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For +more general information, please see the <a href="http://clang.llvm.org">Clang +Web Site</a> or the <a href="http://llvm.org">LLVM Web Site</a>.</p> + +<p>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 <a href="InternalsManual.html">the Clang Internals Manual</a>. If you are +interested in the <a href="http://clang-analyzer.llvm.org">Clang +Static Analyzer</a>, please see its web page.</p> + +<p>Clang is designed to support the C family of programming languages, which +includes <a href="#c">C</a>, <a href="#objc">Objective-C</a>, <a +href="#cxx">C++</a>, and <a href="#objcxx">Objective-C++</a> as well as many +dialects of those. For language-specific information, please see the +corresponding language specific section:</p> + +<ul> +<li><a href="#c">C Language</a>: K&R C, ANSI C89, ISO C90, ISO C94 + (C89+AMD1), ISO C99 (+TC1, TC2, TC3). </li> +<li><a href="#objc">Objective-C Language</a>: ObjC 1, ObjC 2, ObjC 2.1, plus + variants depending on base language.</li> +<li><a href="#cxx">C++ Language</a></li> +<li><a href="#objcxx">Objective C++ Language</a></li> +</ul> + +<p>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".</p> + +<p>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 <a href="#target_features">Target-Specific Features and +Limitations</a> section for more details.</p> + +<p>The rest of the introduction introduces some basic <a +href="#terminology">compiler terminology</a> that is used throughout this manual +and contains a basic <a href="#basicusage">introduction to using Clang</a> +as a command line compiler.</p> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="terminology">Terminology</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p>Front end, parser, backend, preprocessor, undefined behavior, diagnostic, + optimizer</p> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="basicusage">Basic Usage</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p>Intro to how to use a C compiler for newbies.</p> +<p> +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 +</p> + + +<!-- ======================================================================= --> +<h2 id="commandline">Command Line Options</h2> +<!-- ======================================================================= --> + +<p> +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 -c, -g, etc. +</p> + + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="cl_diagnostics">Options to Control Error and Warning Messages</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p><b>-Werror</b>: Turn warnings into errors.</p> +<p><b>-Werror=foo</b>: Turn warning "foo" into an error.</p> +<p><b>-Wno-error=foo</b>: Turn warning "foo" into an warning even if -Werror is + specified.</p> +<p><b>-Wfoo</b>: Enable warning foo</p> +<p><b>-Wno-foo</b>: Disable warning foo</p> +<p><b>-w</b>: Disable all warnings.</p> +<p><b>-pedantic</b>: Warn on language extensions.</p> +<p><b>-pedantic-errors</b>: Error on language extensions.</p> +<p><b>-Wsystem-headers</b>: Enable warnings from system headers.</p> + +<p><b>-ferror-limit=123</b>: Stop emitting diagnostics after 123 errors have + been produced. The default is 20, and the error limit can be disabled with + -ferror-limit=0.</p> + +<p><b>-ftemplate-backtrace-limit=123</b>: 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 -ftemplate-backtrace-limit=0.</p> + +<!-- ================================================= --> +<h4 id="cl_diag_formatting">Formatting of Diagnostics</h4> +<!-- ================================================= --> + +<p>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.</p> + +<dl> + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fshow-column"><b>-f[no-]show-column</b>: Print column number in +diagnostic.</dt> +<dd>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: + +<pre> + test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] + #endif bad + ^ + // +</pre> + +<p>When this is disabled, Clang will print "test.c:28: warning..." with no +column number.</p> +</dd> + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fshow-source-location"><b>-f[no-]show-source-location</b>: Print +source file/line/column information in diagnostic.</dt> +<dd>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: + +<pre> + test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] + #endif bad + ^ + // +</pre> + +<p>When this is disabled, Clang will not print the "test.c:28:8: " part.</p> +</dd> + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fcaret-diagnostics"><b>-f[no-]caret-diagnostics</b>: Print source +line and ranges from source code in diagnostic.</dt> +<dd>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: + +<pre> + test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] + #endif bad + ^ + // +</pre> +</dd> +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fcolor_diagnostics"><b>-f[no-]color-diagnostics</b>: </dt> +<dd>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., + <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> + +<p>When this is disabled, Clang will just print:</p> + +<pre> + test.c:2:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] + #endif bad + ^ + // +</pre> +</dd> +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fdiagnostics-format"><b>-fdiagnostics-format=clang/msvc/vi</b>: +Changes diagnostic output format to better match IDEs and command line tools.</dt> +<dd>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: + + <dl> + <dt><b>clang</b> (default)</dt> + <dd> + <pre>t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int'</pre> + </dd> + + <dt><b>msvc</b></dt> + <dd> + <pre>t.c(3,11) : warning: conversion specifies type 'char *' but the argument has type 'int'</pre> + </dd> + + <dt><b>vi</b></dt> + <dd> + <pre>t.c +3:11: warning: conversion specifies type 'char *' but the argument has type 'int'</pre> + </dd> + </dl> +</dd> + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fdiagnostics-show-name"><b>-f[no-]diagnostics-show-name</b>: +Enable the display of the diagnostic name.</dt> +<dd>This option, which defaults to off, controls whether or not +Clang prints the associated name.<p></p></dd> +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fdiagnostics-show-option"><b>-f[no-]diagnostics-show-option</b>: +Enable <tt>[-Woption]</tt> information in diagnostic line.</dt> +<dd>This option, which defaults to on, +controls whether or not Clang prints the associated <A +href="#cl_diag_warning_groups">warning group</a> option name when outputting +a warning diagnostic. For example, in this output: + +<pre> + test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] + #endif bad + ^ + // +</pre> + +<p>Passing <b>-fno-diagnostics-show-option</b> will prevent Clang from printing +the [<a href="#opt_Wextra-tokens">-Wextra-tokens</a>] information in the +diagnostic. This information tells you the flag needed to enable or disable the +diagnostic, either from the command line or through <a +href="#pragma_GCC_diagnostic">#pragma GCC diagnostic</a>.</dd> + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fdiagnostics-show-category"><b>-fdiagnostics-show-category=none/id/name</b>: +Enable printing category information in diagnostic line.</dt> +<dd>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). + +<p>For example, a format string warning will produce these three renditions +based on the setting of this option:</p> + +<pre> + 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<b>,1</b>] + t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat<b>,Format String</b>] +</pre> + +<p>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.</p> +</dd> + + + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fdiagnostics-fixit-info"><b>-f[no-]diagnostics-fixit-info</b>: +Enable "FixIt" information in the diagnostics output.</dt> +<dd>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: + +<pre> + test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] + #endif bad + ^ + // +</pre> + +<p>Passing <b>-fno-diagnostics-fixit-info</b> 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.</p> +</dd> + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fdiagnostics-print-source-range-info"> +<b>-f[no-]diagnostics-print-source-range-info</b>: +Print machine parsable information about source ranges.</dt> +<dd>This option, which defaults to off, controls whether or not Clang prints +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: + +<pre> +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; + ~~~~~~ ^ ~~~~~~~ +</pre> + +<p>The {}'s are generated by -fdiagnostics-print-source-range-info.</p> +</dd> + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_fdiagnostics-parseable-fixits"> +<b>-fdiagnostics-parseable-fixits</b>: +Print Fix-Its in a machine parseable form.</dt> +<dd><p>This option makes Clang print available Fix-Its in a machine parseable format at the end of diagnostics. The following example illustrates the format:</p> + +<pre> + fix-it:"t.cpp":{7:25-7:29}:"Gamma" +</pre> + +<p>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").</p> +</dd> + +</dl> + + + +<!-- ===================================================== --> +<h4 id="cl_diag_warning_groups">Individual Warning Groups</h4> +<!-- ===================================================== --> + +<p>TODO: Generate this from tblgen. Define one anchor per warning group.</p> + + +<dl> + + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_Wextra-tokens"><b>-Wextra-tokens</b>: Warn about excess tokens at + the end of a preprocessor directive.</dt> +<dd>This option, which defaults to on, enables warnings about extra tokens at +the end of preprocessor directives. For example: + +<pre> + test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens] + #endif bad + ^ +</pre> + +<p>These extra tokens are not strictly conforming, and are usually best handled +by commenting them out.</p> + +<p>This option is also enabled by <a href="">-Wfoo</a>, <a href="">-Wbar</a>, + and <a href="">-Wbaz</a>.</p> +</dd> + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_Wambiguous-member-template"><b>-Wambiguous-member-template</b>: +Warn about unqualified uses of a member template whose name resolves +to another template at the location of the use.</dt> +<dd>This option, which defaults to on, enables a warning in the +following code: + +<pre> +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); +} +</pre> + +<p>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.</p> +</dd> + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dt id="opt_Wbind-to-temporary-copy"><b>-Wbind-to-temporary-copy</b>: Warn about +an unusable copy constructor when binding a reference to a temporary.</dt> +<dd>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: + +<pre> + struct NonCopyable { + NonCopyable(); + private: + NonCopyable(const NonCopyable&); + }; + void foo(const NonCopyable&); + void bar() { + foo(NonCopyable()); // Disallowed in C++98; allowed in C++11. + } +</pre> +<pre> + struct NonCopyable2 { + NonCopyable2(); + NonCopyable2(NonCopyable2&); + }; + void foo(const NonCopyable2&); + void bar() { + foo(NonCopyable2()); // Disallowed in C++98; allowed in C++11. + } +</pre> + +<p>Note that if <tt>NonCopyable2::NonCopyable2()</tt> 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.</p> + +</dd> + +</dl> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="cl_crash_diagnostics">Options to Control Clang Crash Diagnostics</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p>As unbelievable as it may sound, Clang does crash from time to time. +Generally, this only occurs to those living on the +<a href="http://llvm.org/releases/download.html#svn">bleeding edge</a>. 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. +</p> + +<p><b>-fno-crash-diagnostics</b>: Disable auto-generation of preprocessed +source files during a clang crash.</p> + +<p>The -fno-crash-diagnostics flag can be helpful for speeding the process of +generating a delta reduced test case.</p> + + +<!-- ======================================================================= --> +<h2 id="general_features">Language and Target-Independent Features</h2> +<!-- ======================================================================= --> + + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="diagnostics">Controlling Errors and Warnings</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p>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.</p> + +<h4 id="diagnostics_display">Controlling How Clang Displays Diagnostics</h4> + +<p>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:</p> + +<ol> +<li>A file/line/column indicator that shows exactly where the diagnostic occurs + in your code [<a href="#opt_fshow-column">-fshow-column</a>, <a + href="#opt_fshow-source-location">-fshow-source-location</a>].</li> +<li>A categorization of the diagnostic as a note, warning, error, or fatal + error.</li> +<li>A text string that describes what the problem is.</li> +<li>An option that indicates how to control the diagnostic (for diagnostics that + support it) [<a + href="#opt_fdiagnostics-show-option">-fdiagnostics-show-option</a>].</li> +<li>A <a href="#diagnostics_categories">high-level category</a> for the + diagnostic for clients that want to group diagnostics by class (for + diagnostics that support it) [<a + href="#opt_fdiagnostics-show-category">-fdiagnostics-show-category</a>].</li> +<li>The line of source code that the issue occurs on, along with a caret and + ranges that indicate the important locations [<a + href="opt_fcaret-diagnostics">-fcaret-diagnostics</a>].</li> +<li>"FixIt" information, which is a concise explanation of how to fix the + problem (when Clang is certain it knows) [<a + href="opt_fdiagnostics-fixit-info">-fdiagnostics-fixit-info</a>].</li> +<li>A machine-parsable representation of the ranges involved (off by + default) [<a + href="opt_fdiagnostics-print-source-range-info">-fdiagnostics-print-source-range-info</a>].</li> +</ol> + +<p>For more information please see <a href="#cl_diag_formatting">Formatting of +Diagnostics</a>.</p> + + +<h4 id="diagnostics_mappings">Diagnostic Mappings</h4> + +<p>All diagnostics are mapped into one of these 5 classes:</p> + +<ul> +<li>Ignored</li> +<li>Note</li> +<li>Warning</li> +<li>Error</li> +<li>Fatal</li> +</ul> + +<h4 id="diagnostics_categories">Diagnostic Categories</h4> + +<p>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. +</p> + +<p>Categories are not shown by default, but they can be turned on with the +<a href="#opt_fdiagnostics-show-category">-fdiagnostics-show-category</a> option. +When set to "<tt>name</tt>", the category is printed textually in the diagnostic +output. When it is set to "<tt>id</tt>", a category number is printed. The +mapping of category names to category id's can be obtained by running '<tt>clang + --print-diagnostic-categories</tt>'. +</p> + +<h4 id="diagnostics_commandline">Controlling Diagnostics via Command Line + Flags</h4> + +<p>-W flags, -pedantic, etc</p> + +<h4 id="diagnostics_pragmas">Controlling Diagnostics via Pragmas</h4> + +<p>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. </p> + +<p>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:</p> + +<pre> +#pragma GCC diagnostic ignored "-Wall" +</pre> + +<p>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.</p> + +<p>In the below example +-Wmultichar is ignored for only a single line of code, after which the +diagnostics return to whatever state had previously existed.</p> + +<pre> +#pragma clang diagnostic push +#pragma clang diagnostic ignored "-Wmultichar" + +char b = 'df'; // no warning. + +#pragma clang diagnostic pop +</pre> + +<p>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. </p> + +<h4 id="diagnostics_enable_everything">Enabling All Warnings</h4> + +<p>In addition to the traditional <tt>-W</tt> flags, one can enable <b>all</b> + warnings by passing <tt>-Weverything</tt>. + This works as expected with <tt>-Werror</tt>, + and also includes the warnings from <tt>-pedantic</tt>.</p> + +<p>Note that when combined with <tt>-w</tt> (which disables all warnings), that + flag wins.</p> + +<h4 id="analyzer_diagnositics">Controlling Static Analyzer Diagnostics</h4> + +<p>While not strictly part of the compiler, the diagnostics from Clang's <a +href="http://clang-analyzer.llvm.org">static analyzer</a> can also be influenced +by the user via changes to the source code. This can be done in two ways: + +<ul> + +<li id="analyzer_annotations"><b>Annotations</b>: The static analyzer recognizes various GCC-style +attributes (e.g., <tt>__attribute__((nonnull)))</tt>) that can either suppress +static analyzer warnings or teach the analyzer about code invariants which +enable it to find more bugs. While many of these attributes are standard GCC +attributes, additional ones have been added to Clang to specifically support the +static analyzer. Detailed information on these annotations can be found in the +<a href="http://clang-analyzer.llvm.org/annotations.html">analyzer's +documentation</a>.</li> + +<li><b><tt>__clang_analyzer__</tt></b>: When the static analyzer is using Clang +to parse source files, it implicitly defines the preprocessor macro +<tt>__clang_analyzer__</tt>. While discouraged, code can use this macro to +selectively exclude code the analyzer examines. Here is an example: + +<pre> +#ifndef __clang_analyzer__ +// Code not to be analyzed +#endif +</pre> + +In general, this usage is discouraged. Instead, we prefer that users file bugs +against the analyzer when it flags false positives. There is also active +discussion of allowing users in the future to selectively silence specific +analyzer warnings (some of which can already be done using <a +href="#analyzer_annotations">annotations</a>).</li> + +</ul> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="precompiledheaders">Precompiled Headers</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p><a href="http://en.wikipedia.org/wiki/Precompiled_header">Precompiled +headers</a> 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).</p> + +<h4>Generating a PCH File</h4> + +<p>To generate a PCH file using Clang, one invokes Clang with +the <b><tt>-x <i><language></i>-header</tt></b> option. This mirrors the +interface in GCC for generating PCH files:</p> + +<pre> + $ gcc -x c-header test.h -o test.h.gch + $ clang -x c-header test.h -o test.h.pch +</pre> + +<h4>Using a PCH File</h4> + +<p>A PCH file can then be used as a prefix header when a +<b><tt>-include</tt></b> option is passed to <tt>clang</tt>:</p> + +<pre> + $ clang -include test.h test.c -o test +</pre> + +<p>The <tt>clang</tt> driver will first check if a PCH file for <tt>test.h</tt> +is available; if so, the contents of <tt>test.h</tt> (and the files it includes) +will be processed from the PCH file. Otherwise, Clang falls back to +directly processing the content of <tt>test.h</tt>. This mirrors the behavior of +GCC.</p> + +<p><b>NOTE:</b> Clang does <em>not</em> automatically use PCH files +for headers that are directly included within a source file. For example:</p> + +<pre> + $ clang -x c-header test.h -o test.h.pch + $ cat test.c + #include "test.h" + $ clang test.c -o test +</pre> + +<p>In this example, <tt>clang</tt> will not automatically use the PCH file for +<tt>test.h</tt> since <tt>test.h</tt> was included directly in the source file +and not specified on the command line using <tt>-include</tt>.</p> + +<h4>Relocatable PCH Files</h4> +<p>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.</p> + +<p>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 <code>mylib.h</code> that +will be installed into <code>/usr/include</code>, create a subdirectory +<code>build/usr/include</code> and place the header <code>mylib.h</code> into +that subdirectory. If <code>mylib.h</code> depends on other headers, then +they can be stored within <code>build/usr/include</code> in a way that mimics +the installed location.</p> + +<p>Building a relocatable precompiled header requires two additional arguments. +First, pass the <code>--relocatable-pch</code> flag to indicate that the +resulting PCH file should be relocatable. Second, pass +<code>-isysroot /path/to/build</code>, which makes all includes for your +library relative to the build directory. For example:</p> + +<pre> + # clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch +</pre> + +<p>When loading the relocatable PCH file, the various headers used in the PCH +file are found from the system header root. For example, <code>mylib.h</code> +can be found in <code>/usr/include/mylib.h</code>. If the headers are installed +in some other system root, the <code>-isysroot</code> option can be used provide +a different system root from which the headers will be based. For example, +<code>-isysroot /Developer/SDKs/MacOSX10.4u.sdk</code> will look for +<code>mylib.h</code> in +<code>/Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h</code>.</p> + +<p>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. +Relocatable precompiled headers also have some performance impact, because +the difference in location between the header locations at PCH build time vs. +at the time of PCH use requires one of the PCH optimizations, +<code>stat()</code> caching, to be disabled. However, this change is only +likely to affect PCH files that reference a large number of headers.</p> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="codegen">Controlling Code Generation</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p>Clang provides a number of ways to control code generation. The options are listed below.</p> + +<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> +<dl> +<dt id="opt_fcatch-undefined-behavior"><b>-fcatch-undefined-behavior</b>: Turn +on runtime code generation to check for undefined behavior.</dt> + +<dd>This option, which defaults to off, controls whether or not Clang +adds runtime checks for undefined runtime behavior. If a check fails, +<tt>__builtin_trap()</tt> is used to indicate failure. +The checks are: +<ul> +<li>Subscripting where the static type of one operand is a variable + which is decayed from an array type and the other operand is + greater than the size of the array or less than zero.</li> +<li>Shift operators where the amount shifted is greater or equal to the + promoted bit-width of the left-hand-side or less than zero.</li> +<li>If control flow reaches __builtin_unreachable. +<li>When llvm implements more __builtin_object_size support, reads and + writes for objects that __builtin_object_size indicates we aren't + accessing valid memory. Bit-fields and vectors are not yet checked. +</ul> +</dd> + +<dt id="opt_faddress-sanitizer"><b>-f[no-]address-sanitizer</b>: +Turn on <a href="AddressSanitizer.html">AddressSanitizer</a>, +a memory error detector. + +<dt id="opt_fthread-sanitizer"><b>-f[no-]thread-sanitizer</b>: +Turn on ThreadSanitizer, an <em>experimental</em> data race detector. +Not ready for widespread use. + +<dt id="opt_fno-assume-sane-operator-new"><b>-fno-assume-sane-operator-new</b>: +Don't assume that the C++'s new operator is sane.</dt> +<dd>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.</dd> + +<dt id="opt_ftrap-function"><b>-ftrap-function=[name]</b>: Instruct code +generator to emit a function call to the specified function name for +<tt>__builtin_trap()</tt>.</dt> + +<dd>LLVM code generator translates <tt>__builtin_trap()</tt> to a trap +instruction if it is supported by the target ISA. Otherwise, the builtin is +translated into a call to <tt>abort</tt>. 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.</dd> +</dl> + +<!-- ======================================================================= --> +<h2 id="c">C Language Features</h2> +<!-- ======================================================================= --> + +<p>The support for standard C in clang is feature-complete except for the C99 +floating-point pragmas.</p> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="c_ext">Extensions supported by clang</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p>See <a href="LanguageExtensions.html">clang language extensions</a>.</p> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="c_modes">Differences between various standard modes</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p>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. +</p> + +<p>Differences between all c* and gnu* modes:</p> +<ul> +<li>c* modes define "__STRICT_ANSI__".</li> +<li>Target-specific defines not prefixed by underscores, like "linux", are +defined in gnu* modes.</li> +<li>Trigraphs default to being off in gnu* modes; they can be enabled by the +-trigraphs option.</li> +<li>The parser recognizes "asm" and "typeof" as keywords in gnu* modes; the +variants "__asm__" and "__typeof__" are recognized in all modes.</li> +<li>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.</li> +<li>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.</li> +</ul> + +<p>Differences between *89 and *99 modes:</p> +<ul> +<li>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.</li> +<li>Digraphs are not recognized in c89 mode.</li> +<li>The scope of names defined inside a "for", "if", "switch", "while", or "do" +statement is different. (example: "if ((struct x {int x;}*)0) {}".)</li> +<li>__STDC_VERSION__ is not defined in *89 modes.</li> +<li>"inline" is not recognized as a keyword in c89 mode.</li> +<li>"restrict" is not recognized as a keyword in *89 modes.</li> +<li>Commas are allowed in integer constant expressions in *99 modes.</li> +<li>Arrays which are not lvalues are not implicitly promoted to pointers in +*89 modes.</li> +<li>Some warnings are different.</li> +</ul> + +<p>c94 mode is identical to c89 mode except that digraphs are enabled in +c94 mode (FIXME: And __STDC_VERSION__ should be defined!).</p> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="c_unimpl_gcc">GCC extensions not implemented yet</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p>clang tries to be compatible with gcc as much as possible, but some gcc +extensions are not implemented yet:</p> + +<ul> + +<li>clang does not support #pragma weak +(<a href="http://llvm.org/bugs/show_bug.cgi?id=3679">bug 3679</a>). Due to +the uses described in the bug, this is likely to be implemented at some +point, at least partially.</li> + +<li>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.</li> + +<li>clang does not support nested functions; this is a complex feature which +is infrequently used, so it is unlikely to be implemented anytime soon.</li> + +<li>clang does not support global register variables, this is unlikely +to be implemented soon because it requires additional LLVM backend support. +</li> + +<li>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.</li> + +<li>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.</li> + +<li>clang does not support the gcc extension for forward-declaring function +parameters; this has not showed up in any real-world code yet, though, so it +might never be implemented.</li> + +</ul> + +<p>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 <a href="#cxx">C++ Language Features</a>. +Also, this list does not include bugs in mostly-implemented features; please +see the <a href="http://llvm.org/bugs/buglist.cgi?quicksearch=product%3Aclang+component%3A-New%2BBugs%2CAST%2CBasic%2CDriver%2CHeaders%2CLLVM%2BCodeGen%2Cparser%2Cpreprocessor%2CSemantic%2BAnalyzer"> +bug tracker</a> for known existing bugs (FIXME: Is there a section for +bug-reporting guidelines somewhere?).</p> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="c_unsupp_gcc">Intentionally unsupported GCC extensions</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<ul> + +<li>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 <em>does</em> support +flexible array members (arrays with a zero or unspecified size at the end of +a structure).</li> + +<li>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.</li> + +<li>clang does not support __builtin_apply and friends; this extension is +extremely obscure and difficult to implement reliably.</li> + +</ul> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="c_ms">Microsoft extensions</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p>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; enabling Microsoft extensions will silently drop +certain constructs (including __declspec and Microsoft-style asm statements). +</p> + +<ul> +<li>clang allows setting _MSC_VER with -fmsc-version=. It defaults to 1300 which +is the same as Visual C/C++ 2003. Any number is supported and can greatly affect +what Windows SDK and c++stdlib headers clang can compile. This option will be +removed when clang supports the full set of MS extensions required for these +headers.</li> + +<li>clang does not support the Microsoft extension where anonymous +record members can be declared using user defined typedefs.</li> + +<li>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.</li> +</ul> + +<!-- ======================================================================= --> +<h2 id="cxx">C++ Language Features</h2> +<!-- ======================================================================= --> + +<p>clang fully implements all of standard C++98 except for exported templates +(which were removed in C++11), and +<a href="http://clang.llvm.org/cxx_status.html">many C++11 features</a> are also +implemented.</p> + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="cxx_implimits">Controlling implementation limits</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<p><b>-fconstexpr-depth=N</b>: Sets the limit for recursive constexpr function +invocations to N. The default is 512.</p> + +<p><b>-ftemplate-depth=N</b>: Sets the limit for recursively nested template +instantiations to N. The default is 1024.</p> + +<!-- ======================================================================= --> +<h2 id="target_features">Target-Specific Features and Limitations</h2> +<!-- ======================================================================= --> + + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="target_arch">CPU Architectures Features and Limitations</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<!-- ======================== --> +<h4 id="target_arch_x86">X86</h4> +<!-- ======================== --> + +<p>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.</p> + +<p>On x86_64-mingw32, passing i128(by value) is incompatible to Microsoft x64 +calling conversion. You might need to tweak WinX86_64ABIInfo::classify() +in lib/CodeGen/TargetInfo.cpp.</p> + +<!-- ======================== --> +<h4 id="target_arch_arm">ARM</h4> +<!-- ======================== --> + +<p>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.</p> + +<!-- ======================== --> +<h4 id="target_arch_other">Other platforms</h4> +<!-- ======================== --> +clang currently contains some support for PPC and Sparc; however, significant +pieces of code generation are still missing, and they haven't undergone +significant testing. + +<p>clang contains limited support for the MSP430 embedded processor, but both +the clang support and the LLVM backend support are highly experimental. + +<p>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. + +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> +<h3 id="target_os">Operating System Features and Limitations</h3> +<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = --> + +<!-- ======================================= --> +<h4 id="target_os_darwin">Darwin (Mac OS/X)</h4> +<!-- ======================================= --> + +<p>No __thread support, 64-bit ObjC support requires SL tools.</p> + +<!-- ======================================= --> +<h4 id="target_os_win32">Windows</h4> +<!-- ======================================= --> + +<p>Experimental supports are on Cygming.</p> + +<h5>Cygwin</h5> + +<p>Clang works on Cygwin-1.7.</p> + +<h5>MinGW32</h5> + +<p>Clang works on some mingw32 distributions. +Clang assumes directories as below;</p> + +<ul> +<li><tt>C:/mingw/include</tt></li> +<li><tt>C:/mingw/lib</tt></li> +<li><tt>C:/mingw/lib/gcc/mingw32/4.[3-5].0/include/c++</tt></li> +</ul> + +<p>On MSYS, a few tests might fail.</p> + +<h5>MinGW-w64</h5> + +<p>For 32-bit (i686-w64-mingw32), and 64-bit (x86_64-w64-mingw32), Clang assumes as below;<p> + +<ul> +<li><tt>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)</tt></li> +<li><tt>some_directory/bin/gcc.exe</tt></li> +<li><tt>some_directory/bin/clang.exe</tt></li> +<li><tt>some_directory/bin/clang++.exe</tt></li> +<li><tt>some_directory/bin/../include/c++/GCC_version</tt></li> +<li><tt>some_directory/bin/../include/c++/GCC_version/x86_64-w64-mingw32</tt></li> +<li><tt>some_directory/bin/../include/c++/GCC_version/i686-w64-mingw32</tt></li> +<li><tt>some_directory/bin/../include/c++/GCC_version/backward</tt></li> +<li><tt>some_directory/bin/../x86_64-w64-mingw32/include</tt></li> +<li><tt>some_directory/bin/../i686-w64-mingw32/include</tt></li> +<li><tt>some_directory/bin/../include</tt></li> +</ul> + +<p>This directory layout is standard for any toolchain you will find on the official <a href="mingw-w64.sourceforge.net">MinGW-w64 website</a>. + +<p>Clang expects the GCC executable "gcc.exe" compiled for i686-w64-mingw32 (or x86_64-w64-mingw32) to be present on PATH.</p> + +<p><a href="http://llvm.org/bugs/show_bug.cgi?id=9072">Some tests might fail</a> +on x86_64-w64-mingw32.</p> + +</div> +</body> +</html> |