective-C++</a> as well as many
107dialects of those. For language-specific information, please see the
108corresponding language specific section:</p>
109
110<ul>
111<li><a href="#c">C Language</a>: K&amp;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
120broad variety of language extensions, which are documented in the corresponding
121language section. These extensions are provided to be compatible with the GCC,
122Microsoft, and other popular compilers as well as to improve functionality
123through Clang-specific features. The Clang driver and language features are
124intentionally designed to be as compatible with the GNU GCC compiler as
125reasonably 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
129that depend on what CPU architecture or operating system is being compiled for.
Douglas Gregorcd5a5052009-11-09 15:15:41 +0000[diff] [blame]130Please see the <a href="#target_features">Target-Specific Features and
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]131Limitations</a> section for more details.</p>
132
133<p>The rest of the introduction introduces some basic <a
134href="#terminology">compiler terminology</a> that is used throughout this manual
135and contains a basic <a href="#basicusage">introduction to using Clang</a>
136as 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>
151compile + link
152
153compile then link
154
155debug info
156
157enabling optimizations
158
159picking a language to use, defaults to C99 by default. Autosenses based on
160extension.
161
162using a makefile
163</p>
164
165
166<!-- ======================================================================= -->
167<h2 id="commandline">Command Line Options</h2>
168<!-- ======================================================================= -->
169
170<p>
171This section is generally an index into other sections. It does not go into
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]172depth on the ones that are covered by other sections. However, the first part
173introduces the language selection and other high level options like -c, -g, etc.
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]174</p>
175
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]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
197users that first come to Clang. However, different people have different
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]198preferences, and sometimes Clang is driven by another program that wants to
199parse simple and consistent output, not a person. For these cases, Clang
200provides a wide range of options to control the exact output format of the
201diagnostics that it generates.</p>
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]202
203<dl>
204
205<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
206<dt id="opt_fshow-column"><b>-f[no-]show-column</b>: Print column number in
207diagnostic.</dt>
208<dd>This option, which defaults to on, controls whether or not Clang prints the
209column number of a diagnostic. For example, when this is enabled, Clang will
210print 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
220column number.</p>
221</dd>
222
223<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
224<dt id="opt_fshow-source-location"><b>-f[no-]show-source-location</b>: Print
225source file/line/column information in diagnostic.</dt>
226<dd>This option, which defaults to on, controls whether or not Clang prints the
227filename, line number and column number of a diagnostic. For example,
228when 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
242line and ranges from source code in diagnostic.</dt>
243<dd>This option, which defaults to on, controls whether or not Clang prints the
244source line, source ranges, and caret when emitting a diagnostic. For example,
245when 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>:
264Enable <tt>[-Woption]</tt> information in diagnostic line.</dt>
265<dd>This option, which defaults to on,
266controls whether or not Clang prints the associated <A
267href="#cl_diag_warning_groups">warning group</a> option name when outputting
268a 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
278the [<a href="#opt_Wextra-tokens">-Wextra-tokens</a>] information in the
279diagnostic. This information tells you the flag needed to enable or disable the
280diagnostic, either from the command line or through <a
281href="#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>:
286Enable "FixIt" information in the diagnostics output.</dt>
287<dd>This option, which defaults to on, controls whether or not Clang prints the
288information on how to fix a specific diagnostic underneath it when it knows.
289For 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
299the "//" line at the end of the message. This information is useful for users
300who may not understand what is wrong, but can be confusing for machine
301parsing.</p>
302</dd>
303
304<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
Chris Lattner2a9cc232009-04-21 05:35:32 +0000[diff] [blame]305<dt id="opt_fdiagnostics-print-source-range-info">
306<b>-f[no-]diagnostics-print-source-range-info</b>:
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]307Print machine parsable information about source ranges.</dt>
308<dd>This option, which defaults to off, controls whether or not Clang prints
309information about source ranges in a machine parsable format after the
310file/line/column number information. The information is a simple sequence of
311brace enclosed ranges, where each range lists the start and end line/column
312locations. For example, in this output:</p>
313
314<pre>
315exprs.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
Chris Lattner2a9cc232009-04-21 05:35:32 +0000[diff] [blame]320<p>The {}'s are generated by -fdiagnostics-print-source-range-info.</p>
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]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
343the 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
352by 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
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]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
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]369<p>Clang provides a number of ways to control which code constructs cause it to
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]370emit errors and warning messages, and how they are displayed to the console.</p>
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]371
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]372<h4 id="diagnostics_display">Controlling How Clang Displays Diagnostics</h4>
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]373
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]374<p>When Clang emits a diagnostic, it includes rich information in the output,
375and gives you fine-grain control over which information is printed. Clang has
376the ability to print this information, and these are the options that control
377it:</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
Chris Lattner2a9cc232009-04-21 05:35:32 +0000[diff] [blame]398 href="opt_fdiagnostics-print-source-range-info">-fdiagnostics-print-source-range-info</a>].</li>
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]399</ol></p>
400
401<p>For more information please see <a href="#cl_diag_formatting">Formatting of
402Diagnostics</a>.</p>
403
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]404<h4 id="diagnostics_mappings">Diagnostic Mappings</h4>
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]405
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]406<p>All diagnostics are mapped into one of these 5 classes:</p>
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]407
408<p>
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]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>
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]416
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]417<h4 id="diagnostics_commandline">Controlling Diagnostics via Command Line Flags</h4>
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]418
419<p>-W flags, -pedantic, etc</p>
420
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]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
424pragmas in the source code. This is useful for turning off specific warnings
425in a section of source code. Clang supports GCC's pragma for compatibility
426with 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.
429Warnings may be set to ignored, warning, error, or fatal. The following
430example 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
437also allows you to push and pop the current warning state. This is particularly
438useful when writing a header file that will be compiled by other people, because
439you 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
443diagnostics return to whatever state had previously existed.</p>
444
445<pre>
446#pragma clang diagnostic push
447#pragma clang diagnostic ignored "-Wmultichar"
448
449char 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
455the compiler, regardless of how it was set. That means that it is possible to
456use push and pop around GCC compatible diagnostics and Clang will push and pop
457them appropriately, while GCC will ignore the pushes and pops as unknown
458pragmas. It should be noted that while Clang supports the GCC pragma, Clang and
459GCC do not support the exact same set of warnings, so even when using GCC
460compatible #pragmas there is no guarantee that they will have identical behaviour
461on both compilers. </p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]462
463<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
464<h3 id="precompiledheaders">Precompiled Headers</h3>
465<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
466
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]467<p><a href="http://en.wikipedia.org/wiki/Precompiled_header">Precompiled
468headers</a> are a general approach employed by many compilers to reduce
469compilation time. The underlying motivation of the approach is that it is
470common for the same (and often large) header files to be included by
471multiple source files. Consequently, compile times can often be greatly improved
472by caching some of the (redundant) work done by a compiler to process headers.
473Precompiled header files, which represent one of many ways to implement
474this optimization, are literally files that represent an on-disk cache that
475contains the vital information necessary to reduce some of the work
476needed to process a corresponding header file. While details of precompiled
477headers vary between compilers, precompiled headers have been shown to be a
478highly effective at speeding up program compilation on systems with very large
479system headers (e.g., Mac OS/X).</p>
480
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]481<h4>Generating a PCH File</h4>
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]482
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]483<p>To generate a PCH file using Clang, one invokes Clang with
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]484the <b><tt>-x <i>&lt;language&gt;</i>-header</tt></b> option. This mirrors the
485interface in GCC for generating PCH files:</p>
486
487<pre>
488 $ gcc -x c-header test.h -o test.h.gch
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]489 $ clang -x c-header test.h -o test.h.pch
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]490</pre>
491
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]492<h4>Using a PCH File</h4>
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]493
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]494<p>A PCH file can then be used as a prefix header when a
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]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
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]501<p>The <tt>clang</tt> driver will first check if a PCH file for <tt>test.h</tt>
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]502is available; if so, the contents of <tt>test.h</tt> (and the files it includes)
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]503will be processed from the PCH file. Otherwise, Clang falls back to
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]504directly processing the content of <tt>test.h</tt>. This mirrors the behavior of
505GCC.</p>
506
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]507<p><b>NOTE:</b> Clang does <em>not</em> automatically use PCH files
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]508for headers that are directly included within a source file. For example:</p>
509
510<pre>
Chris Lattnere42ec542009-06-13 20:35:58 +0000[diff] [blame]511 $ clang -x c-header test.h -o test.h.pch
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]512 $ cat test.c
513 #include "test.h"
514 $ clang test.c -o test
515</pre>
516
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]517<p>In this example, <tt>clang</tt> will not automatically use the PCH file for
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]518<tt>test.h</tt> since <tt>test.h</tt> was included directly in the source file
519and not specified on the command line using <tt>-include</tt>.</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]520
Douglas Gregore650c8c2009-07-07 00:12:59 +0000[diff] [blame]521<h4>Relocatable PCH Files</h4>
522<p>It is sometimes necessary to build a precompiled header from headers that
523are not yet in their final, installed locations. For example, one might build a
524precompiled header within the build tree that is then meant to be installed
525alongside the headers. Clang permits the creation of "relocatable" precompiled
526headers, which are built with a given path (into the build directory) and can
527later be used from an installed location.</p>
528
529<p>To build a relocatable precompiled header, place your headers into a
530subdirectory whose structure mimics the installed location. For example, if you
531want to build a precompiled header for the header <code>mylib.h</code> that
532will 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
534that subdirectory. If <code>mylib.h</code> depends on other headers, then
535they can be stored within <code>build/usr/include</code> in a way that mimics
536the installed location.</p>
537
538<p>Building a relocatable precompiled header requires two additional arguments.
539First, pass the <code>--relocatable-pch</code> flag to indicate that the
540resulting PCH file should be relocatable. Second, pass
541<code>-isysroot /path/to/build</code>, which makes all includes for your
542library 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
549file are found from the system header root. For example, <code>mylib.h</code>
550can be found in <code>/usr/include/mylib.h</code>. If the headers are installed
551in some other system root, the <code>-isysroot</code> option can be used provide
552a 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
558of cases where the compilation environment is tightly controlled and the
559precompiled header cannot be generated after headers have been installed.
560Relocatable precompiled headers also have some performance impact, because
561the difference in location between the header locations at PCH build time vs.
562at the time of PCH use requires one of the PCH optimizations,
563<code>stat()</code> caching, to be disabled. However, this change is only
564likely to affect PCH files that reference a large number of headers.</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]565
Mike Stump53664ca2009-12-14 23:53:10 +0000[diff] [blame]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
574on runtime code generation to check for undefined behavior.</dt>
575
576<dd>This option, which defaults to off, controls whether or not Clang
577adds runtime checks for undefined runtime behavior. If the check fails,
578<tt>__builtin_trap()</tt> is used to indicate failure.
579The checks are:
580<p>
581<li>Subscripting where the static type of one operand is decayed from an
582 array type and the other operand is greater than the size of the array or
583 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>
Mike Stump8f6a3ed2009-12-16 03:18:14 +0000[diff] [blame^]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.
Mike Stump53664ca2009-12-14 23:53:10 +0000[diff] [blame]590</p>
591</dd>
592
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]593<!-- ======================================================================= -->
594<h2 id="c">C Language Features</h2>
595<!-- ======================================================================= -->
596
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]597<p>The support for standard C in clang is feature-complete except for the C99
598floating-point pragmas.</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]599
600<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]601<h3 id="c_ext">Extensions supported by clang</h3>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]602<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
603
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]604<p>See <a href="LanguageExtensions.html">clang language extensions</a>.</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]605
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]606<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
607<h3 id="c_modes">Differences between various standard modes</h3>
608<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]609
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]610<p>clang supports the -std option, which changes what language mode clang uses.
611The supported modes for C are c89, gnu89, c94, c99, gnu99 and various aliases
612for those modes. If no -std option is specified, clang defaults to gnu99 mode.
613</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]614
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]615<p>Differences between all c* and gnu* modes:</p>
616<ul>
617<li>c* modes define "__STRICT_ANSI__".</li>
Eli Friedman26fa0ed2009-05-27 23:02:57 +0000[diff] [blame]618<li>Target-specific defines not prefixed by underscores, like "linux", are
619defined in gnu* modes.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]620<li>Trigraphs default to being off in gnu* modes; they can be enabled by the
621-trigraphs option.</li>
622<li>The parser recognizes "asm" and "typeof" as keywords in gnu* modes; the
623variants "__asm__" and "__typeof__" are recognized in all modes.</li>
Eli Friedmanb0ac5452009-05-16 23:17:30 +0000[diff] [blame]624<li>The Apple "blocks" extension is recognized by default in gnu* modes
625on some platforms; it can be enabled in any mode with the "-fblocks"
626option.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]627</ul>
628
629<p>Differences between *89 and *99 modes:</p>
630<ul>
631<li>The *99 modes default to implementing "inline" as specified in C99, while
632the *89 modes implement the GNU version. This can be overridden for individual
633functions with the __gnu_inline__ attribute.</li>
Eli Friedman26fa0ed2009-05-27 23:02:57 +0000[diff] [blame]634<li>Digraphs are not recognized in c89 mode.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]635<li>The scope of names defined inside a "for", "if", "switch", "while", or "do"
636statement is different. (example: "if ((struct x {int x;}*)0) {}".)</li>
637<li>__STDC_VERSION__ is not defined in *89 modes.</li>
Eli Friedman26fa0ed2009-05-27 23:02:57 +0000[diff] [blame]638<li>"inline" is not recognized as a keyword in c89 mode.</li>
639<li>"restrict" is not recognized as a keyword in *89 modes.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]640<li>Commas are allowed in integer constant expressions in *99 modes.</li>
641<li>Arrays which are not lvalues are not implicitly promoted to pointers in
642*89 modes.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]643<li>Some warnings are different.</li>
644</ul>
645
646<p>c94 mode is identical to c89 mode except that digraphs are enabled in
647c94 mode (FIXME: And __STDC_VERSION__ should be defined!).</p>
648
649<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
650<h3 id="c_unimpl_gcc">GCC extensions not implemented yet</h3>
651<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
652
653<p>clang tries to be compatible with gcc as much as possible, but some gcc
654extensions are not implemented yet:</p>
655
656<ul>
657<li>clang does not support __label__
658(<a href="http://llvm.org/bugs/show_bug.cgi?id=3429">bug 3429</a>). This is
659a relatively small feature, so it is likely to be implemented relatively
660soon.</li>
661
662<li>clang does not support attributes on function pointers
663(<a href="http://llvm.org/bugs/show_bug.cgi?id=2461">bug 2461</a>). This is
664a relatively important feature, so it is likely to be implemented relatively
665soon.</li>
666
667<li>clang does not support #pragma weak
Eli Friedman4da92552009-06-02 08:21:31 +0000[diff] [blame]668(<a href="http://llvm.org/bugs/show_bug.cgi?id=3679">bug 3679</a>). Due to
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]669the uses described in the bug, this is likely to be implemented at some
670point, at least partially.</li>
671
672<li>clang does not support #pragma align
Eli Friedman4da92552009-06-02 08:21:31 +0000[diff] [blame]673(<a href="http://llvm.org/bugs/show_bug.cgi?id=3811">bug 3811</a>). This is a
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]674relatively small feature, so it is likely to be implemented relatively
675soon.</li>
676
Eli Friedman4da92552009-06-02 08:21:31 +0000[diff] [blame]677<li>clang does not support code generation for local variables pinned to
678registers (<a href="http://llvm.org/bugs/show_bug.cgi?id=3933">bug 3933</a>).
679This is a relatively small feature, so it is likely to be implemented
680relatively soon.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]681
682<li>clang does not support decimal floating point types (_Decimal32 and
683friends) or fixed-point types (_Fract and friends); nobody has expressed
684interest in these features yet, so it's hard to say when they will be
685implemented.</li>
686
687<li>clang does not support nested functions; this is a complex feature which
688is infrequently used, so it is unlikely to be implemented anytime soon.</li>
689
Daniel Dunbar05fa6292009-05-12 23:12:07 +0000[diff] [blame]690<li>clang does not support global register variables, this is unlikely
Eli Friedmanca130bd2009-06-12 20:04:25 +0000[diff] [blame]691to be implemented soon because it requires additional LLVM backend support.
692</li>
Daniel Dunbar05fa6292009-05-12 23:12:07 +0000[diff] [blame]693
694<li>clang does not support static initialization of flexible array
695members. This appears to be a rarely used extension, but could be
696implemented pending user demand.</li>
697
Eli Friedmanca130bd2009-06-12 20:04:25 +0000[diff] [blame]698<li>clang does not support __builtin_va_arg_pack/__builtin_va_arg_pack_len.
699This is used rarely, but in some potentially interesting places, like the
700glibc headers, so it may be implemented pending user demand. Note that
701because clang pretends to be like GCC 4.2, and this extension was introduced
702in 4.3, the glibc headers will not try to use this extension with clang at
703the moment.</li>
704
705<li>clang does not support the gcc extension for forward-declaring function
706parameters; this has not showed up in any real-world code yet, though, so it
707might never be implemented.</li>
708
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]709</ul>
710
711<p>This is not a complete list; if you find an unsupported extension
712missing from this list, please send an e-mail to cfe-dev. This list
713currently excludes C++; see <a href="#cxx">C++ Language Features</a>.
714Also, this list does not include bugs in mostly-implemented features; please
715see 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">
716bug tracker</a> for known existing bugs (FIXME: Is there a section for
717bug-reporting guidelines somewhere?).</p>
718
719<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
720<h3 id="c_unsupp_gcc">Intentionally unsupported GCC extensions</h3>
721<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
722
Eli Friedman0b326002009-06-12 20:11:05 +0000[diff] [blame]723<ul>
724
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]725<li>clang does not support the gcc extension that allows variable-length arrays
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]726in structures. This is for a few of reasons: one, it is tricky
727to implement, two, the extension is completely undocumented, and three, the
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]728extension appears to be rarely used.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]729
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]730<li>clang does not support duplicate definitions of a function where one is
Daniel Dunbarc5f928c2009-06-04 18:37:52 +0000[diff] [blame]731inline. This complicates clients of the AST which normally can expect there is
732at most one definition for each function. Source code using this feature should
733be changed to define the inline and out-of-line definitions in separate
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]734translation units.</li>
Daniel Dunbarc5f928c2009-06-04 18:37:52 +0000[diff] [blame]735
Eli Friedmanca130bd2009-06-12 20:04:25 +0000[diff] [blame]736<li>clang does not have an equivalent to gcc's "fold"; this means that
737clang doesn't accept some constructs gcc might accept in contexts where a
738constant expression is required, like "x-x" where x is a variable, or calls
739to C library functions like strlen.</li>
740
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]741<li>clang does not support multiple alternative constraints in inline asm; this
Eli Friedmanca130bd2009-06-12 20:04:25 +0000[diff] [blame]742is an extremely obscure feature which would be complicated to implement
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]743correctly.</li>
Eli Friedmanca130bd2009-06-12 20:04:25 +0000[diff] [blame]744
745<li>clang does not support __builtin_apply and friends; this extension is
746extremely obscure and difficult to implement reliably.</li>
747
Eli Friedman0b326002009-06-12 20:11:05 +0000[diff] [blame]748</ul>
749
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]750<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
751<h3 id="c_ms">Microsoft extensions</h3>
752<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
753
754<p>clang has some experimental support for extensions from
755Microsoft Visual C++; to enable it, use the -fms-extensions command-line
Eli Friedmana1821b52009-06-08 06:21:03 +0000[diff] [blame]756option. This is the default for Windows targets. Note that the
757support is incomplete; enabling Microsoft extensions will silently drop
758certain constructs (including __declspec and Microsoft-style asm statements).
759</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]760
Daniel Dunbar05fa6292009-05-12 23:12:07 +0000[diff] [blame]761<li>clang does not support the Microsoft extension where anonymous
Daniel Dunbar869e0542009-05-13 00:23:53 +0000[diff] [blame]762record members can be declared using user defined typedefs.</li>
Daniel Dunbar05fa6292009-05-12 23:12:07 +0000[diff] [blame]763
Daniel Dunbar9375ed12009-05-13 21:40:49 +0000[diff] [blame]764<li>clang supports the Microsoft "#pragma pack" feature for
765controlling record layout. GCC also contains support for this feature,
766however where MSVC and GCC are incompatible clang follows the MSVC
767definition.</li>
768
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]769<!-- ======================================================================= -->
770<h2 id="objc">Objective-C Language Features</h2>
771<!-- ======================================================================= -->
772
773
774<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
775<h3 id="objc_incompatibilities">Intentional Incompatibilities with GCC</h3>
776<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
777
778<p>No cast of super, no lvalue casts.</p>
779
780
781
782<!-- ======================================================================= -->
783<h2 id="cxx">C++ Language Features</h2>
784<!-- ======================================================================= -->
785
786<p>At this point, Clang C++ is not generally useful. However, Clang C++ support
787is under active development and is progressing rapidly. Please see the <a
788href="http://clang.llvm.org/cxx_status.html">C++ Status</a> page for details or
789ask on the mailing list about how you can help.</p>
790
791<p>Note that the clang driver will refuse to even try to use clang to compile
792C++ code unless you pass the <tt>-ccc-clang-cxx</tt> option to the driver. If
793you really want to play with Clang's C++ support, please pass that flag. </p>
794
795<!-- ======================================================================= -->
796<h2 id="objcxx">Objective C++ Language Features</h2>
797<!-- ======================================================================= -->
798
799<p>At this point, Clang C++ support is not generally useful (and therefore,
800neither is Objective-C++). Please see the <a href="#cxx">C++ section</a> for
801more information.</p>
802
803<!-- ======================================================================= -->
804<h2 id="target_features">Target-Specific Features and Limitations</h2>
805<!-- ======================================================================= -->
806
807
808<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
809<h3 id="target_arch">CPU Architectures Features and Limitations</h3>
810<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
811
812<!-- ======================== -->
813<h4 id="target_arch_x86">X86</h4>
814<!-- ======================== -->
Eli Friedman3b658d32009-06-08 05:12:39 +0000[diff] [blame]815<p>The support for X86 (both 32-bit and 64-bit) is considered stable
816on Darwin (Mac OS/X), Linux, FreeBSD, and Dragonfly BSD: it has been tested to
817correctly compile large C and Objective-C codebases. (FIXME: Anything specific
818we want to say here? Possibly mention some LLVM x86 limitations?)
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]819
Eli Friedman3b658d32009-06-08 05:12:39 +0000[diff] [blame]820<!-- ======================== -->
821<h4 id="target_arch_arm">ARM</h4>
822<!-- ======================== -->
823ARM support is mostly feature-complete, but still experimental; it hasn't
824undergone significant testing.
825
826<!-- ======================== -->
827<h4 id="target_arch_other">Other platforms</h4>
828<!-- ======================== -->
829clang currently contains some support for PPC and Sparc; however, significant
830pieces of code generation are still missing, and they haven't undergone
831significant testing.
832
833<p>clang contains some support for the embedded PIC16 processor
834(FIXME: I haven't been keeping track of this; what should this say?).
835
836<p>clang contains limited support for the MSP430 embedded processor, but both
837the clang support and the LLVM backend support are highly experimental.
838
839<p>Other platforms are completely unsupported at the moment. Adding the
840minimal support needed for parsing and semantic analysis on a new platform
841is quite easy; see lib/Basic/Targets.cpp in the clang source tree. This level
842of support is also sufficient for conversion to LLVM IR for simple programs.
843Proper support for conversion to LLVM IR requires adding code to
844lib/CodeGen/CGCall.cpp at the moment; this is likely to change soon, though.
845Generating assembly requires a suitable LLVM backend.
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]846
847<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
848<h3 id="target_os">Operating System Features and Limitations</h3>
849<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
850
851<!-- ======================================= -->
852<h4 id="target_os_darwin">Darwin (Mac OS/X)</h4>
853<!-- ======================================= -->
854
855<p>No __thread support, 64-bit ObjC support requires SL tools.</p>
856
857</div>
858</body>
859</html>