============================
Clang Compiler User's Manual
============================
.. contents::
:local:
Introduction
============
The Clang Compiler is an open-source compiler for the C family of
programming languages, aiming to be the best in class implementation of
these languages. Clang builds on the LLVM optimizer and code generator,
allowing it to provide high-quality optimization and code generation
support for many targets. For more general information, please see the
`Clang Web Site <http://clang.llvm.org>`_ or the `LLVM Web
Site <http://llvm.org>`_.
This document describes important notes about using Clang as a compiler
for an end-user, documenting the supported features, command line
options, etc. If you are interested in using Clang to build a tool that
processes code, please see :doc:`InternalsManual`. If you are interested in the
`Clang Static Analyzer <http://clang-analyzer.llvm.org>`_, please see its web
page.
Clang is designed to support the C family of programming languages,
which includes :ref:`C <c>`, :ref:`Objective-C <objc>`, :ref:`C++ <cxx>`, and
:ref:`Objective-C++ <objcxx>` as well as many dialects of those. For
language-specific information, please see the corresponding language
specific section:
- :ref:`C Language <c>`: K&R C, ANSI C89, ISO C90, ISO C94 (C89+AMD1), ISO
C99 (+TC1, TC2, TC3).
- :ref:`Objective-C Language <objc>`: ObjC 1, ObjC 2, ObjC 2.1, plus
variants depending on base language.
- :ref:`C++ Language <cxx>`
- :ref:`Objective C++ Language <objcxx>`
In addition to these base languages and their dialects, Clang supports a
broad variety of language extensions, which are documented in the
corresponding language section. These extensions are provided to be
compatible with the GCC, Microsoft, and other popular compilers as well
as to improve functionality through Clang-specific features. The Clang
driver and language features are intentionally designed to be as
compatible with the GNU GCC compiler as reasonably possible, easing
migration from GCC to Clang. In most cases, code "just works".
Clang also provides an alternative driver, :ref:`clang-cl`, that is designed
to be compatible with the Visual C++ compiler, cl.exe.
In addition to language specific features, Clang has a variety of
features that depend on what CPU architecture or operating system is
being compiled for. Please see the :ref:`Target-Specific Features and
Limitations <target_features>` section for more details.
The rest of the introduction introduces some basic :ref:`compiler
terminology <terminology>` that is used throughout this manual and
contains a basic :ref:`introduction to using Clang <basicusage>` as a
command line compiler.
.. _terminology:
Terminology
-----------
Front end, parser, backend, preprocessor, undefined behavior,
diagnostic, optimizer
.. _basicusage:
Basic Usage
-----------
Intro to how to use a C compiler for newbies.
compile + link compile then link debug info enabling optimizations
picking a language to use, defaults to C99 by default. Autosenses based
on extension. using a makefile
Command Line Options
====================
This section is generally an index into other sections. It does not go
into depth on the ones that are covered by other sections. However, the
first part introduces the language selection and other high level
options like :option:`-c`, :option:`-g`, etc.
Options to Control Error and Warning Messages
---------------------------------------------
.. option:: -Werror
Turn warnings into errors.
.. This is in plain monospaced font because it generates the same label as
.. -Werror, and Sphinx complains.
``-Werror=foo``
Turn warning "foo" into an error.
.. option:: -Wno-error=foo
Turn warning "foo" into an warning even if :option:`-Werror` is specified.
.. option:: -Wfoo
Enable warning "foo".
.. option:: -Wno-foo
Disable warning "foo".
.. option:: -w
Disable all warnings.
.. option:: -Weverything
:ref:`Enable all warnings. <diagnostics_enable_everything>`
.. option:: -pedantic
Warn on language extensions.
.. option:: -pedantic-errors
Error on language extensions.
.. option:: -Wsystem-headers
Enable warnings from system headers.
.. option:: -ferror-limit=123
Stop emitting diagnostics after 123 errors have been produced. The default is
20, and the error limit can be disabled with :option:`-ferror-limit=0`.
.. option:: -ftemplate-backtrace-limit=123
Only emit up to 123 template instantiation notes within the template
instantiation backtrace for a single warning or error. The default is 10, and
the limit can be disabled with :option:`-ftemplate-backtrace-limit=0`.
.. _cl_diag_formatting:
Formatting of Diagnostics
^^^^^^^^^^^^^^^^^^^^^^^^^
Clang aims to produce beautiful diagnostics by default, particularly for
new users that first come to Clang. However, different people have
different preferences, and sometimes Clang is driven by another program
that wants to parse simple and consistent output, not a person. For
these cases, Clang provides a wide range of options to control the exact
output format of the diagnostics that it generates.
.. _opt_fshow-column:
**-f[no-]show-column**
Print column number in diagnostic.
This option, which defaults to on, controls whether or not Clang
prints the column number of a diagnostic. For example, when this is
enabled, Clang will print something like:
::
test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad
^
//
When this is disabled, Clang will print "test.c:28: warning..." with
no column number.
The printed column numbers count bytes from the beginning of the
line; take care if your source contains multibyte characters.
.. _opt_fshow-source-location:
**-f[no-]show-source-location**
Print source file/line/column information in diagnostic.
This option, which defaults to on, controls whether or not Clang
prints the filename, line number and column number of a diagnostic.
For example, when this is enabled, Clang will print something like:
::
test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad
^
//
When this is disabled, Clang will not print the "test.c:28:8: "
part.
.. _opt_fcaret-diagnostics:
**-f[no-]caret-diagnostics**
Print source line and ranges from source code in diagnostic.
This option, which defaults to on, controls whether or not Clang
prints the source line, source ranges, and caret when emitting a
diagnostic. For example, when this is enabled, Clang will print
something like:
::
test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad
^
//
**-f[no-]color-diagnostics**
This option, which defaults to on when a color-capable terminal is
detected, controls whether or not Clang prints diagnostics in color.
When this option is enabled, Clang will use colors to highlight
specific parts of the diagnostic, e.g.,
.. nasty hack to not lose our dignity
.. raw:: html
<pre>
<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>
#endif bad
<span style="color:green">^</span>
<span style="color:green">//</span>
</pre>
When this is disabled, Clang will just print:
::
test.c:2:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad
^
//
**-fansi-escape-codes**
Controls whether ANSI escape codes are used instead of the Windows Console
API to output colored diagnostics. This option is only used on Windows and
defaults to off.
.. option:: -fdiagnostics-format=clang/msvc/vi
Changes diagnostic output format to better match IDEs and command line tools.
This option controls the output format of the filename, line number,
and column printed in diagnostic messages. The options, and their
affect on formatting a simple conversion diagnostic, follow:
**clang** (default)
::
t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int'
**msvc**
::
t.c(3,11) : warning: conversion specifies type 'char *' but the argument has type 'int'
**vi**
::
t.c +3:11: warning: conversion specifies type 'char *' but the argument has type 'int'
.. _opt_fdiagnostics-show-option:
**-f[no-]diagnostics-show-option**
Enable ``[-Woption]`` information in diagnostic line.
This option, which defaults to on, controls whether or not Clang
prints the associated :ref:`warning group <cl_diag_warning_groups>`
option name when outputting a warning diagnostic. For example, in
this output:
::
test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad
^
//
Passing **-fno-diagnostics-show-option** will prevent Clang from
printing the [:ref:`-Wextra-tokens <opt_Wextra-tokens>`] information in
the diagnostic. This information tells you the flag needed to enable
or disable the diagnostic, either from the command line or through
:ref:`#pragma GCC diagnostic <pragma_GCC_diagnostic>`.
.. _opt_fdiagnostics-show-category:
.. option:: -fdiagnostics-show-category=none/id/name
Enable printing category information in diagnostic line.
This option, which defaults to "none", controls whether or not Clang
prints the category associated with a diagnostic when emitting it.
Each diagnostic may or many not have an associated category, if it
has one, it is listed in the diagnostic categorization field of the
diagnostic line (in the []'s).
For example, a format string warning will produce these three
renditions based on the setting of this option:
::
t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat]
t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat,1]
t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat,Format String]
This category can be used by clients that want to group diagnostics
by category, so it should be a high level category. We want dozens
of these, not hundreds or thousands of them.
.. _opt_fdiagnostics-fixit-info:
**-f[no-]diagnostics-fixit-info**
Enable "FixIt" information in the diagnostics output.
This option, which defaults to on, controls whether or not Clang
prints the information on how to fix a specific diagnostic
underneath it when it knows. For example, in this output:
::
test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad
^
//
Passing **-fno-diagnostics-fixit-info** will prevent Clang from
printing the "//" line at the end of the message. This information
is useful for users who may not understand what is wrong, but can be
confusing for machine parsing.
.. _opt_fdiagnostics-print-source-range-info:
**-fdiagnostics-print-source-range-info**
Print machine parsable information about source ranges.
This option makes Clang print information about source ranges in a machine
parsable format after the file/line/column number information. The
information is a simple sequence of brace enclosed ranges, where each range
lists the start and end line/column locations. For example, in this output:
::
exprs.c:47:15:{47:8-47:14}{47:17-47:24}: error: invalid operands to binary expression ('int *' and '_Complex float')
P = (P-42) + Gamma*4;
~~~~~~ ^ ~~~~~~~
The {}'s are generated by -fdiagnostics-print-source-range-info.
The printed column numbers count bytes from the beginning of the
line; take care if your source contains multibyte characters.
.. option:: -fdiagnostics-parseable-fixits
Print Fix-Its in a machine parseable form.
This option makes Clang print available Fix-Its in a machine
parseable format at the end of diagnostics. The following example
illustrates the format:
::
fix-it:"t.cpp":{7:25-7:29}:"Gamma"
The range printed is a half-open range, so in this example the
characters at column 25 up to but not including column 29 on line 7
in t.cpp should be replaced with the string "Gamma". Either the
range or the replacement string may be empty (representing strict
insertions and strict erasures, respectively). Both the file name
and the insertion string escape backslash (as "\\\\"), tabs (as
"\\t"), newlines (as "\\n"), double quotes(as "\\"") and
non-printable characters (as octal "\\xxx").
The printed column numbers count bytes from the beginning of the
line; take care if your source contains multibyte characters.
.. option:: -fno-elide-type
Turns off elision in template type printing.
The default for template type printing is to elide as many template
arguments as possible, removing those which are the same in both
template types, leaving only the differences. Adding this flag will
print all the template arguments. If supported by the terminal,
highlighting will still appear on differing arguments.
Default:
::
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;
-fno-elide-type:
::
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;
.. option:: -fdiagnostics-show-template-tree
Template type diffing prints a text tree.
For diffing large templated types, this option will cause Clang to
display the templates as an indented text tree, one argument per
line, with differences marked inline. This is compatible with
-fno-elide-type.
Default:
::
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;
With :option:`-fdiagnostics-show-template-tree`:
::
t.cc:4:5: note: candidate function not viable: no known conversion for 1st argument;
vector<
map<
[...],
map<
[float != double],
[...]>>>
.. _cl_diag_warning_groups:
Individual Warning Groups
^^^^^^^^^^^^^^^^^^^^^^^^^
TODO: Generate this from tblgen. Define one anchor per warning group.
.. _opt_wextra-tokens:
.. option:: -Wextra-tokens
Warn about excess tokens at the end of a preprocessor directive.
This option, which defaults to on, enables warnings about extra
tokens at the end of preprocessor directives. For example:
::
test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif bad
^
These extra tokens are not strictly conforming, and are usually best
handled by commenting them out.
.. option:: -Wambiguous-member-template
Warn about unqualified uses of a member template whose name resolves to
another template at the location of the use.
This option, which defaults to on, enables a warning in the
following code:
::
template<typename T> struct set{};
template<typename T> struct trait { typedef const T& type; };
struct Value {
template<typename T> void set(typename trait<T>::type value) {}
};
void foo() {
Value v;
v.set<double>(3.2);
}
C++ [basic.lookup.classref] requires this to be an error, but,
because it's hard to work around, Clang downgrades it to a warning
as an extension.
.. option:: -Wbind-to-temporary-copy
Warn about an unusable copy constructor when binding a reference to a
temporary.
This option, which defaults to on, enables warnings about binding a
reference to a temporary when the temporary doesn't have a usable
copy constructor. For example:
::
struct NonCopyable {
NonCopyable();
private:
NonCopyable(const NonCopyable&);
};
void foo(const NonCopyable&);
void bar() {
foo(NonCopyable()); // Disallowed in C++98; allowed in C++11.
}
::
struct NonCopyable2 {
NonCopyable2();
NonCopyable2(NonCopyable2&);
};
void foo(const NonCopyable2&);
void bar() {
foo(NonCopyable2()); // Disallowed in C++98; allowed in C++11.
}
Note that if ``NonCopyable2::NonCopyable2()`` has a default argument
whose instantiation produces a compile error, that error will still
be a hard error in C++98 mode even if this warning is turned off.
Options to Control Clang Crash Diagnostics
------------------------------------------
As unbelievable as it may sound, Clang does crash from time to time.
Generally, this only occurs to those living on the `bleeding
edge <http://llvm.org/releases/download.html#svn>`_. Clang goes to great
lengths to assist you in filing a bug report. Specifically, Clang
generates preprocessed source file(s) and associated run script(s) upon
a crash. These files should be attached to a bug report to ease
reproducibility of the failure. Below are the command line options to
control the crash diagnostics.
.. option:: -fno-crash-diagnostics
Disable auto-generation of preprocessed source files during a clang crash.
The -fno-crash-diagnostics flag can be helpful for speeding the process
of generating a delta reduced test case.
Language and Target-Independent Features
========================================
Controlling Errors and Warnings
-------------------------------
Clang provides a number of ways to control which code constructs cause
it to emit errors and warning messages, and how they are displayed to
the console.
Controlling How Clang Displays Diagnostics
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
When Clang emits a diagnostic, it includes rich information in the
output, and gives you fine-grain control over which information is
printed. Clang has the ability to print this information, and these are
the options that control it:
#. A file/line/column indicator that shows exactly where the diagnostic
occurs in your code [:ref:`-fshow-column <opt_fshow-column>`,
:ref:`-fshow-source-location <opt_fshow-source-location>`].
#. A categorization of the diagnostic as a note, warning, error, or
fatal error.
#. A text string that describes what the problem is.
#. An option that indicates how to control the diagnostic (for
diagnostics that support it)
[:ref:`-fdiagnostics-show-option <opt_fdiagnostics-show-option>`].
#. A :ref:`high-level category <diagnostics_categories>` for the diagnostic
for clients that want to group diagnostics by class (for diagnostics
that support it)
[:ref:`-fdiagnostics-show-category <opt_fdiagnostics-show-category>`].
#. The line of source code that the issue occurs on, along with a caret
and ranges that indicate the important locations
[:ref:`-fcaret-diagnostics <opt_fcaret-diagnostics>`].
#. "FixIt" information, which is a concise explanation of how to fix the
problem (when Clang is certain it knows)
[:ref:`-fdiagnostics-fixit-info <opt_fdiagnostics-fixit-info>`].
#. A machine-parsable representation of the ranges involved (off by
default)
[:ref:`-fdiagnostics-print-source-range-info <opt_fdiagnostics-print-source-range-info>`].
For more information please see :ref:`Formatting of
Diagnostics <cl_diag_formatting>`.
Diagnostic Mappings
^^^^^^^^^^^^^^^^^^^
All diagnostics are mapped into one of these 5 classes:
- Ignored
- Note
- Warning
- Error
- Fatal
.. _diagnostics_categories:
Diagnostic Categories
^^^^^^^^^^^^^^^^^^^^^
Though not shown by default, diagnostics may each be associated with a
high-level category. This category is intended to make it possible to
triage builds that produce a large number of errors or warnings in a
grouped way.
Categories are not shown by default, but they can be turned on with the
:ref:`-fdiagnostics-show-category <opt_fdiagnostics-show-category>` option.
When set to "``name``", the category is printed textually in the
diagnostic output. When it is set to "``id``", a category number is
printed. The mapping of category names to category id's can be obtained
by running '``clang --print-diagnostic-categories``'.
Controlling Diagnostics via Command Line Flags
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TODO: -W flags, -pedantic, etc
.. _pragma_gcc_diagnostic:
Controlling Diagnostics via Pragmas
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Clang can also control what diagnostics are enabled through the use of
pragmas in the source code. This is useful for turning off specific
warnings in a section of source code. Clang supports GCC's pragma for
compatibility with existing source code, as well as several extensions.
The pragma may control any warning that can be used from the command
line. Warnings may be set to ignored, warning, error, or fatal. The
following example code will tell Clang or GCC to ignore the -Wall
warnings:
.. code-block:: c
#pragma GCC diagnostic ignored "-Wall"
In addition to all of the functionality provided by GCC's pragma, Clang
also allows you to push and pop the current warning state. This is
particularly useful when writing a header file that will be compiled by
other people, because you don't know what warning flags they build with.
In the below example :option:`-Wmultichar` is ignored for only a single line of
code, after which the diagnostics return to whatever state had previously
existed.
.. code-block:: c
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wmultichar"
char b = 'df'; // no warning.
#pragma clang diagnostic pop
The push and pop pragmas will save and restore the full diagnostic state
of the compiler, regardless of how it was set. That means that it is
possible to use push and pop around GCC compatible diagnostics and Clang
will push and pop them appropriately, while GCC will ignore the pushes
and pops as unknown pragmas. It should be noted that while Clang
supports the GCC pragma, Clang and GCC do not support the exact same set
of warnings, so even when using GCC compatible #pragmas there is no
guarantee that they will have identical behaviour on both compilers.
In addition to controlling warnings and errors generated by the compiler, it is
possible to generate custom warning and error messages through the following
pragmas:
.. code-block:: c
// The following will produce warning messages
#pragma message "some diagnostic message"
#pragma GCC warning "TODO: replace deprecated feature"
// The following will produce an error message
#pragma GCC error "Not supported"
These pragmas operate similarly to the ``#warning`` and ``#error`` preprocessor
directives, except that they may also be embedded into preprocessor macros via
the C99 ``_Pragma`` operator, for example:
.. code-block:: c
#define STR(X) #X
#define DEFER(M,...) M(__VA_ARGS__)
#define CUSTOM_ERROR(X) _Pragma(STR(GCC error(X " at line " DEFER(STR,__LINE__))))
CUSTOM_ERROR("Feature not available");
Controlling Diagnostics in System Headers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warnings are suppressed when they occur in system headers. By default,
an included file is treated as a system header if it is found in an
include path specified by ``-isystem``, but this can be overridden in
several ways.
The ``system_header`` pragma can be used to mark the current file as
being a system header. No warnings will be produced from the location of
the pragma onwards within the same file.
.. code-block:: c
char a = 'xy'; // warning
#pragma clang system_header
char b = 'ab'; // no warning
The :option:`-isystem-prefix` and :option:`-ino-system-prefix` command-line
arguments can be used to override whether subsets of an include path are
treated as system headers. When the name in a ``#include`` directive is
found within a header search path and starts with a system prefix, the
header is treated as a system header. The last prefix on the
command-line which matches the specified header name takes precedence.
For instance:
.. code-block:: console
$ clang -Ifoo -isystem bar -isystem-prefix x/ -ino-system-prefix x/y/
Here, ``#include "x/a.h"`` is treated as including a system header, even
if the header is found in ``foo``, and ``#include "x/y/b.h"`` is treated
as not including a system header, even if the header is found in
``bar``.
A ``#include`` directive which finds a file relative to the current
directory is treated as including a system header if the including file
is treated as a system header.
.. _diagnostics_enable_everything:
Enabling All Warnings
^^^^^^^^^^^^^^^^^^^^^
In addition to the traditional ``-W`` flags, one can enable **all**
warnings by passing :option:`-Weverything`. This works as expected with
:option:`-Werror`, and also includes the warnings from :option:`-pedantic`.
Note that when combined with :option:`-w` (which disables all warnings), that
flag wins.
Controlling Static Analyzer Diagnostics
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
While not strictly part of the compiler, the diagnostics from Clang's
`static analyzer <http://clang-analyzer.llvm.org>`_ can also be
influenced by the user via changes to the source code. See the available
`annotations <http://clang-analyzer.llvm.org/annotations.html>`_ and the
analyzer's `FAQ
page <http://clang-analyzer.llvm.org/faq.html#exclude_code>`_ for more
information.
.. _usersmanual-precompiled-headers:
Precompiled Headers
-------------------
`Precompiled headers <http://en.wikipedia.org/wiki/Precompiled_header>`__
are a general approach employed by many compilers to reduce compilation
time. The underlying motivation of the approach is that it is common for
the same (and often large) header files to be included by multiple
source files. Consequently, compile times can often be greatly improved
by caching some of the (redundant) work done by a compiler to process
headers. Precompiled header files, which represent one of many ways to
implement this optimization, are literally files that represent an
on-disk cache that contains the vital information necessary to reduce
some of the work needed to process a corresponding header file. While
details of precompiled headers vary between compilers, precompiled
headers have been shown to be highly effective at speeding up program
compilation on systems with very large system headers (e.g., Mac OS/X).
Generating a PCH File
^^^^^^^^^^^^^^^^^^^^^
To generate a PCH file using Clang, one invokes Clang with the
:option:`-x <language>-header` option. This mirrors the interface in GCC
for generating PCH files:
.. code-block:: console
$ gcc -x c-header test.h -o test.h.gch
$ clang -x c-header test.h -o test.h.pch
Using a PCH File
^^^^^^^^^^^^^^^^
A PCH file can then be used as a prefix header when a :option:`-include`
option is passed to ``clang``:
.. code-block:: console
$ clang -include test.h test.c -o test
The ``clang`` driver will first check if a PCH file for ``test.h`` is
available; if so, the contents of ``test.h`` (and the files it includes)
will be processed from the PCH file. Otherwise, Clang falls back to
directly processing the content of ``test.h``. This mirrors the behavior
of GCC.
.. note::
Clang does *not* automatically use PCH files for headers that are directly
included within a source file. For example:
.. code-block:: console
$ clang -x c-header test.h -o test.h.pch
$ cat test.c
#include "test.h"
$ clang test.c -o test
In this example, ``clang`` will not automatically use the PCH file for
``test.h`` since ``test.h`` was included directly in the source file and not
specified on the command line using :option:`-include`.
Relocatable PCH Files
^^^^^^^^^^^^^^^^^^^^^
It is sometimes necessary to build a precompiled header from headers
that are not yet in their final, installed locations. For example, one
might build a precompiled header within the build tree that is then
meant to be installed alongside the headers. Clang permits the creation
of "relocatable" precompiled headers, which are built with a given path
(into the build directory) and can later be used from an installed
location.
To build a relocatable precompiled header, place your headers into a
subdirectory whose structure mimics the installed location. For example,
if you want to build a precompiled header for the header ``mylib.h``
that will be installed into ``/usr/include``, create a subdirectory
``build/usr/include`` and place the header ``mylib.h`` into that
subdirectory. If ``mylib.h`` depends on other headers, then they can be
stored within ``build/usr/include`` in a way that mimics the installed
location.
Building a relocatable precompiled header requires two additional
arguments. First, pass the ``--relocatable-pch`` flag to indicate that
the resulting PCH file should be relocatable. Second, pass
:option:`-isysroot /path/to/build`, which makes all includes for your library
relative to the build directory. For example:
.. code-block:: console
# clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch
When loading the relocatable PCH file, the various headers used in the
PCH file are found from the system header root. For example, ``mylib.h``
can be found in ``/usr/include/mylib.h``. If the headers are installed
in some other system root, the :option:`-isysroot` option can be used provide
a different system root from which the headers will be based. For
example, :option:`-isysroot /Developer/SDKs/MacOSX10.4u.sdk` will look for
``mylib.h`` in ``/Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h``.
Relocatable precompiled headers are intended to be used in a limited
number of cases where the compilation environment is tightly controlled
and the precompiled header cannot be generated after headers have been
installed.
Controlling Code Generation
---------------------------
Clang provides a number of ways to control code generation. The options
are listed below.
**-f[no-]sanitize=check1,check2,...**
Turn on runtime checks for various forms of undefined or suspicious
behavior.
This option controls whether Clang adds runtime checks for various
forms of undefined or suspicious behavior, and is disabled by
default. If a check fails, a diagnostic message is produced at
runtime explaining the problem. The main checks are:
- .. _opt_fsanitize_address:
``-fsanitize=address``:
:doc:`AddressSanitizer`, a memory error
detector.
- ``-fsanitize=init-order``: Make AddressSanitizer check for
dynamic initialization order problems. Implied by ``-fsanitize=address``.
- ``-fsanitize=address-full``: AddressSanitizer with all the
experimental features listed below.
- ``-fsanitize=integer``: Enables checks for undefined or
suspicious integer behavior.
- .. _opt_fsanitize_thread:
``-fsanitize=thread``: :doc:`ThreadSanitizer`, a data race detector.
- .. _opt_fsanitize_memory:
``-fsanitize=memory``: :doc:`MemorySanitizer`,
an *experimental* detector of uninitialized reads. Not ready for
widespread use.
- .. _opt_fsanitize_undefined:
``-fsanitize=undefined``: Fast and compatible undefined behavior
checker. Enables the undefined behavior checks that have small
runtime cost and no impact on address space layout or ABI. This
includes all of the checks listed below other than
``unsigned-integer-overflow``.
- ``-fsanitize=undefined-trap``: This includes all sanitizers
included by ``-fsanitize=undefined``, except those that require
runtime support. This group of sanitizers is intended to be
used in conjunction with the ``-fsanitize-undefined-trap-on-error``
flag. This includes all of the checks listed below other than
``unsigned-integer-overflow`` and ``vptr``.
- ``-fsanitize=dataflow``: :doc:`DataFlowSanitizer`, a general data
flow analysis.
The following more fine-grained checks are also available:
- ``-fsanitize=alignment``: Use of a misaligned pointer or creation
of a misaligned reference.
- ``-fsanitize=bool``: Load of a ``bool`` value which is neither
``true`` nor ``false``.
- ``-fsanitize=bounds``: Out of bounds array indexing, in cases
where the array bound can be statically determined.
- ``-fsanitize=enum``: Load of a value of an enumerated type which
is not in the range of representable values for that enumerated
type.
- ``-fsanitize=float-cast-overflow``: Conversion to, from, or
between floating-point types which would overflow the
destination.
- ``-fsanitize=float-divide-by-zero``: Floating point division by
zero.
- ``-fsanitize=function``: Indirect call of a function through a
function pointer of the wrong type (Linux, C++ and x86/x86_64 only).
- ``-fsanitize=integer-divide-by-zero``: Integer division by zero.
- ``-fsanitize=null``: Use of a null pointer or creation of a null
reference.
- ``-fsanitize=object-size``: An attempt to use bytes which the
optimizer can determine are not part of the object being
accessed. The sizes of objects are determined using
``__builtin_object_size``, and consequently may be able to detect
more problems at higher optimization levels.
- ``-fsanitize=return``: In C++, reaching the end of a
value-returning function without returning a value.
- ``-fsanitize=shift``: Shift operators where the amount shifted is
greater or equal to the promoted bit-width of the left hand side
or less than zero, or where the left hand side is negative. For a
signed left shift, also checks for signed overflow in C, and for
unsigned overflow in C++.
- ``-fsanitize=signed-integer-overflow``: Signed integer overflow,
including all the checks added by ``-ftrapv``, and checking for
overflow in signed division (``INT_MIN / -1``).
- ``-fsanitize=unreachable``: If control flow reaches
``__builtin_unreachable``.
- ``-fsanitize=unsigned-integer-overflow``: Unsigned integer
overflows.
- ``-fsanitize=vla-bound``: A variable-length array whose bound
does not evaluate to a positive value.
- ``-fsanitize=vptr``: Use of an object whose vptr indicates that
it is of the wrong dynamic type, or that its lifetime has not
begun or has ended. Incompatible with ``-fno-rtti``.
You can turn off or modify checks for certain source files, functions
or even variables by providing a special file:
- ``-fsanitize-blacklist=/path/to/blacklist/file``: disable or modify
sanitizer checks for objects listed in the file. See
:doc:`SanitizerSpecialCaseList` for file format description.
- ``-fno-sanitize-blacklist``: don't use blacklist file, if it was
specified earlier in the command line.
Experimental features of AddressSanitizer (not ready for widespread
use, require explicit ``-fsanitize=address``):
- ``-fsanitize=use-after-return``: Check for use-after-return
errors (accessing local variable after the function exit).
- ``-fsanitize=use-after-scope``: Check for use-after-scope errors
(accesing local variable after it went out of scope).
Extra features of MemorySanitizer (require explicit
``-fsanitize=memory``):
- ``-fsanitize-memory-track-origins``: Enables origin tracking in
MemorySanitizer. Adds a second section to MemorySanitizer
reports pointing to the heap or stack allocation the
uninitialized bits came from. Slows down execution by additional
1.5x-2x.
Extra features of UndefinedBehaviorSanitizer:
- ``-fno-sanitize-recover``: By default, after a sanitizer diagnoses
an issue, it will attempt to continue executing the program if there
is a reasonable behavior it can give to the faulting operation. This
option causes the program to abort instead.
- ``-fsanitize-undefined-trap-on-error``: Causes traps to be emitted
rather than calls to runtime libraries when a problem is detected.
This option is intended for use in cases where the sanitizer runtime
cannot be used (for instance, when building libc or a kernel module).
This is only compatible with the sanitizers in the ``undefined-trap``
group.
The ``-fsanitize=`` argument must also be provided when linking, in
order to link to the appropriate runtime library. When using
``-fsanitize=vptr`` (or a group that includes it, such as
``-fsanitize=undefined``) with a C++ program, the link must be
performed by ``clang++``, not ``clang``, in order to link against the
C++-specific parts of the runtime library.
It is not possible to combine more than one of the ``-fsanitize=address``,
``-fsanitize=thread``, and ``-fsanitize=memory`` checkers in the same
program. The ``-fsanitize=undefined`` checks can be combined with other
sanitizers.
**-f[no-]address-sanitizer**
Deprecated synonym for :ref:`-f[no-]sanitize=address
<opt_fsanitize_address>`.
**-f[no-]thread-sanitizer**
Deprecated synonym for :ref:`-f[no-]sanitize=thread
<opt_fsanitize_thread>`.
.. option:: -fcatch-undefined-behavior
Deprecated synonym for :ref:`-fsanitize=undefined
<opt_fsanitize_undefined>`.
.. option:: -fno-assume-sane-operator-new
Don't assume that the C++'s new operator is sane.
This option tells the compiler to do not assume that C++'s global
new operator will always return a pointer that does not alias any
other pointer when the function returns.
.. option:: -ftrap-function=[name]
Instruct code generator to emit a function call to the specified
function name for ``__builtin_trap()``.
LLVM code generator translates ``__builtin_trap()`` to a trap
instruction if it is supported by the target ISA. Otherwise, the
builtin is translated into a call to ``abort``. If this option is
set, then the code generator will always lower the builtin to a call
to the specified function regardless of whether the target ISA has a
trap instruction. This option is useful for environments (e.g.
deeply embedded) where a trap cannot be properly handled, or when
some custom behavior is desired.
.. option:: -ftls-model=[model]
Select which TLS model to use.
Valid values are: ``global-dynamic``, ``local-dynamic``,
``initial-exec`` and ``local-exec``. The default value is
``global-dynamic``. The compiler may use a different model if the
selected model is not supported by the target, or if a more
efficient model can be used. The TLS model can be overridden per
variable using the ``tls_model`` attribute.
.. option:: -mhwdiv=[values]
Select the ARM modes (arm or thumb) that support hardware division
instructions.
Valid values are: ``arm``, ``thumb`` and ``arm,thumb``.
This option is used to indicate which mode (arm or thumb) supports
hardware division instructions. This only applies to the ARM
architecture.
.. option:: -m[no-]crc
Enable or disable CRC instructions.
This option is used to indicate whether CRC instructions are to
be generated. This only applies to the ARM architecture.
CRC instructions are enabled by default on ARMv8.
Controlling Size of Debug Information
-------------------------------------
Debug info kind generated by Clang can be set by one of the flags listed
below. If multiple flags are present, the last one is used.
.. option:: -g0
Don't generate any debug info (default).
.. option:: -gline-tables-only
Generate line number tables only.
This kind of debug info allows to obtain stack traces with function names,
file names and line numbers (by such tools as ``gdb`` or ``addr2line``). It
doesn't contain any other data (e.g. description of local variables or
function parameters).
.. option:: -g
Generate complete debug info.
Comment Parsing Options
--------------------------
Clang parses Doxygen and non-Doxygen style documentation comments and attaches
them to the appropriate declaration nodes. By default, it only parses
Doxygen-style comments and ignores ordinary comments starting with ``//`` and
``/*``.
.. option:: -fparse-all-comments
Parse all comments as documentation comments (including ordinary comments
starting with ``//`` and ``/*``).
.. _c:
C Language Features
===================
The support for standard C in clang is feature-complete except for the
C99 floating-point pragmas.
Extensions supported by clang
-----------------------------
See :doc:`LanguageExtensions`.
Differences between various standard modes
------------------------------------------
clang supports the -std option, which changes what language mode clang
uses. The supported modes for C are c89, gnu89, c94, c99, gnu99 and
various aliases for those modes. If no -std option is specified, clang
defaults to gnu99 mode.
Differences between all ``c*`` and ``gnu*`` modes:
- ``c*`` modes define "``__STRICT_ANSI__``".
- Target-specific defines not prefixed by underscores, like "linux",
are defined in ``gnu*`` modes.
- Trigraphs default to being off in ``gnu*`` modes; they can be enabled by
the -trigraphs option.
- The parser recognizes "asm" and "typeof" as keywords in ``gnu*`` modes;
the variants "``__asm__``" and "``__typeof__``" are recognized in all
modes.
- The Apple "blocks" extension is recognized by default in ``gnu*`` modes
on some platforms; it can be enabled in any mode with the "-fblocks"
option.
- Arrays that are VLA's according to the standard, but which can be
constant folded by the frontend are treated as fixed size arrays.
This occurs for things like "int X[(1, 2)];", which is technically a
VLA. ``c*`` modes are strictly compliant and treat these as VLAs.
Differences between ``*89`` and ``*99`` modes:
- The ``*99`` modes default to implementing "inline" as specified in C99,
while the ``*89`` modes implement the GNU version. This can be
overridden for individual functions with the ``__gnu_inline__``
attribute.
- Digraphs are not recognized in c89 mode.
- The scope of names defined inside a "for", "if", "switch", "while",
or "do" statement is different. (example: "``if ((struct x {int
x;}*)0) {}``".)
- ``__STDC_VERSION__`` is not defined in ``*89`` modes.
- "inline" is not recognized as a keyword in c89 mode.
- "restrict" is not recognized as a keyword in ``*89`` modes.
- Commas are allowed in integer constant expressions in ``*99`` modes.
- Arrays which are not lvalues are not implicitly promoted to pointers
in ``*89`` modes.
- Some warnings are different.
c94 mode is identical to c89 mode except that digraphs are enabled in
c94 mode (FIXME: And ``__STDC_VERSION__`` should be defined!).
GCC extensions not implemented yet
----------------------------------
clang tries to be compatible with gcc as much as possible, but some gcc
extensions are not implemented yet:
- clang does not support #pragma weak (`bug
3679 <http://llvm.org/bugs/show_bug.cgi?id=3679>`_). Due to the uses
described in the bug, this is likely to be implemented at some point,
at least partially.
- clang does not support decimal floating point types (``_Decimal32`` and
friends) or fixed-point types (``_Fract`` and friends); nobody has
expressed interest in these features yet, so it's hard to say when
they will be implemented.
- clang does not support nested functions; this is a complex feature
which is infrequently used, so it is unlikely to be implemented
anytime soon. In C++11 it can be emulated by assigning lambda
functions to local variables, e.g:
.. code-block:: cpp
auto const local_function = [&](int parameter) {
// Do something
};
...
local_function(1);
- clang does not support global register variables; this is unlikely to
be implemented soon because it requires additional LLVM backend
support.
- clang does not support static initialization of flexible array
members. This appears to be a rarely used extension, but could be
implemented pending user demand.
- clang does not support
``__builtin_va_arg_pack``/``__builtin_va_arg_pack_len``. This is
used rarely, but in some potentially interesting places, like the
glibc headers, so it may be implemented pending user demand. Note
that because clang pretends to be like GCC 4.2, and this extension
was introduced in 4.3, the glibc headers will not try to use this
extension with clang at the moment.
- clang does not support the gcc extension for forward-declaring
function parameters; this has not shown up in any real-world code
yet, though, so it might never be implemented.
This is not a complete list; if you find an unsupported extension
missing from this list, please send an e-mail to cfe-dev. This list
currently excludes C++; see :ref:`C++ Language Features <cxx>`. Also, this
list does not include bugs in mostly-implemented features; please see
the `bug
tracker <http://llvm.org/bugs/buglist.cgi?quicksearch=product%3Aclang+component%3A-New%2BBugs%2CAST%2CBasic%2CDriver%2CHeaders%2CLLVM%2BCodeGen%2Cparser%2Cpreprocessor%2CSemantic%2BAnalyzer>`_
for known existing bugs (FIXME: Is there a section for bug-reporting
guidelines somewhere?).
Intentionally unsupported GCC extensions
----------------------------------------
- clang does not support the gcc extension that allows variable-length
arrays in structures. This is for a few reasons: one, it is tricky to
implement, two, the extension is completely undocumented, and three,
the extension appears to be rarely used. Note that clang *does*
support flexible array members (arrays with a zero or unspecified
size at the end of a structure).
- clang does not have an equivalent to gcc's "fold"; this means that
clang doesn't accept some constructs gcc might accept in contexts
where a constant expression is required, like "x-x" where x is a
variable.
- clang does not support ``__builtin_apply`` and friends; this extension
is extremely obscure and difficult to implement reliably.
.. _c_ms:
Microsoft extensions
--------------------
clang has some experimental support for extensions from Microsoft Visual
C++; to enable it, use the ``-fms-extensions`` command-line option. This is
the default for Windows targets. Note that the support is incomplete.
Some constructs such as ``dllexport`` on classes are ignored with a warning,
and others such as `Microsoft IDL annotations
<http://msdn.microsoft.com/en-us/library/8tesw2eh.aspx>`_ are silently
ignored.
clang has a ``-fms-compatibility`` flag that makes clang accept enough
invalid C++ to be able to parse most Microsoft headers. For example, it
allows `unqualified lookup of dependent base class members
<http://clang.llvm.org/compatibility.html#dep_lookup_bases>`_, which is
a common compatibility issue with clang. This flag is enabled by default
for Windows targets.
``-fdelayed-template-parsing`` lets clang delay parsing of function template
definitions until the end of a translation unit. This flag is enabled by
default for Windows targets.
- clang allows setting ``_MSC_VER`` with ``-fmsc-version=``. It defaults to
1700 which is the same as Visual C/C++ 2012. Any number is supported
and can greatly affect what Windows SDK and c++stdlib headers clang
can compile.
- clang does not support the Microsoft extension where anonymous record
members can be declared using user defined typedefs.
- clang supports the Microsoft ``#pragma pack`` feature for controlling
record layout. GCC also contains support for this feature, however
where MSVC and GCC are incompatible clang follows the MSVC
definition.
- clang supports the Microsoft ``#pragma comment(lib, "foo.lib")`` feature for
automatically linking against the specified library. Currently this feature
only works with the Visual C++ linker.
- clang supports the Microsoft ``#pragma comment(linker, "/flag:foo")`` feature
for adding linker flags to COFF object files. The user is responsible for
ensuring that the linker understands the flags.
- clang defaults to C++11 for Windows targets.
.. _cxx:
C++ Language Features
=====================
clang fully implements all of standard C++98 except for exported
templates (which were removed in C++11), and all of standard C++11
and the current draft standard for C++1y.
Controlling implementation limits
---------------------------------
.. option:: -fbracket-depth=N
Sets the limit for nested parentheses, brackets, and braces to N. The
default is 256.
.. option:: -fconstexpr-depth=N
Sets the limit for recursive constexpr function invocations to N. The
default is 512.
.. option:: -ftemplate-depth=N
Sets the limit for recursively nested template instantiations to N. The
default is 256.
.. option:: -foperator-arrow-depth=N
Sets the limit for iterative calls to 'operator->' functions to N. The
default is 256.
.. _objc:
Objective-C Language Features
=============================
.. _objcxx:
Objective-C++ Language Features
===============================
.. _target_features:
Target-Specific Features and Limitations
========================================
CPU Architectures Features and Limitations
------------------------------------------
X86
^^^
The support for X86 (both 32-bit and 64-bit) is considered stable on
Darwin (Mac OS/X), Linux, FreeBSD, and Dragonfly BSD: it has been tested
to correctly compile many large C, C++, Objective-C, and Objective-C++
codebases.
On ``x86_64-mingw32``, passing i128(by value) is incompatible with the
Microsoft x64 calling conversion. You might need to tweak
``WinX86_64ABIInfo::classify()`` in lib/CodeGen/TargetInfo.cpp.
ARM
^^^
The support for ARM (specifically ARMv6 and ARMv7) is considered stable
on Darwin (iOS): it has been tested to correctly compile many large C,
C++, Objective-C, and Objective-C++ codebases. Clang only supports a
limited number of ARM architectures. It does not yet fully support
ARMv5, for example.
PowerPC
^^^^^^^
The support for PowerPC (especially PowerPC64) is considered stable
on Linux and FreeBSD: it has been tested to correctly compile many
large C and C++ codebases. PowerPC (32bit) is still missing certain
features (e.g. PIC code on ELF platforms).
Other platforms
^^^^^^^^^^^^^^^
clang currently contains some support for other architectures (e.g. Sparc);
however, significant pieces of code generation are still missing, and they
haven't undergone significant testing.
clang contains limited support for the MSP430 embedded processor, but
both the clang support and the LLVM backend support are highly
experimental.
Other platforms are completely unsupported at the moment. Adding the
minimal support needed for parsing and semantic analysis on a new
platform is quite easy; see ``lib/Basic/Targets.cpp`` in the clang source
tree. This level of support is also sufficient for conversion to LLVM IR
for simple programs. Proper support for conversion to LLVM IR requires
adding code to ``lib/CodeGen/CGCall.cpp`` at the moment; this is likely to
change soon, though. Generating assembly requires a suitable LLVM
backend.
Operating System Features and Limitations
-----------------------------------------
Darwin (Mac OS/X)
^^^^^^^^^^^^^^^^^
None
Windows
^^^^^^^
Clang has experimental support for targeting "Cygming" (Cygwin / MinGW)
platforms.
See also :ref:`Microsoft Extensions <c_ms>`.
Cygwin
""""""
Clang works on Cygwin-1.7.
MinGW32
"""""""
Clang works on some mingw32 distributions. Clang assumes directories as
below;
- ``C:/mingw/include``
- ``C:/mingw/lib``
- ``C:/mingw/lib/gcc/mingw32/4.[3-5].0/include/c++``
On MSYS, a few tests might fail.
MinGW-w64
"""""""""
For 32-bit (i686-w64-mingw32), and 64-bit (x86\_64-w64-mingw32), Clang
assumes as below;
- ``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)``
- ``some_directory/bin/gcc.exe``
- ``some_directory/bin/clang.exe``
- ``some_directory/bin/clang++.exe``
- ``some_directory/bin/../include/c++/GCC_version``
- ``some_directory/bin/../include/c++/GCC_version/x86_64-w64-mingw32``
- ``some_directory/bin/../include/c++/GCC_version/i686-w64-mingw32``
- ``some_directory/bin/../include/c++/GCC_version/backward``
- ``some_directory/bin/../x86_64-w64-mingw32/include``
- ``some_directory/bin/../i686-w64-mingw32/include``
- ``some_directory/bin/../include``
This directory layout is standard for any toolchain you will find on the
official `MinGW-w64 website <http://mingw-w64.sourceforge.net>`_.
Clang expects the GCC executable "gcc.exe" compiled for
``i686-w64-mingw32`` (or ``x86_64-w64-mingw32``) to be present on PATH.
`Some tests might fail <http://llvm.org/bugs/show_bug.cgi?id=9072>`_ on
``x86_64-w64-mingw32``.
.. _clang-cl:
clang-cl
========
clang-cl is an alternative command-line interface to Clang driver, designed for
compatibility with the Visual C++ compiler, cl.exe.
To enable clang-cl to find system headers, libraries, and the linker when run
from the command-line, it should be executed inside a Visual Studio Native Tools
Command Prompt or a regular Command Prompt where the environment has been set
up using e.g. `vcvars32.bat <http://msdn.microsoft.com/en-us/library/f2ccy3wt.aspx>`_.
clang-cl can also be used from inside Visual Studio by using an LLVM Platform
Toolset.
Command-Line Options
--------------------
To be compatible with cl.exe, clang-cl supports most of the same command-line
options. Those options can start with either ``/`` or ``-``. It also supports
some of Clang's core options, such as the ``-W`` options.
Options that are known to clang-cl, but not currently supported, are ignored
with a warning. For example:
::
clang-cl.exe: warning: argument unused during compilation: '/Zi'
To suppress warnings about unused arguments, use the ``-Qunused-arguments`` option.
Options that are not known to clang-cl will cause errors. If they are spelled with a
leading ``/``, they will be mistaken for a filename:
::
clang-cl.exe: error: no such file or directory: '/foobar'
Please `file a bug <http://llvm.org/bugs/enter_bug.cgi?product=clang&component=Driver>`_
for any valid cl.exe flags that clang-cl does not understand.
Execute ``clang-cl /?`` to see a list of supported options:
::
/? Display available options
/c Compile only
/D <macro[=value]> Define macro
/fallback Fall back to cl.exe if clang-cl fails to compile
/FA Output assembly code file during compilation
/Fa<file or directory> Output assembly code to this file during compilation
/Fe<file or directory> Set output executable file or directory (ends in / or \)
/FI<value> Include file before parsing
/Fo<file or directory> Set output object file, or directory (ends in / or \)
/GF- Disable string pooling
/GR- Disable RTTI
/GR Enable RTTI
/help Display available options
/I <dir> Add directory to include search path
/J Make char type unsigned
/LDd Create debug DLL
/LD Create DLL
/link <options> Forward options to the linker
/MDd Use DLL debug run-time
/MD Use DLL run-time
/MTd Use static debug run-time
/MT Use static run-time
/Ob0 Disable inlining
/Od Disable optimization
/Oi- Disable use of builtin functions
/Oi Enable use of builtin functions
/Os Optimize for size
/Ot Optimize for speed
/Ox Maximum optimization
/Oy- Disable frame pointer omission
/Oy Enable frame pointer omission
/O<n> Optimization level
/P Only run the preprocessor
/showIncludes Print info about included files to stderr
/TC Treat all source files as C
/Tc <filename> Specify a C source file
/TP Treat all source files as C++
/Tp <filename> Specify a C++ source file
/U <macro> Undefine macro
/W0 Disable all warnings
/W1 Enable -Wall
/W2 Enable -Wall
/W3 Enable -Wall
/W4 Enable -Wall
/Wall Enable -Wall
/WX- Do not treat warnings as errors
/WX Treat warnings as errors
/w Disable all warnings
/Zs Syntax-check only
The /fallback Option
^^^^^^^^^^^^^^^^^^^^
When clang-cl is run with the ``/fallback`` option, it will first try to
compile files itself. For any file that it fails to compile, it will fall back
and try to compile the file by invoking cl.exe.
This option is intended to be used as a temporary means to build projects where
clang-cl cannot successfully compile all the files. clang-cl may fail to compile
a file either because it cannot generate code for some C++ feature, or because
it cannot parse some Microsoft language extension.