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|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
<title>Language Compatibility</title>
<link type="text/css" rel="stylesheet" href="menu.css" />
<link type="text/css" rel="stylesheet" href="content.css" />
<style type="text/css">
</style>
</head>
<body>
<!--#include virtual="menu.html.incl"-->
<div id="content">
<!-- ======================================================================= -->
<h1>Language Compatibility</h1>
<!-- ======================================================================= -->
<p>Clang strives to both conform to current language standards (C99,
C++98) and also to implement many widely-used extensions available
in other compilers, so that most correct code will "just work" when
compiler with Clang. However, Clang is more strict than other
popular compilers, and may reject incorrect code that other
compilers allow. This page documents common compatibility and
portability issues with Clang to help you understand and fix the
problem in your code when Clang emits an error message.</p>
<ul>
<li><a href="#c">C compatibility</a>
<ul>
<li><a href="#inline">C99 inline functions</a></li>
<li><a href="#vector_builtins">"missing" vector __builtin functions</a></li>
<li><a href="#lvalue-cast">Lvalue casts</a></li>
<li><a href="#blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</a></li>
</ul>
</li>
<li><a href="#objective-c">Objective-C compatibility</a>
<ul>
<li><a href="#super-cast">Cast of super</a></li>
<li><a href="#sizeof-interface">Size of interfaces</a></li>
<li><a href="#objc_objs-cast">Internal Objective-C types</a></li>
<li><a href="#c_variables-class">C variables in @class or @protocol</a></li>
</ul>
</li>
<li><a href="#c++">C++ compatibility</a>
<ul>
<li><a href="#vla">Variable-length arrays</a></li>
<li><a href="#dep_lookup">Unqualified lookup in templates</a></li>
<li><a href="#dep_lookup_bases">Unqualified lookup into dependent bases of class templates</a></li>
<li><a href="#undep_incomplete">Incomplete types in templates</a></li>
<li><a href="#bad_templates">Templates with no valid instantiations</a></li>
<li><a href="#default_init_const">Default initialization of const
variable of a class type requires user-defined default
constructor</a></li>
</ul>
</li>
<li><a href="#objective-c++">Objective-C++ compatibility</a>
<ul>
<li><a href="#implicit-downcasts">Implicit downcasts</a></li>
</ul>
<ul>
<li><a href="#Use of class as method name">Use of class as method name</a></li>
</ul>
</li>
</ul>
<!-- ======================================================================= -->
<h2 id="c">C compatibility</h3>
<!-- ======================================================================= -->
<!-- ======================================================================= -->
<h3 id="inline">C99 inline functions</h3>
<!-- ======================================================================= -->
<p>By default, Clang builds C code according to the C99 standard,
which provides different inlining semantics than GCC's default
behavior. For example, when compiling the following code with no optimization:</p>
<pre>
inline int add(int i, int j) { return i + j; }
int main() {
int i = add(4, 5);
return i;
}
</pre>
<p>In C99, this is an incomplete (incorrect) program because there is
no external definition of the <code>add</code> function: the inline
definition is only used for optimization, if the compiler decides to
perform inlining. Therefore, we will get a (correct) link-time error
with Clang, e.g.:</p>
<pre>
Undefined symbols:
"_add", referenced from:
_main in cc-y1jXIr.o
</pre>
<p>There are several ways to fix this problem:</p>
<ul>
<li>Change <code>add</code> to a <code>static inline</code>
function. Static inline functions are always resolved within the
translation unit, so you won't have to add an external, non-inline
definition of the function elsewhere in your program.</li>
<li>Provide an external (non-inline) definition of <code>add</code>
somewhere in your program.</li>
<li>Compile with the GNU89 dialect by adding
<code>-std=gnu89</code> to the set of Clang options. This option is
only recommended if the program source cannot be changed or if the
program also relies on additional C89-specific behavior that cannot
be changed.</li>
</ul>
<!-- ======================================================================= -->
<h3 id="vector_builtins">"missing" vector __builtin functions</h3>
<!-- ======================================================================= -->
<p>The Intel and AMD manuals document a number "<tt><*mmintrin.h></tt>"
header files, which define a standardized API for accessing vector operations
on X86 CPUs. These functions have names like <tt>_mm_xor_ps</tt> and
<tt>_mm256_addsub_pd</tt>. Compilers have leeway to implement these functions
however they want. Since Clang supports an excellent set of <a
href="../docs/LanguageExtensions.html#vectors">native vector operations</a>,
the Clang headers implement these interfaces in terms of the native vector
operations.
</p>
<p>In contrast, GCC implements these functions mostly as a 1-to-1 mapping to
builtin function calls, like <tt>__builtin_ia32_paddw128</tt>. These builtin
functions are an internal implementation detail of GCC, and are not portable to
the Intel compiler, the Microsoft compiler, or Clang. If you get build errors
mentioning these, the fix is simple: switch to the *mmintrin.h functions.</p>
<p>The same issue occurs for NEON and Altivec for the ARM and PowerPC
architectures respectively. For these, make sure to use the <arm_neon.h>
and <altivec.h> headers.</p>
<!-- ======================================================================= -->
<h3 id="lvalue-cast">Lvalue casts</h3>
<!-- ======================================================================= -->
<p>Old versions of GCC permit casting the left-hand side of an assignment to a
different type. Clang produces an error on similar code, e.g.,</p>
<pre>
lvalue.c:2:3: error: assignment to cast is illegal, lvalue casts are not
supported
(int*)addr = val;
^~~~~~~~~~ ~
</pre>
<p>To fix this problem, move the cast to the right-hand side. In this
example, one could use:</p>
<pre>
addr = (float *)val;
</pre>
<!-- ======================================================================= -->
<h3 id="blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</h3>
<!-- ======================================================================= -->
<p>Clang disallows jumps into the scope of a <tt>__block</tt> variable, similar
to the manner in which both GCC and Clang disallow jumps into the scope of
variables which have user defined constructors (in C++).</p>
<p>Variables marked with <tt>__block</tt> require special runtime initialization
before they can be used. A jump into the scope of a <tt>__block</tt> variable
would bypass this initialization and therefore the variable cannot safely be
used.</p>
<p>For example, consider the following code fragment:</p>
<pre>
int f0(int c) {
if (c)
goto error;
__block int x;
x = 1;
return x;
error:
x = 0;
return x;
}
</pre>
<p>GCC accepts this code, but it will crash at runtime along the error path,
because the runtime setup for the storage backing the <tt>x</tt> variable will
not have been initialized. Clang rejects this code with a hard error:</p>
<pre>
t.c:3:5: error: goto into protected scope
goto error;
^
t.c:5:15: note: jump bypasses setup of __block variable
__block int x;
^
</pre>
<p>Some instances of this construct may be safe if the variable is never used
after the jump target, however the protected scope checker does not check the
uses of the variable, only the scopes in which it is visible. You should rewrite
your code to put the <tt>__block</tt> variables in a scope which is only visible
where they are used.</p>
<!-- ======================================================================= -->
<h2 id="objective-c">Objective-C compatibility</h3>
<!-- ======================================================================= -->
<!-- ======================================================================= -->
<h3 id="super-cast">Cast of super</h3>
<!-- ======================================================================= -->
<p>GCC treats the <code>super</code> identifier as an expression that
can, among other things, be cast to a different type. Clang treats
<code>super</code> as a context-sensitive keyword, and will reject a
type-cast of <code>super</code>:</p>
<pre>
super.m:11:12: error: cannot cast 'super' (it isn't an expression)
[(Super*)super add:4];
~~~~~~~~^
</pre>
<p>To fix this problem, remove the type cast, e.g.</p>
<pre>
[super add:4];
</pre>
<!-- ======================================================================= -->
<h3 id="sizeof-interface">Size of interfaces</h3>
<!-- ======================================================================= -->
<p>When using the "non-fragile" Objective-C ABI in use, the size of an
Objective-C class may change over time as instance variables are added
(or removed). For this reason, Clang rejects the application of the
<code>sizeof</code> operator to an Objective-C class when using this
ABI:</p>
<pre>
sizeof.m:4:14: error: invalid application of 'sizeof' to interface 'NSArray' in
non-fragile ABI
int size = sizeof(NSArray);
^ ~~~~~~~~~
</pre>
<p>Code that relies on the size of an Objective-C class is likely to
be broken anyway, since that size is not actually constant. To address
this problem, use the Objective-C runtime API function
<code>class_getInstanceSize()</code>:</p>
<pre>
class_getInstanceSize([NSArray class])
</pre>
<!-- ======================================================================= -->
<h3 id="objc_objs-cast">Internal Objective-C types</h3>
<!-- ======================================================================= -->
<p>GCC allows using pointers to internal Objective-C objects, <tt>struct objc_object*</tt>,
<tt>struct objc_selector*</tt>, and <tt>struct objc_class*</tt> in place of the types
<tt>id</tt>, <tt>SEL</tt>, and <tt>Class</tt> respectively. Clang treats the
internal Objective-C structures as implementation detail and won't do implicit conversions:
<pre>
t.mm:11:2: error: no matching function for call to 'f'
f((struct objc_object *)p);
^
t.mm:5:6: note: candidate function not viable: no known conversion from 'struct objc_object *' to 'id' for 1st argument
void f(id x);
^
</pre>
<p>Code s
|
