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<h1>Expressive Diagnostics</h1>
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<p>In addition to being fast and functional, we aim to make Clang extremely user
friendly. As far as a command-line compiler goes, this basically boils down to
making the diagnostics (error and warning messages) generated by the compiler
be as useful as possible. There are several ways that we do this. This section
talks about the experience provided by the command line compiler, contrasting
Clang output to GCC 4.2's output in several examples.
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<h2>Column Numbers and Caret Diagnostics</h2>
<p>First, all diagnostics produced by clang include full column number
information, and use this to print "caret diagnostics". This is a feature
provided by many commercial compilers, but is generally missing from open source
compilers. This is nice because it makes it very easy to understand exactly
what is wrong in a particular piece of code, an example is:</p>
<pre>
$ <b>gcc-4.2 -fsyntax-only -Wformat format-strings.c</b>
format-strings.c:91: warning: too few arguments for format
$ <b>clang -fsyntax-only format-strings.c</b>
format-strings.c:91:13: <font color="magenta">warning:</font> '.*' specified field precision is missing a matching 'int' argument
<font color="darkgreen"> printf("%.*d");</font>
<font color="blue"> ^</font>
</pre>
<p>The caret (the blue "^" character) exactly shows where the problem is, even
inside of the string. This makes it really easy to jump to the problem and
helps when multiple instances of the same character occur on a line. We'll
revisit this more in following examples.</p>
<h2>Range Highlighting for Related Text</h2>
<p>Clang captures and accurately tracks range information for expressions,
statements, and other constructs in your program and uses this to make
diagnostics highlight related information. For example, here's a somewhat
nonsensical example to illustrate this:</p>
<pre>
$ <b>gcc-4.2 -fsyntax-only t.c</b>
t.c:7: error: invalid operands to binary + (have 'int' and 'struct A')
$ <b>clang -fsyntax-only t.c</b>
t.c:7:39: <font color="red">error:</font> invalid operands to binary expression ('int' and 'struct A')
<font color="darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</font>
<font color="blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</font>
</pre>
<p>Here you can see that you don't even need to see the original source code to
understand what is wrong based on the Clang error: Because clang prints a
caret, you know exactly <em>which</em> plus it is complaining about. The range
information highlights the left and right side of the plus which makes it
immediately obvious what the compiler is talking about, which is very useful for
cases involving precedence issues and many other cases.</p>
<h2>Precision in Wording</h2>
<p>A detail is that we have tried really hard to make the diagnostics that come
out of clang contain exactly the pertinent information about what is wrong and
why. In the example above, we tell you what the inferred types are for
the left and right hand sides, and we don't repeat what is obvious from the
caret (that this is a "binary +"). Many other examples abound, here is a simple
one:</p>
<pre>
$ <b>gcc-4.2 -fsyntax-only t.c</b>
t.c:5: error: invalid type argument of 'unary *'
$ <b>clang -fsyntax-only t.c</b>
t.c:5:11: <font color="red">error:</font> indirection requires pointer operand ('int' invalid)
<font color="darkgreen"> int y = *SomeA.X;</font>
<font color="blue"> ^~~~~~~~</font>
</pre>
<p>In this example, not only do we tell you that there is a problem with the *
and point to it, we say exactly why and tell you what the type is (in case it is
a complicated subexpression, such as a call to an overloaded function). This
sort of attention to detail makes it much easier to understand and fix problems
quickly.</p>
<h2>No Pretty Printing of Expressions in Diagnostics</h2>
<p>Since Clang has range highlighting, it never needs to pretty print your code
back out to you. This is particularly bad in G++ (which often emits errors
containing lowered vtable references), but even GCC can produce
inscrutible error messages in some cases when it tries to do this. In this
example P and Q have type "int*":</p>
<pre>
$ <b>gcc-4.2 -fsyntax-only t.c</b>
#'exact_div_expr' not supported by pp_c_expression#'t.c:12: error: called object is not a function
$ <b>clang -fsyntax-only t.c</b>
t.c:12:8: <font color="red">error:</font> called object type 'int' is not a function or function pointer
<font color="darkgreen"> (P-Q)();</font>
<font color="blue"> ~~~~~^</font>
</pre>
<h2>Typedef Preservation and Selective Unwrapping</h2>
<p>Many programmers use high-level user defined types, typedefs, and other
syntactic sugar to refer to types in their program. This is useful because they
can abbreviate otherwise very long types and it is useful to preserve the
typename in diagnostics. However, sometimes very simple typedefs can wrap
trivial types and it is important to strip off the typedef to understand what
is going on. Clang aims to handle both cases well.<p>
<p>For example, here is an example that shows where it is important to preserve
a typedef in C:</p>
<pre>
$ <b>gcc-4.2 -fsyntax-only t.c</b>
t.c:15: error: invalid operands to binary / (have 'float __vector__' and 'const int *')
$ <b>clang -fsyntax-only t.c</b>
t.c:15:11: <font color="red">error:</font> can't convert between vector values of different size ('__m128' and 'int const *')
<font color="darkgreen"> myvec[1]/P;</font>
<font color="blue"> ~~~~~~~~^~</font>
</pre>
<p>Here the type printed by GCC isn't even valid, but if the error were about a
very long and complicated type (as often happens in C++) the error message would
be ugly just because it was long and hard to read. Here's an example where it
is useful for the compiler to expose underlying details of a typedef:</p>
<pre>
$ <b>gcc-4.2 -fsyntax-only t.c</b>
t.c:13: error: request for member 'x' in something not a structure or union
$ <b>clang -fsyntax-only t.c</b>
t.c:13:9: <font color="red">error:</font> member reference base type 'pid_t' (aka 'int') is not a structure or union
<font color="darkgreen"> myvar = myvar.x;</font>
<font color="blue"> ~~~~~ ^</font>
</pre>
<p>If the user was somehow confused about how the system "pid_t" typedef is
defined, Clang helpfully displays it with "aka".</p>
<p>In C++, type preservation includes retaining any qualification written into type names. For example, if we take a small snippet of code such as:
<blockquote>
<pre>
namespace services {
struct WebService { };
}
namespace myapp {
namespace servers {
struct Server { };
}
}
using namespace myapp;
void addHTTPService(servers::Server const &server, ::services::WebService const *http) {
server += http;
}
</pre>
</blockquote>
<p>and then compile it, we see that Clang is both providing more accurate information and is retaining the types as written by the user (e.g., "servers::Server", "::services::WebService"):
<pre>
$ <b>g++-4.2 -fsyntax-only t.cpp</b>
t.cpp:9: error: no match for 'operator+=' in 'server += http'
$ <b>clang -fsyntax-only t.cpp</b>
t.cpp:9:10: <font color="red">error:</font> invalid operands to binary expression ('servers::Server const' and '::services::WebService const *')
<font color="darkgreen">server += http;</font>
<font color="blue">~~~~~~ ^ ~~~~</font>
</pre>
<p>Naturally, type preservation extends to uses of templates, and Clang retains information about how a particular template specialization (like <code>std::vector&lt;Real&gt;</code>) was spelled within the source code. For example:</p>
<pre>
$ <b>g++-4.2 -fsyntax-only t.cpp</b>
t.cpp:12: error: no match for 'operator=' in 'str = vec'
$ <b>clang -fsyntax-only t.cpp</b>
t.cpp:12:7: <font color="red">error:</font> incompatible type assigning 'vector&lt;Real&gt;', expected 'std::string' (aka 'class std::basic_string&lt;char&gt;')
<font color="darkgreen">str = vec</font>;
<font color="blue">^ ~~~</font>
</pre>
<h2>Fix-it Hints</h2>
<p>"Fix-it" hints provide advice for fixing small, localized problems
in source code. When Clang produces a diagnostic about a particular
problem that it can work around (e.g., non-standard or redundant
syntax, missing keywords, common mistakes, etc.), it may also provide
specific guidance in the form of a code transformation to correct the
problem. For example, here Clang warns about the use of a GCC
extension that has been considered obsolete since 1993:</p>
<pre>
$ <b>clang t.c</b>
t.c:5:28: <font color="magenta">warning:</font> use of GNU old-style field designator extension
<font color="darkgreen">struct point origin = { x: 0.0, y: 0.0 };</font>
<font color="red">~~</font> <font color="blue">^</font>
<font color="darkgreen">.x = </font>
t.c:5:36: <font color="magenta">warning:</font> use of GNU old-style field designator extension
<font color="darkgreen">struct point origin = { x: 0.0, y: 0.0 };</font>
<font color="red">~~</font> <font color="blue">^</font>
<font color="darkgreen">.y = </font>
</pre>
<p>The underlined code should be removed, then replaced with the code below the
caret line (".x =" or ".y =", respectively). "Fix-it" hints are most useful for
working around common user errors and misconceptions. For example, C++ users
commonly forget the syntax for explicit specialization of class templates,
as in the following error:</p>
<pre>
$ <b>clang t.cpp</b>
t.cpp:9:3: <font color="red">error:</font> template specialization requires 'template&lt;&gt;'
struct iterator_traits&lt;file_iterator&gt; {
<font color="blue">^</font>
<font color="darkgreen">template&lt;&gt; </font>
</pre>
<p>Again, after describing the problem, Clang provides the fix--add <code>template&lt;&gt;</code>--as part of the diagnostic.<p>
<h2>Automatic Macro Expansion</h2>
<p>Many errors happen in macros that are sometimes deeply nested. With
traditional compilers, you need to dig deep into the definition of the macro to
understand how you got into trouble. Here's a simple example that shows how
Clang helps you out:</p>
<pre>
$ <b>gcc-4.2 -fsyntax-only t.c</b>
t.c: In function 'test':
t.c:80: error: invalid operands to binary &lt; (have 'struct mystruct' and 'float')
$ <b>clang -fsyntax-only t.c</b>
t.c:80:3: <font color="red">error:</font> invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float'))
<font color="darkgreen"> X = MYMAX(P, F);</font>
<font color="blue"> ^~~~~~~~~~~</font>
t.c:76:94: note: instantiated from:
<font color="darkgreen">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a &lt; __b ? __b : __a; })</font>
<font color="blue"> ~~~ ^ ~~~</font>
</pre>
<p>This shows how clang automatically prints instantiation information and
nested range information for diagnostics as they are instantiated through macros
and also shows how some of the other pieces work in a bigger example. Here's
another real world warning that occurs in the "window" Unix package (which
implements the "wwopen" class of APIs):</p>
<pre>
$ <b>clang -fsyntax-only t.c</b>
t.c:22:2: <font color="magenta">warning:</font> type specifier missing, defaults to 'int'
<font color="darkgreen"> ILPAD();</font>
<font color="blue"> ^</font>
t.c:17:17: note: instantiated from:
<font color="darkgreen">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</font>
<font color="blue"> ^</font>
t.c:14:2: note: instantiated from:
<font color="darkgreen"> register i; \</font>
<font color="blue"> ^</font>
</pre>
<p>In practice, we've found that this is actually more useful in multiply nested
macros that in simple ones.</p>
<h2>Quality of Implementation and Attention to Detail</h2>
<p>Finally, we have put a lot of work polishing the little things, because
little things add up over time and contribute to a great user experience. Three
examples are:</p>
<pre>
$ <b>gcc-4.2 t.c</b>
t.c: In function 'foo':
t.c:5: error: expected ';' before '}' token
$ <b>clang t.c</b>
t.c:4:8: <font color="red">error:</font> expected ';' after expression
<font color="darkgreen"> bar()</font>
<font color="blue"> ^</font>
<font color="blue"> ;</font>
</pre>
<p>This shows a trivial little tweak, where we tell you to put the semicolon at
the end of the line that is missing it (line 4) instead of at the beginning of
the following line (line 5). This is particularly important with fixit hints
and caret diagnostics, because otherwise you don't get the important context.
</p>
<pre>
$ <b>gcc-4.2 t.c</b>
t.c:3: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
$ <b>clang t.c</b>
t.c:3:1: <font color="red">error:</font> unknown type name 'foo_t'
<font color="darkgreen">foo_t *P = 0;</font>
<font color="blue">^</font>
</pre>
<p>This shows an example of much better error recovery. The message coming out
of GCC is completely useless for diagnosing the problem, Clang tries much harder
and produces a much more useful diagnosis of the problem.</p>
<pre>
$ <b>cat t.cc</b>
template&lt;class T&gt;
class a {}
class temp {};
a&lt;temp&gt; b;
struct b {
}
$ <b>gcc-4.2 t.cc</b>
t.cc:3: error: multiple types in one declaration
t.cc:4: error: non-template type 'a' used as a template
t.cc:4: error: invalid type in declaration before ';' token
t.cc:6: error: expected unqualified-id at end of input
$ <b>clang t.cc</b>
t.cc:2:11: <font color="red">error:</font> expected ';' after class
<font color="darkgreen">class a {}</font>
<font color="blue"> ^</font>
<font color="blue"> ;</font>
t.cc:6:2: <font color="red">error:</font> expected ';' after struct
<font color="darkgreen">}</font>
<font color="blue"> ^</font>
<font color="blue"> ;</font>
</pre>
<p>This shows that we recover from the simple case of forgetting a ; after
a struct definition much better than GCC.</p>
<p>While each of these details is minor, we feel that they all add up to provide
a much more polished experience.</p>
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