1 ============================
2 Clang Compiler User's Manual
3 ============================
11 The Clang Compiler is an open-source compiler for the C family of
12 programming languages, aiming to be the best in class implementation of
13 these languages. Clang builds on the LLVM optimizer and code generator,
14 allowing it to provide high-quality optimization and code generation
15 support for many targets. For more general information, please see the
16 `Clang Web Site <http://clang.llvm.org>`_ or the `LLVM Web
17 Site <http://llvm.org>`_.
19 This document describes important notes about using Clang as a compiler
20 for an end-user, documenting the supported features, command line
21 options, etc. If you are interested in using Clang to build a tool that
22 processes code, please see :doc:`InternalsManual`. If you are interested in the
23 `Clang Static Analyzer <http://clang-analyzer.llvm.org>`_, please see its web
26 Clang is designed to support the C family of programming languages,
27 which includes :ref:`C <c>`, :ref:`Objective-C <objc>`, :ref:`C++ <cxx>`, and
28 :ref:`Objective-C++ <objcxx>` as well as many dialects of those. For
29 language-specific information, please see the corresponding language
32 - :ref:`C Language <c>`: K&R C, ANSI C89, ISO C90, ISO C94 (C89+AMD1), ISO
34 - :ref:`Objective-C Language <objc>`: ObjC 1, ObjC 2, ObjC 2.1, plus
35 variants depending on base language.
36 - :ref:`C++ Language <cxx>`
37 - :ref:`Objective C++ Language <objcxx>`
39 In addition to these base languages and their dialects, Clang supports a
40 broad variety of language extensions, which are documented in the
41 corresponding language section. These extensions are provided to be
42 compatible with the GCC, Microsoft, and other popular compilers as well
43 as to improve functionality through Clang-specific features. The Clang
44 driver and language features are intentionally designed to be as
45 compatible with the GNU GCC compiler as reasonably possible, easing
46 migration from GCC to Clang. In most cases, code "just works".
47 Clang also provides an alternative driver, :ref:`clang-cl`, that is designed
48 to be compatible with the Visual C++ compiler, cl.exe.
50 In addition to language specific features, Clang has a variety of
51 features that depend on what CPU architecture or operating system is
52 being compiled for. Please see the :ref:`Target-Specific Features and
53 Limitations <target_features>` section for more details.
55 The rest of the introduction introduces some basic :ref:`compiler
56 terminology <terminology>` that is used throughout this manual and
57 contains a basic :ref:`introduction to using Clang <basicusage>` as a
58 command line compiler.
65 Front end, parser, backend, preprocessor, undefined behavior,
73 Intro to how to use a C compiler for newbies.
75 compile + link compile then link debug info enabling optimizations
76 picking a language to use, defaults to C99 by default. Autosenses based
77 on extension. using a makefile
82 This section is generally an index into other sections. It does not go
83 into depth on the ones that are covered by other sections. However, the
84 first part introduces the language selection and other high level
85 options like :option:`-c`, :option:`-g`, etc.
87 Options to Control Error and Warning Messages
88 ---------------------------------------------
92 Turn warnings into errors.
94 .. This is in plain monospaced font because it generates the same label as
95 .. -Werror, and Sphinx complains.
99 Turn warning "foo" into an error.
101 .. option:: -Wno-error=foo
103 Turn warning "foo" into an warning even if :option:`-Werror` is specified.
107 Enable warning "foo".
111 Disable warning "foo".
115 Disable all diagnostics.
117 .. option:: -Weverything
119 :ref:`Enable all diagnostics. <diagnostics_enable_everything>`
121 .. option:: -pedantic
123 Warn on language extensions.
125 .. option:: -pedantic-errors
127 Error on language extensions.
129 .. option:: -Wsystem-headers
131 Enable warnings from system headers.
133 .. option:: -ferror-limit=123
135 Stop emitting diagnostics after 123 errors have been produced. The default is
136 20, and the error limit can be disabled with :option:`-ferror-limit=0`.
138 .. option:: -ftemplate-backtrace-limit=123
140 Only emit up to 123 template instantiation notes within the template
141 instantiation backtrace for a single warning or error. The default is 10, and
142 the limit can be disabled with :option:`-ftemplate-backtrace-limit=0`.
144 .. _cl_diag_formatting:
146 Formatting of Diagnostics
147 ^^^^^^^^^^^^^^^^^^^^^^^^^
149 Clang aims to produce beautiful diagnostics by default, particularly for
150 new users that first come to Clang. However, different people have
151 different preferences, and sometimes Clang is driven by another program
152 that wants to parse simple and consistent output, not a person. For
153 these cases, Clang provides a wide range of options to control the exact
154 output format of the diagnostics that it generates.
156 .. _opt_fshow-column:
158 **-f[no-]show-column**
159 Print column number in diagnostic.
161 This option, which defaults to on, controls whether or not Clang
162 prints the column number of a diagnostic. For example, when this is
163 enabled, Clang will print something like:
167 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
172 When this is disabled, Clang will print "test.c:28: warning..." with
175 The printed column numbers count bytes from the beginning of the
176 line; take care if your source contains multibyte characters.
178 .. _opt_fshow-source-location:
180 **-f[no-]show-source-location**
181 Print source file/line/column information in diagnostic.
183 This option, which defaults to on, controls whether or not Clang
184 prints the filename, line number and column number of a diagnostic.
185 For example, when this is enabled, Clang will print something like:
189 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
194 When this is disabled, Clang will not print the "test.c:28:8: "
197 .. _opt_fcaret-diagnostics:
199 **-f[no-]caret-diagnostics**
200 Print source line and ranges from source code in diagnostic.
201 This option, which defaults to on, controls whether or not Clang
202 prints the source line, source ranges, and caret when emitting a
203 diagnostic. For example, when this is enabled, Clang will print
208 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
213 **-f[no-]color-diagnostics**
214 This option, which defaults to on when a color-capable terminal is
215 detected, controls whether or not Clang prints diagnostics in color.
217 When this option is enabled, Clang will use colors to highlight
218 specific parts of the diagnostic, e.g.,
220 .. nasty hack to not lose our dignity
225 <b><span style="color:black">test.c:28:8: <span style="color:magenta">warning</span>: extra tokens at end of #endif directive [-Wextra-tokens]</span></b>
227 <span style="color:green">^</span>
228 <span style="color:green">//</span>
231 When this is disabled, Clang will just print:
235 test.c:2:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
240 **-fansi-escape-codes**
241 Controls whether ANSI escape codes are used instead of the Windows Console
242 API to output colored diagnostics. This option is only used on Windows and
245 .. option:: -fdiagnostics-format=clang/msvc/vi
247 Changes diagnostic output format to better match IDEs and command line tools.
249 This option controls the output format of the filename, line number,
250 and column printed in diagnostic messages. The options, and their
251 affect on formatting a simple conversion diagnostic, follow:
256 t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int'
261 t.c(3,11) : warning: conversion specifies type 'char *' but the argument has type 'int'
266 t.c +3:11: warning: conversion specifies type 'char *' but the argument has type 'int'
268 .. _opt_fdiagnostics-show-option:
270 **-f[no-]diagnostics-show-option**
271 Enable ``[-Woption]`` information in diagnostic line.
273 This option, which defaults to on, controls whether or not Clang
274 prints the associated :ref:`warning group <cl_diag_warning_groups>`
275 option name when outputting a warning diagnostic. For example, in
280 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
285 Passing **-fno-diagnostics-show-option** will prevent Clang from
286 printing the [:ref:`-Wextra-tokens <opt_Wextra-tokens>`] information in
287 the diagnostic. This information tells you the flag needed to enable
288 or disable the diagnostic, either from the command line or through
289 :ref:`#pragma GCC diagnostic <pragma_GCC_diagnostic>`.
291 .. _opt_fdiagnostics-show-category:
293 .. option:: -fdiagnostics-show-category=none/id/name
295 Enable printing category information in diagnostic line.
297 This option, which defaults to "none", controls whether or not Clang
298 prints the category associated with a diagnostic when emitting it.
299 Each diagnostic may or many not have an associated category, if it
300 has one, it is listed in the diagnostic categorization field of the
301 diagnostic line (in the []'s).
303 For example, a format string warning will produce these three
304 renditions based on the setting of this option:
308 t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat]
309 t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat,1]
310 t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat,Format String]
312 This category can be used by clients that want to group diagnostics
313 by category, so it should be a high level category. We want dozens
314 of these, not hundreds or thousands of them.
316 .. _opt_fdiagnostics-fixit-info:
318 **-f[no-]diagnostics-fixit-info**
319 Enable "FixIt" information in the diagnostics output.
321 This option, which defaults to on, controls whether or not Clang
322 prints the information on how to fix a specific diagnostic
323 underneath it when it knows. For example, in this output:
327 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
332 Passing **-fno-diagnostics-fixit-info** will prevent Clang from
333 printing the "//" line at the end of the message. This information
334 is useful for users who may not understand what is wrong, but can be
335 confusing for machine parsing.
337 .. _opt_fdiagnostics-print-source-range-info:
339 **-fdiagnostics-print-source-range-info**
340 Print machine parsable information about source ranges.
341 This option makes Clang print information about source ranges in a machine
342 parsable format after the file/line/column number information. The
343 information is a simple sequence of brace enclosed ranges, where each range
344 lists the start and end line/column locations. For example, in this output:
348 exprs.c:47:15:{47:8-47:14}{47:17-47:24}: error: invalid operands to binary expression ('int *' and '_Complex float')
349 P = (P-42) + Gamma*4;
352 The {}'s are generated by -fdiagnostics-print-source-range-info.
354 The printed column numbers count bytes from the beginning of the
355 line; take care if your source contains multibyte characters.
357 .. option:: -fdiagnostics-parseable-fixits
359 Print Fix-Its in a machine parseable form.
361 This option makes Clang print available Fix-Its in a machine
362 parseable format at the end of diagnostics. The following example
363 illustrates the format:
367 fix-it:"t.cpp":{7:25-7:29}:"Gamma"
369 The range printed is a half-open range, so in this example the
370 characters at column 25 up to but not including column 29 on line 7
371 in t.cpp should be replaced with the string "Gamma". Either the
372 range or the replacement string may be empty (representing strict
373 insertions and strict erasures, respectively). Both the file name
374 and the insertion string escape backslash (as "\\\\"), tabs (as
375 "\\t"), newlines (as "\\n"), double quotes(as "\\"") and
376 non-printable characters (as octal "\\xxx").
378 The printed column numbers count bytes from the beginning of the
379 line; take care if your source contains multibyte characters.
381 .. option:: -fno-elide-type
383 Turns off elision in template type printing.
385 The default for template type printing is to elide as many template
386 arguments as possible, removing those which are the same in both
387 template types, leaving only the differences. Adding this flag will
388 print all the template arguments. If supported by the terminal,
389 highlighting will still appear on differing arguments.
395 t.cc:4:5: note: candidate function not viable: no known conversion from 'vector<map<[...], map<float, [...]>>>' to 'vector<map<[...], map<double, [...]>>>' for 1st argument;
401 t.cc:4:5: note: candidate function not viable: no known conversion from 'vector<map<int, map<float, int>>>' to 'vector<map<int, map<double, int>>>' for 1st argument;
403 .. option:: -fdiagnostics-show-template-tree
405 Template type diffing prints a text tree.
407 For diffing large templated types, this option will cause Clang to
408 display the templates as an indented text tree, one argument per
409 line, with differences marked inline. This is compatible with
416 t.cc:4:5: note: candidate function not viable: no known conversion from 'vector<map<[...], map<float, [...]>>>' to 'vector<map<[...], map<double, [...]>>>' for 1st argument;
418 With :option:`-fdiagnostics-show-template-tree`:
422 t.cc:4:5: note: candidate function not viable: no known conversion for 1st argument;
430 .. _cl_diag_warning_groups:
432 Individual Warning Groups
433 ^^^^^^^^^^^^^^^^^^^^^^^^^
435 TODO: Generate this from tblgen. Define one anchor per warning group.
437 .. _opt_wextra-tokens:
439 .. option:: -Wextra-tokens
441 Warn about excess tokens at the end of a preprocessor directive.
443 This option, which defaults to on, enables warnings about extra
444 tokens at the end of preprocessor directives. For example:
448 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
452 These extra tokens are not strictly conforming, and are usually best
453 handled by commenting them out.
455 .. option:: -Wambiguous-member-template
457 Warn about unqualified uses of a member template whose name resolves to
458 another template at the location of the use.
460 This option, which defaults to on, enables a warning in the
465 template<typename T> struct set{};
466 template<typename T> struct trait { typedef const T& type; };
468 template<typename T> void set(typename trait<T>::type value) {}
475 C++ [basic.lookup.classref] requires this to be an error, but,
476 because it's hard to work around, Clang downgrades it to a warning
479 .. option:: -Wbind-to-temporary-copy
481 Warn about an unusable copy constructor when binding a reference to a
484 This option, which defaults to on, enables warnings about binding a
485 reference to a temporary when the temporary doesn't have a usable
486 copy constructor. For example:
493 NonCopyable(const NonCopyable&);
495 void foo(const NonCopyable&);
497 foo(NonCopyable()); // Disallowed in C++98; allowed in C++11.
502 struct NonCopyable2 {
504 NonCopyable2(NonCopyable2&);
506 void foo(const NonCopyable2&);
508 foo(NonCopyable2()); // Disallowed in C++98; allowed in C++11.
511 Note that if ``NonCopyable2::NonCopyable2()`` has a default argument
512 whose instantiation produces a compile error, that error will still
513 be a hard error in C++98 mode even if this warning is turned off.
515 Options to Control Clang Crash Diagnostics
516 ------------------------------------------
518 As unbelievable as it may sound, Clang does crash from time to time.
519 Generally, this only occurs to those living on the `bleeding
520 edge <http://llvm.org/releases/download.html#svn>`_. Clang goes to great
521 lengths to assist you in filing a bug report. Specifically, Clang
522 generates preprocessed source file(s) and associated run script(s) upon
523 a crash. These files should be attached to a bug report to ease
524 reproducibility of the failure. Below are the command line options to
525 control the crash diagnostics.
527 .. option:: -fno-crash-diagnostics
529 Disable auto-generation of preprocessed source files during a clang crash.
531 The -fno-crash-diagnostics flag can be helpful for speeding the process
532 of generating a delta reduced test case.
534 Language and Target-Independent Features
535 ========================================
537 Controlling Errors and Warnings
538 -------------------------------
540 Clang provides a number of ways to control which code constructs cause
541 it to emit errors and warning messages, and how they are displayed to
544 Controlling How Clang Displays Diagnostics
545 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
547 When Clang emits a diagnostic, it includes rich information in the
548 output, and gives you fine-grain control over which information is
549 printed. Clang has the ability to print this information, and these are
550 the options that control it:
552 #. A file/line/column indicator that shows exactly where the diagnostic
553 occurs in your code [:ref:`-fshow-column <opt_fshow-column>`,
554 :ref:`-fshow-source-location <opt_fshow-source-location>`].
555 #. A categorization of the diagnostic as a note, warning, error, or
557 #. A text string that describes what the problem is.
558 #. An option that indicates how to control the diagnostic (for
559 diagnostics that support it)
560 [:ref:`-fdiagnostics-show-option <opt_fdiagnostics-show-option>`].
561 #. A :ref:`high-level category <diagnostics_categories>` for the diagnostic
562 for clients that want to group diagnostics by class (for diagnostics
564 [:ref:`-fdiagnostics-show-category <opt_fdiagnostics-show-category>`].
565 #. The line of source code that the issue occurs on, along with a caret
566 and ranges that indicate the important locations
567 [:ref:`-fcaret-diagnostics <opt_fcaret-diagnostics>`].
568 #. "FixIt" information, which is a concise explanation of how to fix the
569 problem (when Clang is certain it knows)
570 [:ref:`-fdiagnostics-fixit-info <opt_fdiagnostics-fixit-info>`].
571 #. A machine-parsable representation of the ranges involved (off by
573 [:ref:`-fdiagnostics-print-source-range-info <opt_fdiagnostics-print-source-range-info>`].
575 For more information please see :ref:`Formatting of
576 Diagnostics <cl_diag_formatting>`.
581 All diagnostics are mapped into one of these 5 classes:
590 .. _diagnostics_categories:
592 Diagnostic Categories
593 ^^^^^^^^^^^^^^^^^^^^^
595 Though not shown by default, diagnostics may each be associated with a
596 high-level category. This category is intended to make it possible to
597 triage builds that produce a large number of errors or warnings in a
600 Categories are not shown by default, but they can be turned on with the
601 :ref:`-fdiagnostics-show-category <opt_fdiagnostics-show-category>` option.
602 When set to "``name``", the category is printed textually in the
603 diagnostic output. When it is set to "``id``", a category number is
604 printed. The mapping of category names to category id's can be obtained
605 by running '``clang --print-diagnostic-categories``'.
607 Controlling Diagnostics via Command Line Flags
608 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
610 TODO: -W flags, -pedantic, etc
612 .. _pragma_gcc_diagnostic:
614 Controlling Diagnostics via Pragmas
615 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
617 Clang can also control what diagnostics are enabled through the use of
618 pragmas in the source code. This is useful for turning off specific
619 warnings in a section of source code. Clang supports GCC's pragma for
620 compatibility with existing source code, as well as several extensions.
622 The pragma may control any warning that can be used from the command
623 line. Warnings may be set to ignored, warning, error, or fatal. The
624 following example code will tell Clang or GCC to ignore the -Wall
629 #pragma GCC diagnostic ignored "-Wall"
631 In addition to all of the functionality provided by GCC's pragma, Clang
632 also allows you to push and pop the current warning state. This is
633 particularly useful when writing a header file that will be compiled by
634 other people, because you don't know what warning flags they build with.
636 In the below example :option:`-Wmultichar` is ignored for only a single line of
637 code, after which the diagnostics return to whatever state had previously
642 #pragma clang diagnostic push
643 #pragma clang diagnostic ignored "-Wmultichar"
645 char b = 'df'; // no warning.
647 #pragma clang diagnostic pop
649 The push and pop pragmas will save and restore the full diagnostic state
650 of the compiler, regardless of how it was set. That means that it is
651 possible to use push and pop around GCC compatible diagnostics and Clang
652 will push and pop them appropriately, while GCC will ignore the pushes
653 and pops as unknown pragmas. It should be noted that while Clang
654 supports the GCC pragma, Clang and GCC do not support the exact same set
655 of warnings, so even when using GCC compatible #pragmas there is no
656 guarantee that they will have identical behaviour on both compilers.
658 In addition to controlling warnings and errors generated by the compiler, it is
659 possible to generate custom warning and error messages through the following
664 // The following will produce warning messages
665 #pragma message "some diagnostic message"
666 #pragma GCC warning "TODO: replace deprecated feature"
668 // The following will produce an error message
669 #pragma GCC error "Not supported"
671 These pragmas operate similarly to the ``#warning`` and ``#error`` preprocessor
672 directives, except that they may also be embedded into preprocessor macros via
673 the C99 ``_Pragma`` operator, for example:
678 #define DEFER(M,...) M(__VA_ARGS__)
679 #define CUSTOM_ERROR(X) _Pragma(STR(GCC error(X " at line " DEFER(STR,__LINE__))))
681 CUSTOM_ERROR("Feature not available");
683 Controlling Diagnostics in System Headers
684 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
686 Warnings are suppressed when they occur in system headers. By default,
687 an included file is treated as a system header if it is found in an
688 include path specified by ``-isystem``, but this can be overridden in
691 The ``system_header`` pragma can be used to mark the current file as
692 being a system header. No warnings will be produced from the location of
693 the pragma onwards within the same file.
697 char a = 'xy'; // warning
699 #pragma clang system_header
701 char b = 'ab'; // no warning
703 The :option:`-isystem-prefix` and :option:`-ino-system-prefix` command-line
704 arguments can be used to override whether subsets of an include path are
705 treated as system headers. When the name in a ``#include`` directive is
706 found within a header search path and starts with a system prefix, the
707 header is treated as a system header. The last prefix on the
708 command-line which matches the specified header name takes precedence.
711 .. code-block:: console
713 $ clang -Ifoo -isystem bar -isystem-prefix x/ -ino-system-prefix x/y/
715 Here, ``#include "x/a.h"`` is treated as including a system header, even
716 if the header is found in ``foo``, and ``#include "x/y/b.h"`` is treated
717 as not including a system header, even if the header is found in
720 A ``#include`` directive which finds a file relative to the current
721 directory is treated as including a system header if the including file
722 is treated as a system header.
724 .. _diagnostics_enable_everything:
726 Enabling All Diagnostics
727 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
729 In addition to the traditional ``-W`` flags, one can enable **all**
730 diagnostics by passing :option:`-Weverything`. This works as expected
732 :option:`-Werror`, and also includes the warnings from :option:`-pedantic`.
734 Note that when combined with :option:`-w` (which disables all warnings), that
737 Controlling Static Analyzer Diagnostics
738 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
740 While not strictly part of the compiler, the diagnostics from Clang's
741 `static analyzer <http://clang-analyzer.llvm.org>`_ can also be
742 influenced by the user via changes to the source code. See the available
743 `annotations <http://clang-analyzer.llvm.org/annotations.html>`_ and the
745 page <http://clang-analyzer.llvm.org/faq.html#exclude_code>`_ for more
748 .. _usersmanual-precompiled-headers:
753 `Precompiled headers <http://en.wikipedia.org/wiki/Precompiled_header>`__
754 are a general approach employed by many compilers to reduce compilation
755 time. The underlying motivation of the approach is that it is common for
756 the same (and often large) header files to be included by multiple
757 source files. Consequently, compile times can often be greatly improved
758 by caching some of the (redundant) work done by a compiler to process
759 headers. Precompiled header files, which represent one of many ways to
760 implement this optimization, are literally files that represent an
761 on-disk cache that contains the vital information necessary to reduce
762 some of the work needed to process a corresponding header file. While
763 details of precompiled headers vary between compilers, precompiled
764 headers have been shown to be highly effective at speeding up program
765 compilation on systems with very large system headers (e.g., Mac OS X).
767 Generating a PCH File
768 ^^^^^^^^^^^^^^^^^^^^^
770 To generate a PCH file using Clang, one invokes Clang with the
771 :option:`-x <language>-header` option. This mirrors the interface in GCC
772 for generating PCH files:
774 .. code-block:: console
776 $ gcc -x c-header test.h -o test.h.gch
777 $ clang -x c-header test.h -o test.h.pch
782 A PCH file can then be used as a prefix header when a :option:`-include`
783 option is passed to ``clang``:
785 .. code-block:: console
787 $ clang -include test.h test.c -o test
789 The ``clang`` driver will first check if a PCH file for ``test.h`` is
790 available; if so, the contents of ``test.h`` (and the files it includes)
791 will be processed from the PCH file. Otherwise, Clang falls back to
792 directly processing the content of ``test.h``. This mirrors the behavior
797 Clang does *not* automatically use PCH files for headers that are directly
798 included within a source file. For example:
800 .. code-block:: console
802 $ clang -x c-header test.h -o test.h.pch
805 $ clang test.c -o test
807 In this example, ``clang`` will not automatically use the PCH file for
808 ``test.h`` since ``test.h`` was included directly in the source file and not
809 specified on the command line using :option:`-include`.
811 Relocatable PCH Files
812 ^^^^^^^^^^^^^^^^^^^^^
814 It is sometimes necessary to build a precompiled header from headers
815 that are not yet in their final, installed locations. For example, one
816 might build a precompiled header within the build tree that is then
817 meant to be installed alongside the headers. Clang permits the creation
818 of "relocatable" precompiled headers, which are built with a given path
819 (into the build directory) and can later be used from an installed
822 To build a relocatable precompiled header, place your headers into a
823 subdirectory whose structure mimics the installed location. For example,
824 if you want to build a precompiled header for the header ``mylib.h``
825 that will be installed into ``/usr/include``, create a subdirectory
826 ``build/usr/include`` and place the header ``mylib.h`` into that
827 subdirectory. If ``mylib.h`` depends on other headers, then they can be
828 stored within ``build/usr/include`` in a way that mimics the installed
831 Building a relocatable precompiled header requires two additional
832 arguments. First, pass the ``--relocatable-pch`` flag to indicate that
833 the resulting PCH file should be relocatable. Second, pass
834 :option:`-isysroot /path/to/build`, which makes all includes for your library
835 relative to the build directory. For example:
837 .. code-block:: console
839 # clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch
841 When loading the relocatable PCH file, the various headers used in the
842 PCH file are found from the system header root. For example, ``mylib.h``
843 can be found in ``/usr/include/mylib.h``. If the headers are installed
844 in some other system root, the :option:`-isysroot` option can be used provide
845 a different system root from which the headers will be based. For
846 example, :option:`-isysroot /Developer/SDKs/MacOSX10.4u.sdk` will look for
847 ``mylib.h`` in ``/Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h``.
849 Relocatable precompiled headers are intended to be used in a limited
850 number of cases where the compilation environment is tightly controlled
851 and the precompiled header cannot be generated after headers have been
854 Controlling Code Generation
855 ---------------------------
857 Clang provides a number of ways to control code generation. The options
860 **-f[no-]sanitize=check1,check2,...**
861 Turn on runtime checks for various forms of undefined or suspicious
864 This option controls whether Clang adds runtime checks for various
865 forms of undefined or suspicious behavior, and is disabled by
866 default. If a check fails, a diagnostic message is produced at
867 runtime explaining the problem. The main checks are:
869 - .. _opt_fsanitize_address:
871 ``-fsanitize=address``:
872 :doc:`AddressSanitizer`, a memory error
874 - ``-fsanitize=init-order``: Make AddressSanitizer check for
875 dynamic initialization order problems. Implied by ``-fsanitize=address``.
876 - ``-fsanitize=address-full``: AddressSanitizer with all the
877 experimental features listed below.
878 - ``-fsanitize=integer``: Enables checks for undefined or
879 suspicious integer behavior.
880 - .. _opt_fsanitize_thread:
882 ``-fsanitize=thread``: :doc:`ThreadSanitizer`, a data race detector.
883 - .. _opt_fsanitize_memory:
885 ``-fsanitize=memory``: :doc:`MemorySanitizer`,
886 an *experimental* detector of uninitialized reads. Not ready for
888 - .. _opt_fsanitize_undefined:
890 ``-fsanitize=undefined``: Fast and compatible undefined behavior
891 checker. Enables the undefined behavior checks that have small
892 runtime cost and no impact on address space layout or ABI. This
893 includes all of the checks listed below other than
894 ``unsigned-integer-overflow``.
896 - ``-fsanitize=undefined-trap``: This includes all sanitizers
897 included by ``-fsanitize=undefined``, except those that require
898 runtime support. This group of sanitizers is intended to be
899 used in conjunction with the ``-fsanitize-undefined-trap-on-error``
900 flag. This includes all of the checks listed below other than
901 ``unsigned-integer-overflow`` and ``vptr``.
902 - ``-fsanitize=dataflow``: :doc:`DataFlowSanitizer`, a general data
905 The following more fine-grained checks are also available:
907 - ``-fsanitize=alignment``: Use of a misaligned pointer or creation
908 of a misaligned reference.
909 - ``-fsanitize=bool``: Load of a ``bool`` value which is neither
910 ``true`` nor ``false``.
911 - ``-fsanitize=bounds``: Out of bounds array indexing, in cases
912 where the array bound can be statically determined.
913 - ``-fsanitize=enum``: Load of a value of an enumerated type which
914 is not in the range of representable values for that enumerated
916 - ``-fsanitize=float-cast-overflow``: Conversion to, from, or
917 between floating-point types which would overflow the
919 - ``-fsanitize=float-divide-by-zero``: Floating point division by
921 - ``-fsanitize=function``: Indirect call of a function through a
922 function pointer of the wrong type (Linux, C++ and x86/x86_64 only).
923 - ``-fsanitize=integer-divide-by-zero``: Integer division by zero.
924 - ``-fsanitize=null``: Use of a null pointer or creation of a null
926 - ``-fsanitize=object-size``: An attempt to use bytes which the
927 optimizer can determine are not part of the object being
928 accessed. The sizes of objects are determined using
929 ``__builtin_object_size``, and consequently may be able to detect
930 more problems at higher optimization levels.
931 - ``-fsanitize=return``: In C++, reaching the end of a
932 value-returning function without returning a value.
933 - ``-fsanitize=shift``: Shift operators where the amount shifted is
934 greater or equal to the promoted bit-width of the left hand side
935 or less than zero, or where the left hand side is negative. For a
936 signed left shift, also checks for signed overflow in C, and for
937 unsigned overflow in C++.
938 - ``-fsanitize=signed-integer-overflow``: Signed integer overflow,
939 including all the checks added by ``-ftrapv``, and checking for
940 overflow in signed division (``INT_MIN / -1``).
941 - ``-fsanitize=unreachable``: If control flow reaches
942 ``__builtin_unreachable``.
943 - ``-fsanitize=unsigned-integer-overflow``: Unsigned integer
945 - ``-fsanitize=vla-bound``: A variable-length array whose bound
946 does not evaluate to a positive value.
947 - ``-fsanitize=vptr``: Use of an object whose vptr indicates that
948 it is of the wrong dynamic type, or that its lifetime has not
949 begun or has ended. Incompatible with ``-fno-rtti``.
951 You can turn off or modify checks for certain source files, functions
952 or even variables by providing a special file:
954 - ``-fsanitize-blacklist=/path/to/blacklist/file``: disable or modify
955 sanitizer checks for objects listed in the file. See
956 :doc:`SanitizerSpecialCaseList` for file format description.
957 - ``-fno-sanitize-blacklist``: don't use blacklist file, if it was
958 specified earlier in the command line.
960 Experimental features of AddressSanitizer (not ready for widespread
961 use, require explicit ``-fsanitize=address``):
963 - ``-fsanitize=use-after-return``: Check for use-after-return
964 errors (accessing local variable after the function exit).
965 - ``-fsanitize=use-after-scope``: Check for use-after-scope errors
966 (accesing local variable after it went out of scope).
968 Extra features of MemorySanitizer (require explicit
969 ``-fsanitize=memory``):
971 - ``-fsanitize-memory-track-origins``: Enables origin tracking in
972 MemorySanitizer. Adds a second section to MemorySanitizer
973 reports pointing to the heap or stack allocation the
974 uninitialized bits came from. Slows down execution by additional
977 Extra features of UndefinedBehaviorSanitizer:
979 - ``-fno-sanitize-recover``: By default, after a sanitizer diagnoses
980 an issue, it will attempt to continue executing the program if there
981 is a reasonable behavior it can give to the faulting operation. This
982 option causes the program to abort instead.
983 - ``-fsanitize-undefined-trap-on-error``: Causes traps to be emitted
984 rather than calls to runtime libraries when a problem is detected.
985 This option is intended for use in cases where the sanitizer runtime
986 cannot be used (for instance, when building libc or a kernel module).
987 This is only compatible with the sanitizers in the ``undefined-trap``
990 The ``-fsanitize=`` argument must also be provided when linking, in
991 order to link to the appropriate runtime library. When using
992 ``-fsanitize=vptr`` (or a group that includes it, such as
993 ``-fsanitize=undefined``) with a C++ program, the link must be
994 performed by ``clang++``, not ``clang``, in order to link against the
995 C++-specific parts of the runtime library.
997 It is not possible to combine more than one of the ``-fsanitize=address``,
998 ``-fsanitize=thread``, and ``-fsanitize=memory`` checkers in the same
999 program. The ``-fsanitize=undefined`` checks can be combined with other
1002 **-f[no-]address-sanitizer**
1003 Deprecated synonym for :ref:`-f[no-]sanitize=address
1004 <opt_fsanitize_address>`.
1005 **-f[no-]thread-sanitizer**
1006 Deprecated synonym for :ref:`-f[no-]sanitize=thread
1007 <opt_fsanitize_thread>`.
1009 .. option:: -fcatch-undefined-behavior
1011 Deprecated synonym for :ref:`-fsanitize=undefined
1012 <opt_fsanitize_undefined>`.
1014 .. option:: -fno-assume-sane-operator-new
1016 Don't assume that the C++'s new operator is sane.
1018 This option tells the compiler to do not assume that C++'s global
1019 new operator will always return a pointer that does not alias any
1020 other pointer when the function returns.
1022 .. option:: -ftrap-function=[name]
1024 Instruct code generator to emit a function call to the specified
1025 function name for ``__builtin_trap()``.
1027 LLVM code generator translates ``__builtin_trap()`` to a trap
1028 instruction if it is supported by the target ISA. Otherwise, the
1029 builtin is translated into a call to ``abort``. If this option is
1030 set, then the code generator will always lower the builtin to a call
1031 to the specified function regardless of whether the target ISA has a
1032 trap instruction. This option is useful for environments (e.g.
1033 deeply embedded) where a trap cannot be properly handled, or when
1034 some custom behavior is desired.
1036 .. option:: -ftls-model=[model]
1038 Select which TLS model to use.
1040 Valid values are: ``global-dynamic``, ``local-dynamic``,
1041 ``initial-exec`` and ``local-exec``. The default value is
1042 ``global-dynamic``. The compiler may use a different model if the
1043 selected model is not supported by the target, or if a more
1044 efficient model can be used. The TLS model can be overridden per
1045 variable using the ``tls_model`` attribute.
1047 .. option:: -mhwdiv=[values]
1049 Select the ARM modes (arm or thumb) that support hardware division
1052 Valid values are: ``arm``, ``thumb`` and ``arm,thumb``.
1053 This option is used to indicate which mode (arm or thumb) supports
1054 hardware division instructions. This only applies to the ARM
1057 .. option:: -m[no-]crc
1059 Enable or disable CRC instructions.
1061 This option is used to indicate whether CRC instructions are to
1062 be generated. This only applies to the ARM architecture.
1064 CRC instructions are enabled by default on ARMv8.
1066 .. option:: -mgeneral-regs-only
1068 Generate code which only uses the general purpose registers.
1070 This option restricts the generated code to use general registers
1071 only. This only applies to the AArch64 architecture.
1074 Controlling Size of Debug Information
1075 -------------------------------------
1077 Debug info kind generated by Clang can be set by one of the flags listed
1078 below. If multiple flags are present, the last one is used.
1082 Don't generate any debug info (default).
1084 .. option:: -gline-tables-only
1086 Generate line number tables only.
1088 This kind of debug info allows to obtain stack traces with function names,
1089 file names and line numbers (by such tools as ``gdb`` or ``addr2line``). It
1090 doesn't contain any other data (e.g. description of local variables or
1091 function parameters).
1095 Generate complete debug info.
1097 Comment Parsing Options
1098 -----------------------
1100 Clang parses Doxygen and non-Doxygen style documentation comments and attaches
1101 them to the appropriate declaration nodes. By default, it only parses
1102 Doxygen-style comments and ignores ordinary comments starting with ``//`` and
1105 .. option:: -Wdocumentation
1107 Emit warnings about use of documentation comments. This warning group is off
1110 This includes checking that ``\param`` commands name parameters that actually
1111 present in the function signature, checking that ``\returns`` is used only on
1112 functions that actually return a value etc.
1114 .. option:: -Wno-documentation-unknown-command
1116 Don't warn when encountering an unknown Doxygen command.
1118 .. option:: -fparse-all-comments
1120 Parse all comments as documentation comments (including ordinary comments
1121 starting with ``//`` and ``/*``).
1123 .. option:: -fcomment-block-commands=[commands]
1125 Define custom documentation commands as block commands. This allows Clang to
1126 construct the correct AST for these custom commands, and silences warnings
1127 about unknown commands. Several commands must be separated by a comma
1128 *without trailing space*; e.g. ``-fcomment-block-commands=foo,bar`` defines
1129 custom commands ``\foo`` and ``\bar``.
1131 It is also possible to use ``-fcomment-block-commands`` several times; e.g.
1132 ``-fcomment-block-commands=foo -fcomment-block-commands=bar`` does the same
1140 The support for standard C in clang is feature-complete except for the
1141 C99 floating-point pragmas.
1143 Extensions supported by clang
1144 -----------------------------
1146 See :doc:`LanguageExtensions`.
1148 Differences between various standard modes
1149 ------------------------------------------
1151 clang supports the -std option, which changes what language mode clang
1152 uses. The supported modes for C are c89, gnu89, c94, c99, gnu99 and
1153 various aliases for those modes. If no -std option is specified, clang
1154 defaults to gnu99 mode.
1156 Differences between all ``c*`` and ``gnu*`` modes:
1158 - ``c*`` modes define "``__STRICT_ANSI__``".
1159 - Target-specific defines not prefixed by underscores, like "linux",
1160 are defined in ``gnu*`` modes.
1161 - Trigraphs default to being off in ``gnu*`` modes; they can be enabled by
1162 the -trigraphs option.
1163 - The parser recognizes "asm" and "typeof" as keywords in ``gnu*`` modes;
1164 the variants "``__asm__``" and "``__typeof__``" are recognized in all
1166 - The Apple "blocks" extension is recognized by default in ``gnu*`` modes
1167 on some platforms; it can be enabled in any mode with the "-fblocks"
1169 - Arrays that are VLA's according to the standard, but which can be
1170 constant folded by the frontend are treated as fixed size arrays.
1171 This occurs for things like "int X[(1, 2)];", which is technically a
1172 VLA. ``c*`` modes are strictly compliant and treat these as VLAs.
1174 Differences between ``*89`` and ``*99`` modes:
1176 - The ``*99`` modes default to implementing "inline" as specified in C99,
1177 while the ``*89`` modes implement the GNU version. This can be
1178 overridden for individual functions with the ``__gnu_inline__``
1180 - Digraphs are not recognized in c89 mode.
1181 - The scope of names defined inside a "for", "if", "switch", "while",
1182 or "do" statement is different. (example: "``if ((struct x {int
1184 - ``__STDC_VERSION__`` is not defined in ``*89`` modes.
1185 - "inline" is not recognized as a keyword in c89 mode.
1186 - "restrict" is not recognized as a keyword in ``*89`` modes.
1187 - Commas are allowed in integer constant expressions in ``*99`` modes.
1188 - Arrays which are not lvalues are not implicitly promoted to pointers
1190 - Some warnings are different.
1192 c94 mode is identical to c89 mode except that digraphs are enabled in
1193 c94 mode (FIXME: And ``__STDC_VERSION__`` should be defined!).
1195 GCC extensions not implemented yet
1196 ----------------------------------
1198 clang tries to be compatible with gcc as much as possible, but some gcc
1199 extensions are not implemented yet:
1201 - clang does not support #pragma weak (`bug
1202 3679 <http://llvm.org/bugs/show_bug.cgi?id=3679>`_). Due to the uses
1203 described in the bug, this is likely to be implemented at some point,
1205 - clang does not support decimal floating point types (``_Decimal32`` and
1206 friends) or fixed-point types (``_Fract`` and friends); nobody has
1207 expressed interest in these features yet, so it's hard to say when
1208 they will be implemented.
1209 - clang does not support nested functions; this is a complex feature
1210 which is infrequently used, so it is unlikely to be implemented
1211 anytime soon. In C++11 it can be emulated by assigning lambda
1212 functions to local variables, e.g:
1216 auto const local_function = [&](int parameter) {
1222 - clang does not support global register variables; this is unlikely to
1223 be implemented soon because it requires additional LLVM backend
1225 - clang does not support static initialization of flexible array
1226 members. This appears to be a rarely used extension, but could be
1227 implemented pending user demand.
1228 - clang does not support
1229 ``__builtin_va_arg_pack``/``__builtin_va_arg_pack_len``. This is
1230 used rarely, but in some potentially interesting places, like the
1231 glibc headers, so it may be implemented pending user demand. Note
1232 that because clang pretends to be like GCC 4.2, and this extension
1233 was introduced in 4.3, the glibc headers will not try to use this
1234 extension with clang at the moment.
1235 - clang does not support the gcc extension for forward-declaring
1236 function parameters; this has not shown up in any real-world code
1237 yet, though, so it might never be implemented.
1239 This is not a complete list; if you find an unsupported extension
1240 missing from this list, please send an e-mail to cfe-dev. This list
1241 currently excludes C++; see :ref:`C++ Language Features <cxx>`. Also, this
1242 list does not include bugs in mostly-implemented features; please see
1244 tracker <http://llvm.org/bugs/buglist.cgi?quicksearch=product%3Aclang+component%3A-New%2BBugs%2CAST%2CBasic%2CDriver%2CHeaders%2CLLVM%2BCodeGen%2Cparser%2Cpreprocessor%2CSemantic%2BAnalyzer>`_
1245 for known existing bugs (FIXME: Is there a section for bug-reporting
1246 guidelines somewhere?).
1248 Intentionally unsupported GCC extensions
1249 ----------------------------------------
1251 - clang does not support the gcc extension that allows variable-length
1252 arrays in structures. This is for a few reasons: one, it is tricky to
1253 implement, two, the extension is completely undocumented, and three,
1254 the extension appears to be rarely used. Note that clang *does*
1255 support flexible array members (arrays with a zero or unspecified
1256 size at the end of a structure).
1257 - clang does not have an equivalent to gcc's "fold"; this means that
1258 clang doesn't accept some constructs gcc might accept in contexts
1259 where a constant expression is required, like "x-x" where x is a
1261 - clang does not support ``__builtin_apply`` and friends; this extension
1262 is extremely obscure and difficult to implement reliably.
1266 Microsoft extensions
1267 --------------------
1269 clang has some experimental support for extensions from Microsoft Visual
1270 C++; to enable it, use the ``-fms-extensions`` command-line option. This is
1271 the default for Windows targets. Note that the support is incomplete.
1272 Some constructs such as ``dllexport`` on classes are ignored with a warning,
1273 and others such as `Microsoft IDL annotations
1274 <http://msdn.microsoft.com/en-us/library/8tesw2eh.aspx>`_ are silently
1277 clang has a ``-fms-compatibility`` flag that makes clang accept enough
1278 invalid C++ to be able to parse most Microsoft headers. For example, it
1279 allows `unqualified lookup of dependent base class members
1280 <http://clang.llvm.org/compatibility.html#dep_lookup_bases>`_, which is
1281 a common compatibility issue with clang. This flag is enabled by default
1282 for Windows targets.
1284 ``-fdelayed-template-parsing`` lets clang delay parsing of function template
1285 definitions until the end of a translation unit. This flag is enabled by
1286 default for Windows targets.
1288 - clang allows setting ``_MSC_VER`` with ``-fmsc-version=``. It defaults to
1289 1700 which is the same as Visual C/C++ 2012. Any number is supported
1290 and can greatly affect what Windows SDK and c++stdlib headers clang
1292 - clang does not support the Microsoft extension where anonymous record
1293 members can be declared using user defined typedefs.
1294 - clang supports the Microsoft ``#pragma pack`` feature for controlling
1295 record layout. GCC also contains support for this feature, however
1296 where MSVC and GCC are incompatible clang follows the MSVC
1298 - clang supports the Microsoft ``#pragma comment(lib, "foo.lib")`` feature for
1299 automatically linking against the specified library. Currently this feature
1300 only works with the Visual C++ linker.
1301 - clang supports the Microsoft ``#pragma comment(linker, "/flag:foo")`` feature
1302 for adding linker flags to COFF object files. The user is responsible for
1303 ensuring that the linker understands the flags.
1304 - clang defaults to C++11 for Windows targets.
1308 C++ Language Features
1309 =====================
1311 clang fully implements all of standard C++98 except for exported
1312 templates (which were removed in C++11), and all of standard C++11
1313 and the current draft standard for C++1y.
1315 Controlling implementation limits
1316 ---------------------------------
1318 .. option:: -fbracket-depth=N
1320 Sets the limit for nested parentheses, brackets, and braces to N. The
1323 .. option:: -fconstexpr-depth=N
1325 Sets the limit for recursive constexpr function invocations to N. The
1328 .. option:: -ftemplate-depth=N
1330 Sets the limit for recursively nested template instantiations to N. The
1333 .. option:: -foperator-arrow-depth=N
1335 Sets the limit for iterative calls to 'operator->' functions to N. The
1340 Objective-C Language Features
1341 =============================
1345 Objective-C++ Language Features
1346 ===============================
1349 .. _target_features:
1351 Target-Specific Features and Limitations
1352 ========================================
1354 CPU Architectures Features and Limitations
1355 ------------------------------------------
1360 The support for X86 (both 32-bit and 64-bit) is considered stable on
1361 Darwin (Mac OS X), Linux, FreeBSD, and Dragonfly BSD: it has been tested
1362 to correctly compile many large C, C++, Objective-C, and Objective-C++
1365 On ``x86_64-mingw32``, passing i128(by value) is incompatible with the
1366 Microsoft x64 calling convention. You might need to tweak
1367 ``WinX86_64ABIInfo::classify()`` in lib/CodeGen/TargetInfo.cpp.
1369 For the X86 target, clang supports the :option:`-m16` command line
1370 argument which enables 16-bit code output. This is broadly similar to
1371 using ``asm(".code16gcc")`` with the GNU toolchain. The generated code
1372 and the ABI remains 32-bit but the assembler emits instructions
1373 appropriate for a CPU running in 16-bit mode, with address-size and
1374 operand-size prefixes to enable 32-bit addressing and operations.
1379 The support for ARM (specifically ARMv6 and ARMv7) is considered stable
1380 on Darwin (iOS): it has been tested to correctly compile many large C,
1381 C++, Objective-C, and Objective-C++ codebases. Clang only supports a
1382 limited number of ARM architectures. It does not yet fully support
1388 The support for PowerPC (especially PowerPC64) is considered stable
1389 on Linux and FreeBSD: it has been tested to correctly compile many
1390 large C and C++ codebases. PowerPC (32bit) is still missing certain
1391 features (e.g. PIC code on ELF platforms).
1396 clang currently contains some support for other architectures (e.g. Sparc);
1397 however, significant pieces of code generation are still missing, and they
1398 haven't undergone significant testing.
1400 clang contains limited support for the MSP430 embedded processor, but
1401 both the clang support and the LLVM backend support are highly
1404 Other platforms are completely unsupported at the moment. Adding the
1405 minimal support needed for parsing and semantic analysis on a new
1406 platform is quite easy; see ``lib/Basic/Targets.cpp`` in the clang source
1407 tree. This level of support is also sufficient for conversion to LLVM IR
1408 for simple programs. Proper support for conversion to LLVM IR requires
1409 adding code to ``lib/CodeGen/CGCall.cpp`` at the moment; this is likely to
1410 change soon, though. Generating assembly requires a suitable LLVM
1413 Operating System Features and Limitations
1414 -----------------------------------------
1424 Clang has experimental support for targeting "Cygming" (Cygwin / MinGW)
1427 See also :ref:`Microsoft Extensions <c_ms>`.
1432 Clang works on Cygwin-1.7.
1437 Clang works on some mingw32 distributions. Clang assumes directories as
1440 - ``C:/mingw/include``
1442 - ``C:/mingw/lib/gcc/mingw32/4.[3-5].0/include/c++``
1444 On MSYS, a few tests might fail.
1449 For 32-bit (i686-w64-mingw32), and 64-bit (x86\_64-w64-mingw32), Clang
1452 - ``GCC versions 4.5.0 to 4.5.3, 4.6.0 to 4.6.2, or 4.7.0 (for the C++ header search path)``
1453 - ``some_directory/bin/gcc.exe``
1454 - ``some_directory/bin/clang.exe``
1455 - ``some_directory/bin/clang++.exe``
1456 - ``some_directory/bin/../include/c++/GCC_version``
1457 - ``some_directory/bin/../include/c++/GCC_version/x86_64-w64-mingw32``
1458 - ``some_directory/bin/../include/c++/GCC_version/i686-w64-mingw32``
1459 - ``some_directory/bin/../include/c++/GCC_version/backward``
1460 - ``some_directory/bin/../x86_64-w64-mingw32/include``
1461 - ``some_directory/bin/../i686-w64-mingw32/include``
1462 - ``some_directory/bin/../include``
1464 This directory layout is standard for any toolchain you will find on the
1465 official `MinGW-w64 website <http://mingw-w64.sourceforge.net>`_.
1467 Clang expects the GCC executable "gcc.exe" compiled for
1468 ``i686-w64-mingw32`` (or ``x86_64-w64-mingw32``) to be present on PATH.
1470 `Some tests might fail <http://llvm.org/bugs/show_bug.cgi?id=9072>`_ on
1471 ``x86_64-w64-mingw32``.
1478 clang-cl is an alternative command-line interface to Clang driver, designed for
1479 compatibility with the Visual C++ compiler, cl.exe.
1481 To enable clang-cl to find system headers, libraries, and the linker when run
1482 from the command-line, it should be executed inside a Visual Studio Native Tools
1483 Command Prompt or a regular Command Prompt where the environment has been set
1484 up using e.g. `vcvars32.bat <http://msdn.microsoft.com/en-us/library/f2ccy3wt.aspx>`_.
1486 clang-cl can also be used from inside Visual Studio by using an LLVM Platform
1489 Command-Line Options
1490 --------------------
1492 To be compatible with cl.exe, clang-cl supports most of the same command-line
1493 options. Those options can start with either ``/`` or ``-``. It also supports
1494 some of Clang's core options, such as the ``-W`` options.
1496 Options that are known to clang-cl, but not currently supported, are ignored
1497 with a warning. For example:
1501 clang-cl.exe: warning: argument unused during compilation: '/Zi'
1503 To suppress warnings about unused arguments, use the ``-Qunused-arguments`` option.
1505 Options that are not known to clang-cl will cause errors. If they are spelled with a
1506 leading ``/``, they will be mistaken for a filename:
1510 clang-cl.exe: error: no such file or directory: '/foobar'
1512 Please `file a bug <http://llvm.org/bugs/enter_bug.cgi?product=clang&component=Driver>`_
1513 for any valid cl.exe flags that clang-cl does not understand.
1515 Execute ``clang-cl /?`` to see a list of supported options:
1519 /? Display available options
1521 /D <macro[=value]> Define macro
1522 /fallback Fall back to cl.exe if clang-cl fails to compile
1523 /FA Output assembly code file during compilation
1524 /Fa<file or directory> Output assembly code to this file during compilation
1525 /Fe<file or directory> Set output executable file or directory (ends in / or \)
1526 /FI<value> Include file before parsing
1527 /Fo<file or directory> Set output object file, or directory (ends in / or \)
1528 /GF- Disable string pooling
1531 /help Display available options
1532 /I <dir> Add directory to include search path
1533 /J Make char type unsigned
1534 /LDd Create debug DLL
1536 /link <options> Forward options to the linker
1537 /MDd Use DLL debug run-time
1538 /MD Use DLL run-time
1539 /MTd Use static debug run-time
1540 /MT Use static run-time
1541 /Ob0 Disable inlining
1542 /Od Disable optimization
1543 /Oi- Disable use of builtin functions
1544 /Oi Enable use of builtin functions
1545 /Os Optimize for size
1546 /Ot Optimize for speed
1547 /Ox Maximum optimization
1548 /Oy- Disable frame pointer omission
1549 /Oy Enable frame pointer omission
1550 /O<n> Optimization level
1551 /P Only run the preprocessor
1552 /showIncludes Print info about included files to stderr
1553 /TC Treat all source files as C
1554 /Tc <filename> Specify a C source file
1555 /TP Treat all source files as C++
1556 /Tp <filename> Specify a C++ source file
1557 /U <macro> Undefine macro
1558 /W0 Disable all warnings
1564 /WX- Do not treat warnings as errors
1565 /WX Treat warnings as errors
1566 /w Disable all warnings
1567 /Zs Syntax-check only
1569 The /fallback Option
1570 ^^^^^^^^^^^^^^^^^^^^
1572 When clang-cl is run with the ``/fallback`` option, it will first try to
1573 compile files itself. For any file that it fails to compile, it will fall back
1574 and try to compile the file by invoking cl.exe.
1576 This option is intended to be used as a temporary means to build projects where
1577 clang-cl cannot successfully compile all the files. clang-cl may fail to compile
1578 a file either because it cannot generate code for some C++ feature, or because
1579 it cannot parse some Microsoft language extension.