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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>
43 </ul>
44</li>
45<li><a href="#c">C Language Features</a>
46 <ul>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]47 <li><a href="#c_ext">Extensions supported by clang</a></li>
48 <li><a href="#c_modes">Differences between various standard modes</a></li>
49 <li><a href="#c_unimpl_gcc">GCC extensions not implemented yet</a></li>
50 <li><a href="#c_unsupp_gcc">Intentionally unsupported GCC extensions</a></li>
51 <li><a href="#c_ms">Microsoft extensions</a></li>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]52 </ul>
53</li>
54<li><a href="#objc">Objective-C Language Features</a>
55 <ul>
56 <li><a href="#objc_incompatibilities">Intentional Incompatibilities with
57 GCC</a></li>
58 </ul>
59</li>
60<li><a href="#cxx">C++ Language Features</a>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]61</li>
62<li><a href="#objcxx">Objective C++ Language Features</a>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]63</li>
64<li><a href="#target_features">Target-Specific Features and Limitations</a>
65 <ul>
66 <li><a href="#target_arch">CPU Architectures Features and Limitations</a>
67 <ul>
68 <li><a href="#target_arch_x86">X86</a></li>
Eli Friedman3b658d32009-06-08 05:12:39 +0000[diff] [blame]69 <li><a href="#target_arch_arm">ARM</a></li>
70 <li><a href="#target_arch_other">Other platforms</a></li>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]71 </ul>
72 </li>
73 <li><a href="#target_os">Operating System Features and Limitations</a>
74 <ul>
75 <li><a href="#target_os_darwin">Darwin (Mac OS/X)</a></li>
76 <li>Linux, etc.</li>
77 </ul>
78
79 </li>
80 </ul>
81</li>
82</ul>
83
84
85<!-- ======================================================================= -->
86<h2 id="intro">Introduction</h2>
87<!-- ======================================================================= -->
88
89<p>The Clang Compiler is an open-source compiler for the C family of programming
90languages, aiming to be the best in class implementation of these languages.
91Clang builds on the LLVM optimizer and code generator, allowing it to provide
92high-quality optimization and code generation support for many targets. For
93more general information, please see the <a href="http://clang.llvm.org">Clang
94Web Site</a> or the <a href="http://llvm.org">LLVM Web Site</a>.</p>
95
96<p>This document describes important notes about using Clang as a compiler for
97an end-user, documenting the supported features, command line options, etc. If
98you are interested in using Clang to build a tool that processes code, please
99see <a href="InternalsManual.html">the Clang Internals Manual</a>. If you are
100interested in the <a href="http://clang.llvm.org/StaticAnalysis.html">Clang
101Static Analyzer</a>, please see its web page.</p>
102
103<p>Clang is designed to support the C family of programming languages, which
104includes <a href="#c">C</a>, <a href="#objc">Objective-C</a>, <a
105href="#cxx">C++</a>, and <a href="#objcxx">Objective-C++</a> as well as many
106dialects of those. For language-specific information, please see the
107corresponding language specific section:</p>
108
109<ul>
110<li><a href="#c">C Language</a>: K&amp;R C, ANSI C89, ISO C90, ISO C94
111 (C89+AMD1), ISO C99 (+TC1, TC2, TC3). </li>
112<li><a href="#objc">Objective-C Language</a>: ObjC 1, ObjC 2, ObjC 2.1, plus
113 variants depending on base language.</li>
114<li><a href="#cxx">C++ Language Features</a></li>
115<li><a href="#objcxx">Objective C++ Language</a></li>
116</ul>
117
118<p>In addition to these base languages and their dialects, Clang supports a
119broad variety of language extensions, which are documented in the corresponding
120language section. These extensions are provided to be compatible with the GCC,
121Microsoft, and other popular compilers as well as to improve functionality
122through Clang-specific features. The Clang driver and language features are
123intentionally designed to be as compatible with the GNU GCC compiler as
124reasonably possible, easing migration from GCC to Clang. In most cases, code
125"just works".</p>
126
127<p>In addition to language specific features, Clang has a variety of features
128that depend on what CPU architecture or operating system is being compiled for.
129Please see the <a href="target_features">Target-Specific Features and
130Limitations</a> section for more details.</p>
131
132<p>The rest of the introduction introduces some basic <a
133href="#terminology">compiler terminology</a> that is used throughout this manual
134and contains a basic <a href="#basicusage">introduction to using Clang</a>
135as a command line compiler.</p>
136
137<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
138<h3 id="terminology">Terminology</h3>
139<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
140
141<p>Front end, parser, backend, preprocessor, undefined behavior, diagnostic,
142 optimizer</p>
143
144<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
145<h3 id="basicusage">Basic Usage</h3>
146<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
147
148<p>Intro to how to use a C compiler for newbies.</p>
149<p>
150compile + link
151
152compile then link
153
154debug info
155
156enabling optimizations
157
158picking a language to use, defaults to C99 by default. Autosenses based on
159extension.
160
161using a makefile
162</p>
163
164
165<!-- ======================================================================= -->
166<h2 id="commandline">Command Line Options</h2>
167<!-- ======================================================================= -->
168
169<p>
170This section is generally an index into other sections. It does not go into
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]171depth on the ones that are covered by other sections. However, the first part
172introduces the language selection and other high level options like -c, -g, etc.
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]173</p>
174
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]175
176<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
177<h3 id="cl_diagnostics">Options to Control Error and Warning Messages</h3>
178<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
179
180<p><b>-Werror</b>: Turn warnings into errors.</p>
181<p><b>-Werror=foo</b>: Turn warning "foo" into an error.</p>
182<p><b>-Wno-error=foo</b>: Turn warning "foo" into an warning even if -Werror is
183 specified.</p>
184<p><b>-Wfoo</b>: Enable warning foo</p>
185<p><b>-Wno-foo</b>: Disable warning foo</p>
186<p><b>-w</b>: Disable all warnings.</p>
187<p><b>-pedantic</b>: Warn on language extensions.</p>
188<p><b>-pedantic-errors</b>: Error on language extensions.</p>
189<p><b>-Wsystem-headers</b>: Enable warnings from system headers.</p>
190
191<!-- ================================================= -->
192<h4 id="cl_diag_formatting">Formatting of Diagnostics</h4>
193<!-- ================================================= -->
194
195<p>Clang aims to produce beautiful diagnostics by default, particularly for new
196users that first come to Clang. However, different people have different
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]197preferences, and sometimes Clang is driven by another program that wants to
198parse simple and consistent output, not a person. For these cases, Clang
199provides a wide range of options to control the exact output format of the
200diagnostics that it generates.</p>
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]201
202<dl>
203
204<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
205<dt id="opt_fshow-column"><b>-f[no-]show-column</b>: Print column number in
206diagnostic.</dt>
207<dd>This option, which defaults to on, controls whether or not Clang prints the
208column number of a diagnostic. For example, when this is enabled, Clang will
209print something like:</p>
210
211<pre>
212 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
213 #endif bad
214 ^
215 //
216</pre>
217
218<p>When this is disabled, Clang will print "test.c:28: warning..." with no
219column number.</p>
220</dd>
221
222<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
223<dt id="opt_fshow-source-location"><b>-f[no-]show-source-location</b>: Print
224source file/line/column information in diagnostic.</dt>
225<dd>This option, which defaults to on, controls whether or not Clang prints the
226filename, line number and column number of a diagnostic. For example,
227when this is enabled, Clang will print something like:</p>
228
229<pre>
230 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
231 #endif bad
232 ^
233 //
234</pre>
235
236<p>When this is disabled, Clang will not print the "test.c:28:8: " part.</p>
237</dd>
238
239<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
240<dt id="opt_fcaret-diagnostics"><b>-f[no-]caret-diagnostics</b>: Print source
241line and ranges from source code in diagnostic.</dt>
242<dd>This option, which defaults to on, controls whether or not Clang prints the
243source line, source ranges, and caret when emitting a diagnostic. For example,
244when this is enabled, Clang will print something like:</p>
245
246<pre>
247 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
248 #endif bad
249 ^
250 //
251</pre>
252
253<p>When this is disabled, Clang will just print:</p>
254
255<pre>
256 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
257</pre>
258
259</dd>
260
261<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
262<dt id="opt_fdiagnostics-show-option"><b>-f[no-]diagnostics-show-option</b>:
263Enable <tt>[-Woption]</tt> information in diagnostic line.</dt>
264<dd>This option, which defaults to on,
265controls whether or not Clang prints the associated <A
266href="#cl_diag_warning_groups">warning group</a> option name when outputting
267a warning diagnostic. For example, in this output:</p>
268
269<pre>
270 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
271 #endif bad
272 ^
273 //
274</pre>
275
276<p>Passing <b>-fno-diagnostics-show-option</b> will prevent Clang from printing
277the [<a href="#opt_Wextra-tokens">-Wextra-tokens</a>] information in the
278diagnostic. This information tells you the flag needed to enable or disable the
279diagnostic, either from the command line or through <a
280href="#pragma_GCC_diagnostic">#pragma GCC diagnostic</a>.</dd>
281
282
283<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
284<dt id="opt_fdiagnostics-fixit-info"><b>-f[no-]diagnostics-fixit-info</b>:
285Enable "FixIt" information in the diagnostics output.</dt>
286<dd>This option, which defaults to on, controls whether or not Clang prints the
287information on how to fix a specific diagnostic underneath it when it knows.
288For example, in this output:</p>
289
290<pre>
291 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
292 #endif bad
293 ^
294 //
295</pre>
296
297<p>Passing <b>-fno-diagnostics-fixit-info</b> will prevent Clang from printing
298the "//" line at the end of the message. This information is useful for users
299who may not understand what is wrong, but can be confusing for machine
300parsing.</p>
301</dd>
302
303<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
Chris Lattner2a9cc232009-04-21 05:35:32 +0000[diff] [blame]304<dt id="opt_fdiagnostics-print-source-range-info">
305<b>-f[no-]diagnostics-print-source-range-info</b>:
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]306Print machine parsable information about source ranges.</dt>
307<dd>This option, which defaults to off, controls whether or not Clang prints
308information about source ranges in a machine parsable format after the
309file/line/column number information. The information is a simple sequence of
310brace enclosed ranges, where each range lists the start and end line/column
311locations. For example, in this output:</p>
312
313<pre>
314exprs.c:47:15:{47:8-47:14}{47:17-47:24}: error: invalid operands to binary expression ('int *' and '_Complex float')
315 P = (P-42) + Gamma*4;
316 ~~~~~~ ^ ~~~~~~~
317</pre>
318
Chris Lattner2a9cc232009-04-21 05:35:32 +0000[diff] [blame]319<p>The {}'s are generated by -fdiagnostics-print-source-range-info.</p>
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]320</dd>
321
322
323</dl>
324
325
326
327
328<!-- ===================================================== -->
329<h4 id="cl_diag_warning_groups">Individual Warning Groups</h4>
330<!-- ===================================================== -->
331
332<p>TODO: Generate this from tblgen. Define one anchor per warning group.</p>
333
334
335<dl>
336
337
338<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
339<dt id="opt_Wextra-tokens"><b>-Wextra-tokens</b>: Warn about excess tokens at
340 the end of a preprocessor directive.</dt>
341<dd>This option, which defaults to on, enables warnings about extra tokens at
342the end of preprocessor directives. For example:</p>
343
344<pre>
345 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
346 #endif bad
347 ^
348</pre>
349
350<p>These extra tokens are not strictly conforming, and are usually best handled
351by commenting them out.</p>
352
353<p>This option is also enabled by <a href="">-Wfoo</a>, <a href="">-Wbar</a>,
354 and <a href="">-Wbaz</a>.</p>
355</dd>
356
357</dl>
358
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]359<!-- ======================================================================= -->
360<h2 id="general_features">Language and Target-Independent Features</h2>
361<!-- ======================================================================= -->
362
363
364<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
365<h3 id="diagnostics">Controlling Errors and Warnings</h3>
366<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
367
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]368<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]369emit errors and warning messages, and how they are displayed to the console.</p>
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]370
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]371<h4 id="diagnostics_display">Controlling How Clang Displays Diagnostics</h4>
Chris Lattner65a795b2009-04-20 06:00:23 +0000[diff] [blame]372
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]373<p>When Clang emits a diagnostic, it includes rich information in the output,
374and gives you fine-grain control over which information is printed. Clang has
375the ability to print this information, and these are the options that control
376it:</p>
377
378<p>
379<ol>
380<li>A file/line/column indicator that shows exactly where the diagnostic occurs
381 in your code [<a href="#opt_fshow-column">-fshow-column</a>, <a
382 href="#opt_fshow-source-location">-fshow-source-location</a>].</li>
383<li>A categorization of the diagnostic as a note, warning, error, or fatal
384 error.</li>
385<li>A text string that describes what the problem is.</li>
386<li>An option that indicates how to control the diagnostic (for diagnostics that
387 support it) [<a
388 href="#opt_fdiagnostics-show-option">-fdiagnostics-show-option</a>].</li>
389<li>The line of source code that the issue occurs on, along with a caret and
390 ranges that indicate the important locations [<a
391 href="opt_fcaret-diagnostics">-fcaret-diagnostics</a>].</li>
392<li>"FixIt" information, which is a concise explanation of how to fix the
393 problem (when Clang is certain it knows) [<a
394 href="opt_fdiagnostics-fixit-info">-fdiagnostics-fixit-info</a>].</li>
395<li>A machine-parsable representation of the ranges involved (off by
396 default) [<a
Chris Lattner2a9cc232009-04-21 05:35:32 +0000[diff] [blame]397 href="opt_fdiagnostics-print-source-range-info">-fdiagnostics-print-source-range-info</a>].</li>
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]398</ol></p>
399
400<p>For more information please see <a href="#cl_diag_formatting">Formatting of
401Diagnostics</a>.</p>
402
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]403<h4 id="diagnostics_mappings">Diagnostic Mappings</h4>
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]404
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]405<p>All diagnostics are mapped into one of these 5 classes:</p>
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]406
407<p>
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]408<ul>
409<li>Ignored</li>
410<li>Note</li>
411<li>Warning</li>
412<li>Error</li>
413<li>Fatal</li>
414</ul></p>
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]415
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]416<h4 id="diagnostics_commandline">Controlling Diagnostics via Command Line Flags</h4>
Chris Lattner8217f4e2009-04-20 06:26:18 +0000[diff] [blame]417
418<p>-W flags, -pedantic, etc</p>
419
Chris Lattner3401cf82009-07-12 21:22:10 +0000[diff] [blame]420<h4 id="diagnostics_pragmas">Controlling Diagnostics via Pragmas</h4>
421
422<p>Clang can also control what diagnostics are enabled through the use of
423pragmas in the source code. This is useful for turning off specific warnings
424in a section of source code. Clang supports GCC's pragma for compatibility
425with existing source code, as well as several extensions. </p>
426
427<p>The pragma may control any warning that can be used from the command line.
428Warnings may be set to ignored, warning, error, or fatal. The following
429example code will tell Clang or GCC to ignore the -Wall warnings:</p>
430
431<pre>
432#pragma GCC diagnostic ignored "-Wall"
433</pre>
434
435<p>In addition to all of the functionality of provided by GCC's pragma, Clang
436also allows you to push and pop the current warning state. This is particularly
437useful when writing a header file that will be compiled by other people, because
438you don't know what warning flags they build with.</p>
439
440<p>In the below example
441-Wmultichar is ignored for only a single line of code, after which the
442diagnostics return to whatever state had previously existed.</p>
443
444<pre>
445#pragma clang diagnostic push
446#pragma clang diagnostic ignored "-Wmultichar"
447
448char b = 'df'; // no warning.
449
450#pragma clang diagnostic pop
451</pre>
452
453<p>The push and pop pragmas will save and restore the full diagnostic state of
454the compiler, regardless of how it was set. That means that it is possible to
455use push and pop around GCC compatible diagnostics and Clang will push and pop
456them appropriately, while GCC will ignore the pushes and pops as unknown
457pragmas. It should be noted that while Clang supports the GCC pragma, Clang and
458GCC do not support the exact same set of warnings, so even when using GCC
459compatible #pragmas there is no guarantee that they will have identical behaviour
460on both compilers. </p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]461
462<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
463<h3 id="precompiledheaders">Precompiled Headers</h3>
464<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
465
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]466<p><a href="http://en.wikipedia.org/wiki/Precompiled_header">Precompiled
467headers</a> are a general approach employed by many compilers to reduce
468compilation time. The underlying motivation of the approach is that it is
469common for the same (and often large) header files to be included by
470multiple source files. Consequently, compile times can often be greatly improved
471by caching some of the (redundant) work done by a compiler to process headers.
472Precompiled header files, which represent one of many ways to implement
473this optimization, are literally files that represent an on-disk cache that
474contains the vital information necessary to reduce some of the work
475needed to process a corresponding header file. While details of precompiled
476headers vary between compilers, precompiled headers have been shown to be a
477highly effective at speeding up program compilation on systems with very large
478system headers (e.g., Mac OS/X).</p>
479
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]480<h4>Generating a PCH File</h4>
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]481
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]482<p>To generate a PCH file using Clang, one invokes Clang with
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]483the <b><tt>-x <i>&lt;language&gt;</i>-header</tt></b> option. This mirrors the
484interface in GCC for generating PCH files:</p>
485
486<pre>
487 $ gcc -x c-header test.h -o test.h.gch
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]488 $ clang -x c-header test.h -o test.h.pch
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]489</pre>
490
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]491<h4>Using a PCH File</h4>
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]492
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]493<p>A PCH file can then be used as a prefix header when a
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]494<b><tt>-include</tt></b> option is passed to <tt>clang</tt>:</p>
495
496<pre>
497 $ clang -include test.h test.c -o test
498</pre>
499
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]500<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]501is 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]502will be processed from the PCH file. Otherwise, Clang falls back to
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]503directly processing the content of <tt>test.h</tt>. This mirrors the behavior of
504GCC.</p>
505
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]506<p><b>NOTE:</b> Clang does <em>not</em> automatically use PCH files
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]507for headers that are directly included within a source file. For example:</p>
508
509<pre>
Chris Lattnere42ec542009-06-13 20:35:58 +0000[diff] [blame]510 $ clang -x c-header test.h -o test.h.pch
Chris Lattner5c3074f2009-04-20 04:37:38 +0000[diff] [blame]511 $ cat test.c
512 #include "test.h"
513 $ clang test.c -o test
514</pre>
515
Douglas Gregorf4d59532009-06-03 22:37:00 +0000[diff] [blame]516<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]517<tt>test.h</tt> since <tt>test.h</tt> was included directly in the source file
518and not specified on the command line using <tt>-include</tt>.</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]519
Douglas Gregore650c8c2009-07-07 00:12:59 +0000[diff] [blame]520<h4>Relocatable PCH Files</h4>
521<p>It is sometimes necessary to build a precompiled header from headers that
522are not yet in their final, installed locations. For example, one might build a
523precompiled header within the build tree that is then meant to be installed
524alongside the headers. Clang permits the creation of "relocatable" precompiled
525headers, which are built with a given path (into the build directory) and can
526later be used from an installed location.</p>
527
528<p>To build a relocatable precompiled header, place your headers into a
529subdirectory whose structure mimics the installed location. For example, if you
530want to build a precompiled header for the header <code>mylib.h</code> that
531will be installed into <code>/usr/include</code>, create a subdirectory
532<code>build/usr/include</code> and place the header <code>mylib.h</code> into
533that subdirectory. If <code>mylib.h</code> depends on other headers, then
534they can be stored within <code>build/usr/include</code> in a way that mimics
535the installed location.</p>
536
537<p>Building a relocatable precompiled header requires two additional arguments.
538First, pass the <code>--relocatable-pch</code> flag to indicate that the
539resulting PCH file should be relocatable. Second, pass
540<code>-isysroot /path/to/build</code>, which makes all includes for your
541library relative to the build directory. For example:</p>
542
543<pre>
544 # clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch
545</pre>
546
547<p>When loading the relocatable PCH file, the various headers used in the PCH
548file are found from the system header root. For example, <code>mylib.h</code>
549can be found in <code>/usr/include/mylib.h</code>. If the headers are installed
550in some other system root, the <code>-isysroot</code> option can be used provide
551a different system root from which the headers will be based. For example,
552<code>-isysroot /Developer/SDKs/MacOSX10.4u.sdk</code> will look for
553<code>mylib.h</code> in
554<code>/Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h</code>.</p>
555
556<p>Relocatable precompiled headers are intended to be used in a limited number
557of cases where the compilation environment is tightly controlled and the
558precompiled header cannot be generated after headers have been installed.
559Relocatable precompiled headers also have some performance impact, because
560the difference in location between the header locations at PCH build time vs.
561at the time of PCH use requires one of the PCH optimizations,
562<code>stat()</code> caching, to be disabled. However, this change is only
563likely to affect PCH files that reference a large number of headers.</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]564
565<!-- ======================================================================= -->
566<h2 id="c">C Language Features</h2>
567<!-- ======================================================================= -->
568
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]569<p>The support for standard C in clang is feature-complete except for the C99
570floating-point pragmas.</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]571
572<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]573<h3 id="c_ext">Extensions supported by clang</h3>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]574<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
575
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]576<p>See <a href="LanguageExtensions.html">clang language extensions</a>.</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]577
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]578<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
579<h3 id="c_modes">Differences between various standard modes</h3>
580<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]581
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]582<p>clang supports the -std option, which changes what language mode clang uses.
583The supported modes for C are c89, gnu89, c94, c99, gnu99 and various aliases
584for those modes. If no -std option is specified, clang defaults to gnu99 mode.
585</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]586
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]587<p>Differences between all c* and gnu* modes:</p>
588<ul>
589<li>c* modes define "__STRICT_ANSI__".</li>
Eli Friedman26fa0ed2009-05-27 23:02:57 +0000[diff] [blame]590<li>Target-specific defines not prefixed by underscores, like "linux", are
591defined in gnu* modes.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]592<li>Trigraphs default to being off in gnu* modes; they can be enabled by the
593-trigraphs option.</li>
594<li>The parser recognizes "asm" and "typeof" as keywords in gnu* modes; the
595variants "__asm__" and "__typeof__" are recognized in all modes.</li>
Eli Friedmanb0ac5452009-05-16 23:17:30 +0000[diff] [blame]596<li>The Apple "blocks" extension is recognized by default in gnu* modes
597on some platforms; it can be enabled in any mode with the "-fblocks"
598option.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]599</ul>
600
601<p>Differences between *89 and *99 modes:</p>
602<ul>
603<li>The *99 modes default to implementing "inline" as specified in C99, while
604the *89 modes implement the GNU version. This can be overridden for individual
605functions with the __gnu_inline__ attribute.</li>
Eli Friedman26fa0ed2009-05-27 23:02:57 +0000[diff] [blame]606<li>Digraphs are not recognized in c89 mode.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]607<li>The scope of names defined inside a "for", "if", "switch", "while", or "do"
608statement is different. (example: "if ((struct x {int x;}*)0) {}".)</li>
609<li>__STDC_VERSION__ is not defined in *89 modes.</li>
Eli Friedman26fa0ed2009-05-27 23:02:57 +0000[diff] [blame]610<li>"inline" is not recognized as a keyword in c89 mode.</li>
611<li>"restrict" is not recognized as a keyword in *89 modes.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]612<li>Commas are allowed in integer constant expressions in *99 modes.</li>
613<li>Arrays which are not lvalues are not implicitly promoted to pointers in
614*89 modes.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]615<li>Some warnings are different.</li>
616</ul>
617
618<p>c94 mode is identical to c89 mode except that digraphs are enabled in
619c94 mode (FIXME: And __STDC_VERSION__ should be defined!).</p>
620
621<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
622<h3 id="c_unimpl_gcc">GCC extensions not implemented yet</h3>
623<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
624
625<p>clang tries to be compatible with gcc as much as possible, but some gcc
626extensions are not implemented yet:</p>
627
628<ul>
629<li>clang does not support __label__
630(<a href="http://llvm.org/bugs/show_bug.cgi?id=3429">bug 3429</a>). This is
631a relatively small feature, so it is likely to be implemented relatively
632soon.</li>
633
634<li>clang does not support attributes on function pointers
635(<a href="http://llvm.org/bugs/show_bug.cgi?id=2461">bug 2461</a>). This is
636a relatively important feature, so it is likely to be implemented relatively
637soon.</li>
638
639<li>clang does not support #pragma weak
Eli Friedman4da92552009-06-02 08:21:31 +0000[diff] [blame]640(<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]641the uses described in the bug, this is likely to be implemented at some
642point, at least partially.</li>
643
644<li>clang does not support #pragma align
Eli Friedman4da92552009-06-02 08:21:31 +0000[diff] [blame]645(<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]646relatively small feature, so it is likely to be implemented relatively
647soon.</li>
648
Eli Friedman4da92552009-06-02 08:21:31 +0000[diff] [blame]649<li>clang does not support code generation for local variables pinned to
650registers (<a href="http://llvm.org/bugs/show_bug.cgi?id=3933">bug 3933</a>).
651This is a relatively small feature, so it is likely to be implemented
652relatively soon.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]653
654<li>clang does not support decimal floating point types (_Decimal32 and
655friends) or fixed-point types (_Fract and friends); nobody has expressed
656interest in these features yet, so it's hard to say when they will be
657implemented.</li>
658
659<li>clang does not support nested functions; this is a complex feature which
660is infrequently used, so it is unlikely to be implemented anytime soon.</li>
661
Daniel Dunbar05fa6292009-05-12 23:12:07 +0000[diff] [blame]662<li>clang does not support global register variables, this is unlikely
Eli Friedmanca130bd2009-06-12 20:04:25 +0000[diff] [blame]663to be implemented soon because it requires additional LLVM backend support.
664</li>
Daniel Dunbar05fa6292009-05-12 23:12:07 +0000[diff] [blame]665
666<li>clang does not support static initialization of flexible array
667members. This appears to be a rarely used extension, but could be
668implemented pending user demand.</li>
669
Eli Friedmanca130bd2009-06-12 20:04:25 +0000[diff] [blame]670<li>clang does not support __builtin_va_arg_pack/__builtin_va_arg_pack_len.
671This is used rarely, but in some potentially interesting places, like the
672glibc headers, so it may be implemented pending user demand. Note that
673because clang pretends to be like GCC 4.2, and this extension was introduced
674in 4.3, the glibc headers will not try to use this extension with clang at
675the moment.</li>
676
677<li>clang does not support the gcc extension for forward-declaring function
678parameters; this has not showed up in any real-world code yet, though, so it
679might never be implemented.</li>
680
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]681</ul>
682
683<p>This is not a complete list; if you find an unsupported extension
684missing from this list, please send an e-mail to cfe-dev. This list
685currently excludes C++; see <a href="#cxx">C++ Language Features</a>.
686Also, this list does not include bugs in mostly-implemented features; please
687see 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">
688bug tracker</a> for known existing bugs (FIXME: Is there a section for
689bug-reporting guidelines somewhere?).</p>
690
691<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
692<h3 id="c_unsupp_gcc">Intentionally unsupported GCC extensions</h3>
693<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
694
Eli Friedman0b326002009-06-12 20:11:05 +0000[diff] [blame]695<ul>
696
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]697<li>clang does not support the gcc extension that allows variable-length arrays
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]698in structures. This is for a few of reasons: one, it is tricky
699to implement, two, the extension is completely undocumented, and three, the
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]700extension appears to be rarely used.</li>
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]701
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]702<li>clang does not support duplicate definitions of a function where one is
Daniel Dunbarc5f928c2009-06-04 18:37:52 +0000[diff] [blame]703inline. This complicates clients of the AST which normally can expect there is
704at most one definition for each function. Source code using this feature should
705be changed to define the inline and out-of-line definitions in separate
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]706translation units.</li>
Daniel Dunbarc5f928c2009-06-04 18:37:52 +0000[diff] [blame]707
Eli Friedmanca130bd2009-06-12 20:04:25 +0000[diff] [blame]708<li>clang does not have an equivalent to gcc's "fold"; this means that
709clang doesn't accept some constructs gcc might accept in contexts where a
710constant expression is required, like "x-x" where x is a variable, or calls
711to C library functions like strlen.</li>
712
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]713<li>clang does not support multiple alternative constraints in inline asm; this
Eli Friedmanca130bd2009-06-12 20:04:25 +0000[diff] [blame]714is an extremely obscure feature which would be complicated to implement
Eli Friedman2f3fca02009-06-12 20:08:48 +0000[diff] [blame]715correctly.</li>
Eli Friedmanca130bd2009-06-12 20:04:25 +0000[diff] [blame]716
717<li>clang does not support __builtin_apply and friends; this extension is
718extremely obscure and difficult to implement reliably.</li>
719
Eli Friedman0b326002009-06-12 20:11:05 +0000[diff] [blame]720</ul>
721
Eli Friedman660a5ac2009-04-28 18:48:34 +0000[diff] [blame]722<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
723<h3 id="c_ms">Microsoft extensions</h3>
724<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
725
726<p>clang has some experimental support for extensions from
727Microsoft Visual C++; to enable it, use the -fms-extensions command-line
Eli Friedmana1821b52009-06-08 06:21:03 +0000[diff] [blame]728option. This is the default for Windows targets. Note that the
729support is incomplete; enabling Microsoft extensions will silently drop
730certain constructs (including __declspec and Microsoft-style asm statements).
731</p>
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]732
Daniel Dunbar05fa6292009-05-12 23:12:07 +0000[diff] [blame]733<li>clang does not support the Microsoft extension where anonymous
Daniel Dunbar869e0542009-05-13 00:23:53 +0000[diff] [blame]734record members can be declared using user defined typedefs.</li>
Daniel Dunbar05fa6292009-05-12 23:12:07 +0000[diff] [blame]735
Daniel Dunbar9375ed12009-05-13 21:40:49 +0000[diff] [blame]736<li>clang supports the Microsoft "#pragma pack" feature for
737controlling record layout. GCC also contains support for this feature,
738however where MSVC and GCC are incompatible clang follows the MSVC
739definition.</li>
740
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]741<!-- ======================================================================= -->
742<h2 id="objc">Objective-C Language Features</h2>
743<!-- ======================================================================= -->
744
745
746<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
747<h3 id="objc_incompatibilities">Intentional Incompatibilities with GCC</h3>
748<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
749
750<p>No cast of super, no lvalue casts.</p>
751
752
753
754<!-- ======================================================================= -->
755<h2 id="cxx">C++ Language Features</h2>
756<!-- ======================================================================= -->
757
758<p>At this point, Clang C++ is not generally useful. However, Clang C++ support
759is under active development and is progressing rapidly. Please see the <a
760href="http://clang.llvm.org/cxx_status.html">C++ Status</a> page for details or
761ask on the mailing list about how you can help.</p>
762
763<p>Note that the clang driver will refuse to even try to use clang to compile
764C++ code unless you pass the <tt>-ccc-clang-cxx</tt> option to the driver. If
765you really want to play with Clang's C++ support, please pass that flag. </p>
766
767<!-- ======================================================================= -->
768<h2 id="objcxx">Objective C++ Language Features</h2>
769<!-- ======================================================================= -->
770
771<p>At this point, Clang C++ support is not generally useful (and therefore,
772neither is Objective-C++). Please see the <a href="#cxx">C++ section</a> for
773more information.</p>
774
775<!-- ======================================================================= -->
776<h2 id="target_features">Target-Specific Features and Limitations</h2>
777<!-- ======================================================================= -->
778
779
780<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
781<h3 id="target_arch">CPU Architectures Features and Limitations</h3>
782<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
783
784<!-- ======================== -->
785<h4 id="target_arch_x86">X86</h4>
786<!-- ======================== -->
Eli Friedman3b658d32009-06-08 05:12:39 +0000[diff] [blame]787<p>The support for X86 (both 32-bit and 64-bit) is considered stable
788on Darwin (Mac OS/X), Linux, FreeBSD, and Dragonfly BSD: it has been tested to
789correctly compile large C and Objective-C codebases. (FIXME: Anything specific
790we want to say here? Possibly mention some LLVM x86 limitations?)
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]791
Eli Friedman3b658d32009-06-08 05:12:39 +0000[diff] [blame]792<!-- ======================== -->
793<h4 id="target_arch_arm">ARM</h4>
794<!-- ======================== -->
795ARM support is mostly feature-complete, but still experimental; it hasn't
796undergone significant testing.
797
798<!-- ======================== -->
799<h4 id="target_arch_other">Other platforms</h4>
800<!-- ======================== -->
801clang currently contains some support for PPC and Sparc; however, significant
802pieces of code generation are still missing, and they haven't undergone
803significant testing.
804
805<p>clang contains some support for the embedded PIC16 processor
806(FIXME: I haven't been keeping track of this; what should this say?).
807
808<p>clang contains limited support for the MSP430 embedded processor, but both
809the clang support and the LLVM backend support are highly experimental.
810
811<p>Other platforms are completely unsupported at the moment. Adding the
812minimal support needed for parsing and semantic analysis on a new platform
813is quite easy; see lib/Basic/Targets.cpp in the clang source tree. This level
814of support is also sufficient for conversion to LLVM IR for simple programs.
815Proper support for conversion to LLVM IR requires adding code to
816lib/CodeGen/CGCall.cpp at the moment; this is likely to change soon, though.
817Generating assembly requires a suitable LLVM backend.
Chris Lattnercf17d9d2009-04-20 04:23:09 +0000[diff] [blame]818
819<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
820<h3 id="target_os">Operating System Features and Limitations</h3>
821<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
822
823<!-- ======================================= -->
824<h4 id="target_os_darwin">Darwin (Mac OS/X)</h4>
825<!-- ======================================= -->
826
827<p>No __thread support, 64-bit ObjC support requires SL tools.</p>
828
829</div>
830</body>
831</html>