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23 <!--=======================================================================-->
24 <h1>Expressive Diagnostics</h1>
25 <!--=======================================================================-->
27 <p>In addition to being fast and functional, we aim to make Clang extremely user
28 friendly. As far as a command-line compiler goes, this basically boils down to
29 making the diagnostics (error and warning messages) generated by the compiler
30 be as useful as possible. There are several ways that we do this. This section
31 talks about the experience provided by the command line compiler, contrasting
32 Clang output to GCC 4.2's output in several examples.
35 that embed Clang and extract equivalent information through internal APIs.-->
38 <h2>Column Numbers and Caret Diagnostics</h2>
40 <p>First, all diagnostics produced by clang include full column number
41 information. The clang command-line compiler driver uses this information
42 to print "point diagnostics".
43 (IDEs can use the information to display in-line error markup.)
44 Precise error location in the source is a feature provided by many commercial
45 compilers, but is generally missing from open source
46 compilers. This is nice because it makes it very easy to understand exactly
47 what is wrong in a particular piece of code</p>
49 <p>The point (the blue "^" character) exactly shows where the problem is, even
50 inside of a string. This makes it really easy to jump to the problem and
51 helps when multiple instances of the same character occur on a line. (We'll
52 revisit this more in following examples.)</p>
55 $ <b>gcc-4.2 -fsyntax-only -Wformat format-strings.c</b>
56 format-strings.c:91: warning: too few arguments for format
57 $ <b>clang -fsyntax-only format-strings.c</b>
58 format-strings.c:91:13: <span class="warn">warning:</span> '.*' specified field precision is missing a matching 'int' argument
59 <span class="snip"> printf("%.*d");</span>
60 <span class="point"> ^</span>
63 <h2>Range Highlighting for Related Text</h2>
65 <p>Clang captures and accurately tracks range information for expressions,
66 statements, and other constructs in your program and uses this to make
67 diagnostics highlight related information. In the following somewhat
68 nonsensical example you can see that you don't even need to see the original source code to
69 understand what is wrong based on the Clang error. Because clang prints a
70 point, you know exactly <em>which</em> plus it is complaining about. The range
71 information highlights the left and right side of the plus which makes it
72 immediately obvious what the compiler is talking about.
73 Range information is very useful for
74 cases involving precedence issues and many other cases.</p>
77 $ <b>gcc-4.2 -fsyntax-only t.c</b>
78 t.c:7: error: invalid operands to binary + (have 'int' and 'struct A')
79 $ <b>clang -fsyntax-only t.c</b>
80 t.c:7:39: <span class="err">error:</span> invalid operands to binary expression ('int' and 'struct A')
81 <span class="snip"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</span>
82 <span class="point"> ~~~~~~~~~~~~~~ ^ ~~~~~</span>
85 <h2>Precision in Wording</h2>
87 <p>A detail is that we have tried really hard to make the diagnostics that come
88 out of clang contain exactly the pertinent information about what is wrong and
89 why. In the example above, we tell you what the inferred types are for
90 the left and right hand sides, and we don't repeat what is obvious from the
91 point (e.g., that this is a "binary +").</p>
93 <p>Many other examples abound. In the following example, not only do we tell you that there is a problem with the *
94 and point to it, we say exactly why and tell you what the type is (in case it is
95 a complicated subexpression, such as a call to an overloaded function). This
96 sort of attention to detail makes it much easier to understand and fix problems
100 $ <b>gcc-4.2 -fsyntax-only t.c</b>
101 t.c:5: error: invalid type argument of 'unary *'
102 $ <b>clang -fsyntax-only t.c</b>
103 t.c:5:11: <span class="err">error:</span> indirection requires pointer operand ('int' invalid)
104 <span class="snip"> int y = *SomeA.X;</span>
105 <span class="point"> ^~~~~~~~</span>
108 <h2>No Pretty Printing of Expressions in Diagnostics</h2>
110 <p>Since Clang has range highlighting, it never needs to pretty print your code
111 back out to you. GCC can produce inscrutible error messages in some cases when
112 it tries to do this. In this example P and Q have type "int*":</p>
115 $ <b>gcc-4.2 -fsyntax-only t.c</b>
116 #'exact_div_expr' not supported by pp_c_expression#'t.c:12: error: called object is not a function
117 $ <b>clang -fsyntax-only t.c</b>
118 t.c:12:8: <span class="err">error:</span> called object type 'int' is not a function or function pointer
119 <span class="snip"> (P-Q)();</span>
120 <span class="point"> ~~~~~^</span>
123 <p>This can be particularly bad in G++, which often emits errors
124 containing lowered vtable references. For example:</p>
132 struct foo : public virtual a {
136 return P->bar() + *P;
138 $ <b>gcc-4.2 t.cc</b>
139 t.cc: In function 'void test(foo*)':
140 t.cc:9: error: no match for 'operator+' in '(((a*)P) + (*(long int*)(P->foo::<anonymous>.a::_vptr$a + -0x00000000000000020)))->a::bar() + * P'
141 t.cc:9: error: return-statement with a value, in function returning 'void'
143 t.cc:9:18: <span class="err">error:</span> invalid operands to binary expression ('int' and 'foo')
144 <span class="snip"> return P->bar() + *P;</span>
145 <span class="point"> ~~~~~~~~ ^ ~~</span>
149 <h2>Typedef Preservation and Selective Unwrapping</h2>
151 <p>Many programmers use high-level user defined types, typedefs, and other
152 syntactic sugar to refer to types in their program. This is useful because they
153 can abbreviate otherwise very long types and it is useful to preserve the
154 typename in diagnostics. However, sometimes very simple typedefs can wrap
155 trivial types and it is important to strip off the typedef to understand what
156 is going on. Clang aims to handle both cases well.<p>
158 <p>The following example shows where it is important to preserve
159 a typedef in C. Here the type printed by GCC isn't even valid, but if the error
160 were about a very long and complicated type (as often happens in C++) the error
161 message would be ugly just because it was long and hard to read.</p>
164 $ <b>gcc-4.2 -fsyntax-only t.c</b>
165 t.c:15: error: invalid operands to binary / (have 'float __vector__' and 'const int *')
166 $ <b>clang -fsyntax-only t.c</b>
167 t.c:15:11: <span class="err">error:</span> can't convert between vector values of different size ('__m128' and 'int const *')
168 <span class="snip"> myvec[1]/P;</span>
169 <span class="point"> ~~~~~~~~^~</span>
172 <p>The following example shows where it is useful for the compiler to expose
173 underlying details of a typedef. If the user was somehow confused about how the
174 system "pid_t" typedef is defined, Clang helpfully displays it with "aka".</p>
177 $ <b>gcc-4.2 -fsyntax-only t.c</b>
178 t.c:13: error: request for member 'x' in something not a structure or union
179 $ <b>clang -fsyntax-only t.c</b>
180 t.c:13:9: <span class="err">error:</span> member reference base type 'pid_t' (aka 'int') is not a structure or union
181 <span class="snip"> myvar = myvar.x;</span>
182 <span class="point"> ~~~~~ ^</span>
185 <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:
190 struct WebService { };
198 using namespace myapp;
199 void addHTTPService(servers::Server const &server, ::services::WebService const *http) {
205 <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"):
208 $ <b>g++-4.2 -fsyntax-only t.cpp</b>
209 t.cpp:9: error: no match for 'operator+=' in 'server += http'
210 $ <b>clang -fsyntax-only t.cpp</b>
211 t.cpp:9:10: <span class="err">error:</span> invalid operands to binary expression ('servers::Server const' and '::services::WebService const *')
212 <span class="snip">server += http;</span>
213 <span class="point">~~~~~~ ^ ~~~~</span>
216 <p>Naturally, type preservation extends to uses of templates, and Clang retains information about how a particular template specialization (like <code>std::vector<Real></code>) was spelled within the source code. For example:</p>
219 $ <b>g++-4.2 -fsyntax-only t.cpp</b>
220 t.cpp:12: error: no match for 'operator=' in 'str = vec'
221 $ <b>clang -fsyntax-only t.cpp</b>
222 t.cpp:12:7: <span class="err">error:</span> incompatible type assigning 'vector<Real>', expected 'std::string' (aka 'class std::basic_string<char>')
223 <span class="snip">str = vec</span>;
224 <span class="point">^ ~~~</span>
227 <h2>Fix-it Hints</h2>
229 <p>"Fix-it" hints provide advice for fixing small, localized problems
230 in source code. When Clang produces a diagnostic about a particular
231 problem that it can work around (e.g., non-standard or redundant
232 syntax, missing keywords, common mistakes, etc.), it may also provide
233 specific guidance in the form of a code transformation to correct the
234 problem. In the following example, Clang warns about the use of a GCC
235 extension that has been considered obsolete since 1993. The underlined
236 code should be removed, then replaced with the code below the
237 point line (".x =" or ".y =", respectively).</p>
241 t.c:5:28: <span class="warn">warning:</span> use of GNU old-style field designator extension
242 <span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span>
243 <span class="err">~~</span> <span class="point">^</span>
244 <span class="snip">.x = </span>
245 t.c:5:36: <span class="warn">warning:</span> use of GNU old-style field designator extension
246 <span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span>
247 <span class="err">~~</span> <span class="point">^</span>
248 <span class="snip">.y = </span>
251 <p>"Fix-it" hints are most useful for
252 working around common user errors and misconceptions. For example, C++ users
253 commonly forget the syntax for explicit specialization of class templates,
254 as in the error in the following example. Again, after describing the problem,
255 Clang provides the fix--add <code>template<></code>--as part of the
260 t.cpp:9:3: <span class="err">error:</span> template specialization requires 'template<>'
261 struct iterator_traits<file_iterator> {
262 <span class="point">^</span>
263 <span class="snip">template<> </span>
266 <h2>Template Type Diffing</h2>
268 <p>Templates types can be long and difficult to read. Moreso when part of an
269 error message. Instead of just printing out the type name, Clang has enough
270 information to remove the common elements and highlight the differences. To
271 show the template structure more clearly, the templated type can also be
272 printed as an indented text tree.</p>
274 Default: template diff with type elision
276 t.cc:4:5: <span class="note">note:</span> candidate function not viable: no known conversion from 'vector<map<[...], <span class="template-highlight">float</span>>>' to 'vector<map<[...], <span class="template-highlight">double</span>>>' for 1st argument;
278 -fno-elide-type: template diff without elision
280 t.cc:4:5: <span class="note">note:</span> candidate function not viable: no known conversion from 'vector<map<int, <span class="template-highlight">float</span>>>' to 'vector<map<int, <span class="template-highlight">double</span>>>' for 1st argument;
282 -fdiagnostics-show-template-tree: template tree printing with elision
284 t.cc:4:5: <span class="note">note:</span> candidate function not viable: no known conversion for 1st argument;
288 [<span class="template-highlight">float</span> != <span class="template-highlight">double</span>]>>
290 -fdiagnostics-show-template-tree -fno-elide-type: template tree printing with no elision
292 t.cc:4:5: <span class="note">note:M</span> candidate function not viable: no known conversion for 1st argument;
296 [<span class="template-highlight">float</span> != <span class="template-highlight">double</span>]>>
299 <h2>Automatic Macro Expansion</h2>
301 <p>Many errors happen in macros that are sometimes deeply nested. With
302 traditional compilers, you need to dig deep into the definition of the macro to
303 understand how you got into trouble. The following simple example shows how
304 Clang helps you out by automatically printing instantiation information and
305 nested range information for diagnostics as they are instantiated through macros
306 and also shows how some of the other pieces work in a bigger example.</p>
309 $ <b>gcc-4.2 -fsyntax-only t.c</b>
310 t.c: In function 'test':
311 t.c:80: error: invalid operands to binary < (have 'struct mystruct' and 'float')
312 $ <b>clang -fsyntax-only t.c</b>
313 t.c:80:3: <span class="err">error:</span> invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float'))
314 <span class="snip"> X = MYMAX(P, F);</span>
315 <span class="point"> ^~~~~~~~~~~</span>
316 t.c:76:94: note: expanded from:
317 <span class="snip">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })</span>
318 <span class="point"> ~~~ ^ ~~~</span>
321 <p>Here's another real world warning that occurs in the "window" Unix package (which
322 implements the "wwopen" class of APIs):</p>
325 $ <b>clang -fsyntax-only t.c</b>
326 t.c:22:2: <span class="warn">warning:</span> type specifier missing, defaults to 'int'
327 <span class="snip"> ILPAD();</span>
328 <span class="point"> ^</span>
329 t.c:17:17: note: expanded from:
330 <span class="snip">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</span>
331 <span class="point"> ^</span>
332 t.c:14:2: note: expanded from:
333 <span class="snip"> register i; \</span>
334 <span class="point"> ^</span>
337 <p>In practice, we've found that Clang's treatment of macros is actually more useful in multiply nested
338 macros that in simple ones.</p>
340 <h2>Quality of Implementation and Attention to Detail</h2>
342 <p>Finally, we have put a lot of work polishing the little things, because
343 little things add up over time and contribute to a great user experience.</p>
345 <p>The following example shows a trivial little tweak, where we tell you to put the semicolon at
346 the end of the line that is missing it (line 4) instead of at the beginning of
347 the following line (line 5). This is particularly important with fixit hints
348 and point diagnostics, because otherwise you don't get the important context.
353 t.c: In function 'foo':
354 t.c:5: error: expected ';' before '}' token
356 t.c:4:8: <span class="err">error:</span> expected ';' after expression
357 <span class="snip"> bar()</span>
358 <span class="point"> ^</span>
359 <span class="point"> ;</span>
362 <p>The following example shows much better error recovery than GCC. The message coming out
363 of GCC is completely useless for diagnosing the problem. Clang tries much harder
364 and produces a much more useful diagnosis of the problem.</p>
368 t.c:3: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
370 t.c:3:1: <span class="err">error:</span> unknown type name 'foo_t'
371 <span class="snip">foo_t *P = 0;</span>
372 <span class="point">^</span>
375 <p>The following example shows that we recover from the simple case of
376 forgetting a ; after a struct definition much better than GCC.</p>
380 template<class T>
386 $ <b>gcc-4.2 t.cc</b>
387 t.cc:3: error: multiple types in one declaration
388 t.cc:4: error: non-template type 'a' used as a template
389 t.cc:4: error: invalid type in declaration before ';' token
390 t.cc:6: error: expected unqualified-id at end of input
392 t.cc:2:11: <span class="err">error:</span> expected ';' after class
393 <span class="snip">class a {}</span>
394 <span class="point"> ^</span>
395 <span class="point"> ;</span>
396 t.cc:6:2: <span class="err">error:</span> expected ';' after struct
397 <span class="snip">}</span>
398 <span class="point"> ^</span>
399 <span class="point"> ;</span>
402 <p>While each of these details is minor, we feel that they all add up to provide
403 a much more polished experience.</p>