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19<!--=======================================================================-->
20<h1>Expressive Diagnostics</h1>
21<!--=======================================================================-->
22
23<p>In addition to being fast and functional, we aim to make Clang extremely user
24friendly. As far as a command-line compiler goes, this basically boils down to
25making the diagnostics (error and warning messages) generated by the compiler
26be as useful as possible. There are several ways that we do this. This section
27talks about the experience provided by the command line compiler, contrasting
28Clang output to GCC 4.2's output in several examples.
29<!--
30Other clients
31that embed Clang and extract equivalent information through internal APIs.-->
32</p>
33
34<h2>Column Numbers and Caret Diagnostics</h2>
35
36<p>First, all diagnostics produced by clang include full column number
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]37information. The clang command-line compiler driver uses this information
38to print "caret diagnostics".
39(IDEs can use the information to display in-line error markup.)
40Precise error location in the source is a feature provided by many commercial
41compilers, but is generally missing from open source
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]42compilers. This is nice because it makes it very easy to understand exactly
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]43what is wrong in a particular piece of code</p>
44
45<p>The caret (the blue "^" character) exactly shows where the problem is, even
46inside of a string. This makes it really easy to jump to the problem and
47helps when multiple instances of the same character occur on a line. (We'll
48revisit this more in following examples.)</p>
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]49
50<pre>
51 $ <b>gcc-4.2 -fsyntax-only -Wformat format-strings.c</b>
52 format-strings.c:91: warning: too few arguments for format
53 $ <b>clang -fsyntax-only format-strings.c</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]54 format-strings.c:91:13: <font color="magenta">warning:</font> '.*' specified field precision is missing a matching 'int' argument
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]55 <font color="darkgreen"> printf("%.*d");</font>
56 <font color="blue"> ^</font>
57</pre>
58
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]59<h2>Range Highlighting for Related Text</h2>
60
61<p>Clang captures and accurately tracks range information for expressions,
62statements, and other constructs in your program and uses this to make
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]63diagnostics highlight related information. In the following somewhat
64nonsensical example you can see that you don't even need to see the original source code to
65understand what is wrong based on the Clang error. Because clang prints a
66caret, you know exactly <em>which</em> plus it is complaining about. The range
67information highlights the left and right side of the plus which makes it
68immediately obvious what the compiler is talking about.
69Range information is very useful for
70cases involving precedence issues and many other cases.</p>
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]71
72<pre>
73 $ <b>gcc-4.2 -fsyntax-only t.c</b>
74 t.c:7: error: invalid operands to binary + (have 'int' and 'struct A')
75 $ <b>clang -fsyntax-only t.c</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]76 t.c:7:39: <font color="red">error:</font> invalid operands to binary expression ('int' and 'struct A')
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]77 <font color="darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</font>
78 <font color="blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</font>
79</pre>
80
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]81<h2>Precision in Wording</h2>
82
83<p>A detail is that we have tried really hard to make the diagnostics that come
84out of clang contain exactly the pertinent information about what is wrong and
85why. In the example above, we tell you what the inferred types are for
86the left and right hand sides, and we don't repeat what is obvious from the
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]87caret (e.g., that this is a "binary +").</p>
88
89<p>Many other examples abound. In the following example, not only do we tell you that there is a problem with the *
90and point to it, we say exactly why and tell you what the type is (in case it is
91a complicated subexpression, such as a call to an overloaded function). This
92sort of attention to detail makes it much easier to understand and fix problems
93quickly.</p>
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]94
95<pre>
96 $ <b>gcc-4.2 -fsyntax-only t.c</b>
97 t.c:5: error: invalid type argument of 'unary *'
98 $ <b>clang -fsyntax-only t.c</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]99 t.c:5:11: <font color="red">error:</font> indirection requires pointer operand ('int' invalid)
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]100 <font color="darkgreen"> int y = *SomeA.X;</font>
101 <font color="blue"> ^~~~~~~~</font>
102</pre>
103
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]104<h2>No Pretty Printing of Expressions in Diagnostics</h2>
105
106<p>Since Clang has range highlighting, it never needs to pretty print your code
Chris Lattnerb3de1122011-08-03 15:54:40 +0000[diff] [blame]107back out to you. GCC can produce inscrutible error messages in some cases when
108it tries to do this. In this example P and Q have type "int*":</p>
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]109
110<pre>
111 $ <b>gcc-4.2 -fsyntax-only t.c</b>
112 #'exact_div_expr' not supported by pp_c_expression#'t.c:12: error: called object is not a function
113 $ <b>clang -fsyntax-only t.c</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]114 t.c:12:8: <font color="red">error:</font> called object type 'int' is not a function or function pointer
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]115 <font color="darkgreen"> (P-Q)();</font>
116 <font color="blue"> ~~~~~^</font>
117</pre>
118
Chris Lattnerb3de1122011-08-03 15:54:40 +0000[diff] [blame]119<p>This can be particularly bad in G++, which often emits errors
120 containing lowered vtable references. For example:</p>
121
122<pre>
123 $ <b>cat t.cc</b>
124 struct a {
125 virtual int bar();
126 };
127
128 struct foo : public virtual a {
129 };
130
131 void test(foo *P) {
132 return P->bar() + *P;
133 }
134 $ <b>gcc-4.2 t.cc</b>
135 t.cc: In function 'void test(foo*)':
136 t.cc:9: error: no match for 'operator+' in '(((a*)P) + (*(long int*)(P-&gt;foo::&lt;anonymous&gt;.a::_vptr$a + -0x00000000000000020)))-&gt;a::bar() + * P'
137 t.cc:9: error: return-statement with a value, in function returning 'void'
138 $ <b>clang t.cc</b>
139 t.cc:9:18: <font color="red">error:</font> invalid operands to binary expression ('int' and 'foo')
140 <font color="darkgreen"> return P->bar() + *P;</font>
141 <font color="blue"> ~~~~~~~~ ^ ~~</font>
142</pre>
143
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]144
145<h2>Typedef Preservation and Selective Unwrapping</h2>
146
147<p>Many programmers use high-level user defined types, typedefs, and other
148syntactic sugar to refer to types in their program. This is useful because they
149can abbreviate otherwise very long types and it is useful to preserve the
150typename in diagnostics. However, sometimes very simple typedefs can wrap
151trivial types and it is important to strip off the typedef to understand what
152is going on. Clang aims to handle both cases well.<p>
153
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]154<p>The following example shows where it is important to preserve
155a typedef in C. Here the type printed by GCC isn't even valid, but if the error
156were about a very long and complicated type (as often happens in C++) the error
157message would be ugly just because it was long and hard to read.</p>
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]158
159<pre>
160 $ <b>gcc-4.2 -fsyntax-only t.c</b>
161 t.c:15: error: invalid operands to binary / (have 'float __vector__' and 'const int *')
162 $ <b>clang -fsyntax-only t.c</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]163 t.c:15:11: <font color="red">error:</font> can't convert between vector values of different size ('__m128' and 'int const *')
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]164 <font color="darkgreen"> myvec[1]/P;</font>
165 <font color="blue"> ~~~~~~~~^~</font>
166</pre>
167
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]168<p>The following example shows where it is useful for the compiler to expose
169underlying details of a typedef. If the user was somehow confused about how the
170system "pid_t" typedef is defined, Clang helpfully displays it with "aka".</p>
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]171
172<pre>
173 $ <b>gcc-4.2 -fsyntax-only t.c</b>
174 t.c:13: error: request for member 'x' in something not a structure or union
175 $ <b>clang -fsyntax-only t.c</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]176 t.c:13:9: <font color="red">error:</font> member reference base type 'pid_t' (aka 'int') is not a structure or union
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]177 <font color="darkgreen"> myvar = myvar.x;</font>
178 <font color="blue"> ~~~~~ ^</font>
179</pre>
180
Douglas Gregor969c6892009-04-01 15:47:24 +0000[diff] [blame]181<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:
182
183<blockquote>
184<pre>
185namespace services {
186 struct WebService { };
187}
188namespace myapp {
189 namespace servers {
190 struct Server { };
191 }
192}
193
194using namespace myapp;
Benjamin Kramerd8ee4072011-08-06 03:04:45 +0000[diff] [blame]195void addHTTPService(servers::Server const &amp;server, ::services::WebService const *http) {
Douglas Gregor969c6892009-04-01 15:47:24 +0000[diff] [blame]196 server += http;
197}
198</pre>
199</blockquote>
200
201<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"):
202
203<pre>
204 $ <b>g++-4.2 -fsyntax-only t.cpp</b>
205 t.cpp:9: error: no match for 'operator+=' in 'server += http'
206 $ <b>clang -fsyntax-only t.cpp</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]207 t.cpp:9:10: <font color="red">error:</font> invalid operands to binary expression ('servers::Server const' and '::services::WebService const *')
Douglas Gregor969c6892009-04-01 15:47:24 +0000[diff] [blame]208 <font color="darkgreen">server += http;</font>
209 <font color="blue">~~~~~~ ^ ~~~~</font>
210</pre>
211
212<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>
213
214<pre>
215 $ <b>g++-4.2 -fsyntax-only t.cpp</b>
216 t.cpp:12: error: no match for 'operator=' in 'str = vec'
217 $ <b>clang -fsyntax-only t.cpp</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]218 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;')
Douglas Gregor969c6892009-04-01 15:47:24 +0000[diff] [blame]219 <font color="darkgreen">str = vec</font>;
220 <font color="blue">^ ~~~</font>
221</pre>
222
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]223<h2>Fix-it Hints</h2>
224
Douglas Gregoreff49c62009-04-01 16:24:40 +0000[diff] [blame]225<p>"Fix-it" hints provide advice for fixing small, localized problems
226in source code. When Clang produces a diagnostic about a particular
227problem that it can work around (e.g., non-standard or redundant
228syntax, missing keywords, common mistakes, etc.), it may also provide
229specific guidance in the form of a code transformation to correct the
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]230problem. In the following example, Clang warns about the use of a GCC
231extension that has been considered obsolete since 1993. The underlined
232code should be removed, then replaced with the code below the
233caret line (".x =" or ".y =", respectively).</p>
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]234
Douglas Gregoreff49c62009-04-01 16:24:40 +0000[diff] [blame]235<pre>
236 $ <b>clang t.c</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]237 t.c:5:28: <font color="magenta">warning:</font> use of GNU old-style field designator extension
Douglas Gregoreff49c62009-04-01 16:24:40 +0000[diff] [blame]238 <font color="darkgreen">struct point origin = { x: 0.0, y: 0.0 };</font>
239 <font color="red">~~</font> <font color="blue">^</font>
240 <font color="darkgreen">.x = </font>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]241 t.c:5:36: <font color="magenta">warning:</font> use of GNU old-style field designator extension
Douglas Gregoreff49c62009-04-01 16:24:40 +0000[diff] [blame]242 <font color="darkgreen">struct point origin = { x: 0.0, y: 0.0 };</font>
243 <font color="red">~~</font> <font color="blue">^</font>
244 <font color="darkgreen">.y = </font>
245</pre>
246
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]247<p>"Fix-it" hints are most useful for
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]248working around common user errors and misconceptions. For example, C++ users
249commonly forget the syntax for explicit specialization of class templates,
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]250as in the error in the following example. Again, after describing the problem,
251Clang provides the fix--add <code>template&lt;&gt;</code>--as part of the
252diagnostic.<p>
Douglas Gregoreff49c62009-04-01 16:24:40 +0000[diff] [blame]253
254<pre>
255 $ <b>clang t.cpp</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]256 t.cpp:9:3: <font color="red">error:</font> template specialization requires 'template&lt;&gt;'
Douglas Gregoreff49c62009-04-01 16:24:40 +0000[diff] [blame]257 struct iterator_traits&lt;file_iterator&gt; {
258 <font color="blue">^</font>
259 <font color="darkgreen">template&lt;&gt; </font>
260</pre>
261
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]262<h2>Automatic Macro Expansion</h2>
263
264<p>Many errors happen in macros that are sometimes deeply nested. With
265traditional compilers, you need to dig deep into the definition of the macro to
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]266understand how you got into trouble. The following simple example shows how
267Clang helps you out by automatically printing instantiation information and
268nested range information for diagnostics as they are instantiated through macros
269and also shows how some of the other pieces work in a bigger example.</p>
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]270
271<pre>
272 $ <b>gcc-4.2 -fsyntax-only t.c</b>
273 t.c: In function 'test':
274 t.c:80: error: invalid operands to binary &lt; (have 'struct mystruct' and 'float')
275 $ <b>clang -fsyntax-only t.c</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]276 t.c:80:3: <font color="red">error:</font> invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float'))
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]277 <font color="darkgreen"> X = MYMAX(P, F);</font>
278 <font color="blue"> ^~~~~~~~~~~</font>
279 t.c:76:94: note: instantiated from:
280 <font color="darkgreen">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a &lt; __b ? __b : __a; })</font>
281 <font color="blue"> ~~~ ^ ~~~</font>
282</pre>
283
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]284<p>Here's another real world warning that occurs in the "window" Unix package (which
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]285implements the "wwopen" class of APIs):</p>
286
287<pre>
288 $ <b>clang -fsyntax-only t.c</b>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]289 t.c:22:2: <font color="magenta">warning:</font> type specifier missing, defaults to 'int'
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]290 <font color="darkgreen"> ILPAD();</font>
291 <font color="blue"> ^</font>
292 t.c:17:17: note: instantiated from:
293 <font color="darkgreen">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</font>
294 <font color="blue"> ^</font>
295 t.c:14:2: note: instantiated from:
296 <font color="darkgreen"> register i; \</font>
297 <font color="blue"> ^</font>
298</pre>
299
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]300<p>In practice, we've found that Clang's treatment of macros is actually more useful in multiply nested
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]301macros that in simple ones.</p>
302
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]303<h2>Quality of Implementation and Attention to Detail</h2>
304
305<p>Finally, we have put a lot of work polishing the little things, because
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]306little things add up over time and contribute to a great user experience.</p>
307
308<p>The following example shows a trivial little tweak, where we tell you to put the semicolon at
309the end of the line that is missing it (line 4) instead of at the beginning of
310the following line (line 5). This is particularly important with fixit hints
311and caret diagnostics, because otherwise you don't get the important context.
312</p>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]313
314<pre>
315 $ <b>gcc-4.2 t.c</b>
316 t.c: In function 'foo':
317 t.c:5: error: expected ';' before '}' token
318 $ <b>clang t.c</b>
319 t.c:4:8: <font color="red">error:</font> expected ';' after expression
320 <font color="darkgreen"> bar()</font>
321 <font color="blue"> ^</font>
322 <font color="blue"> ;</font>
323</pre>
324
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]325<p>The following example shows much better error recovery than GCC. The message coming out
326of GCC is completely useless for diagnosing the problem. Clang tries much harder
327and produces a much more useful diagnosis of the problem.</p>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]328
329<pre>
330 $ <b>gcc-4.2 t.c</b>
331 t.c:3: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
332 $ <b>clang t.c</b>
333 t.c:3:1: <font color="red">error:</font> unknown type name 'foo_t'
334 <font color="darkgreen">foo_t *P = 0;</font>
335 <font color="blue">^</font>
336</pre>
337
Douglas Gregord89df5a2011-06-01 22:45:49 +0000[diff] [blame]338<p>The following example shows that we recover from the simple case of
339forgetting a ; after a struct definition much better than GCC.</p>
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]340
Chris Lattner94634cc2010-02-02 01:35:23 +0000[diff] [blame]341<pre>
342 $ <b>cat t.cc</b>
343 template&lt;class T&gt;
344 class a {}
345 class temp {};
346 a&lt;temp&gt; b;
347 struct b {
348 }
349 $ <b>gcc-4.2 t.cc</b>
350 t.cc:3: error: multiple types in one declaration
351 t.cc:4: error: non-template type 'a' used as a template
352 t.cc:4: error: invalid type in declaration before ';' token
353 t.cc:6: error: expected unqualified-id at end of input
354 $ <b>clang t.cc</b>
355 t.cc:2:11: <font color="red">error:</font> expected ';' after class
356 <font color="darkgreen">class a {}</font>
357 <font color="blue"> ^</font>
358 <font color="blue"> ;</font>
359 t.cc:6:2: <font color="red">error:</font> expected ';' after struct
360 <font color="darkgreen">}</font>
361 <font color="blue"> ^</font>
362 <font color="blue"> ;</font>
363</pre>
364
Chris Lattner0e3b44b2009-09-30 20:19:10 +0000[diff] [blame]365<p>While each of these details is minor, we feel that they all add up to provide
366a much more polished experience.</p>
367
Chris Lattner76766012009-03-19 18:52:17 +0000[diff] [blame]368</div>
369</body>
370</html>