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20 <h1>Expressive Diagnostics</h1>
21 <!--=======================================================================-->
23 <p>In addition to being fast and functional, we aim to make Clang extremely user
24 friendly. As far as a command-line compiler goes, this basically boils down to
25 making the diagnostics (error and warning messages) generated by the compiler
26 be as useful as possible. There are several ways that we do this. This section
27 talks about the experience provided by the command line compiler, contrasting
28 Clang output to GCC 4.2's output in several examples.
31 that embed Clang and extract equivalent information through internal APIs.-->
34 <h2>Column Numbers and Caret Diagnostics</h2>
36 <p>First, all diagnostics produced by clang include full column number
37 information. The clang command-line compiler driver uses this information
38 to print "caret diagnostics".
39 (IDEs can use the information to display in-line error markup.)
40 Precise error location in the source is a feature provided by many commercial
41 compilers, but is generally missing from open source
42 compilers. This is nice because it makes it very easy to understand exactly
43 what is wrong in a particular piece of code</p>
45 <p>The caret (the blue "^" character) exactly shows where the problem is, even
46 inside of a string. This makes it really easy to jump to the problem and
47 helps when multiple instances of the same character occur on a line. (We'll
48 revisit this more in following examples.)</p>
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>
54 format-strings.c:91:13: <font color="magenta">warning:</font> '.*' specified field precision is missing a matching 'int' argument
55 <font color="darkgreen"> printf("%.*d");</font>
56 <font color="blue"> ^</font>
59 <h2>Range Highlighting for Related Text</h2>
61 <p>Clang captures and accurately tracks range information for expressions,
62 statements, and other constructs in your program and uses this to make
63 diagnostics highlight related information. In the following somewhat
64 nonsensical example you can see that you don't even need to see the original source code to
65 understand what is wrong based on the Clang error. Because clang prints a
66 caret, you know exactly <em>which</em> plus it is complaining about. The range
67 information highlights the left and right side of the plus which makes it
68 immediately obvious what the compiler is talking about.
69 Range information is very useful for
70 cases involving precedence issues and many other cases.</p>
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>
76 t.c:7:39: <font color="red">error:</font> invalid operands to binary expression ('int' and 'struct A')
77 <font color="darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</font>
78 <font color="blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</font>
81 <h2>Precision in Wording</h2>
83 <p>A detail is that we have tried really hard to make the diagnostics that come
84 out of clang contain exactly the pertinent information about what is wrong and
85 why. In the example above, we tell you what the inferred types are for
86 the left and right hand sides, and we don't repeat what is obvious from the
87 caret (e.g., that this is a "binary +").</p>
89 <p>Many other examples abound. In the following example, not only do we tell you that there is a problem with the *
90 and point to it, we say exactly why and tell you what the type is (in case it is
91 a complicated subexpression, such as a call to an overloaded function). This
92 sort of attention to detail makes it much easier to understand and fix problems
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>
99 t.c:5:11: <font color="red">error:</font> indirection requires pointer operand ('int' invalid)
100 <font color="darkgreen"> int y = *SomeA.X;</font>
101 <font color="blue"> ^~~~~~~~</font>
104 <h2>No Pretty Printing of Expressions in Diagnostics</h2>
106 <p>Since Clang has range highlighting, it never needs to pretty print your code
107 back out to you. GCC can produce inscrutible error messages in some cases when
108 it tries to do this. In this example P and Q have type "int*":</p>
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>
114 t.c:12:8: <font color="red">error:</font> called object type 'int' is not a function or function pointer
115 <font color="darkgreen"> (P-Q)();</font>
116 <font color="blue"> ~~~~~^</font>
119 <p>This can be particularly bad in G++, which often emits errors
120 containing lowered vtable references. For example:</p>
128 struct foo : public virtual a {
132 return P->bar() + *P;
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->foo::<anonymous>.a::_vptr$a + -0x00000000000000020)))->a::bar() + * P'
137 t.cc:9: error: return-statement with a value, in function returning 'void'
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>
145 <h2>Typedef Preservation and Selective Unwrapping</h2>
147 <p>Many programmers use high-level user defined types, typedefs, and other
148 syntactic sugar to refer to types in their program. This is useful because they
149 can abbreviate otherwise very long types and it is useful to preserve the
150 typename in diagnostics. However, sometimes very simple typedefs can wrap
151 trivial types and it is important to strip off the typedef to understand what
152 is going on. Clang aims to handle both cases well.<p>
154 <p>The following example shows where it is important to preserve
155 a typedef in C. Here the type printed by GCC isn't even valid, but if the error
156 were about a very long and complicated type (as often happens in C++) the error
157 message would be ugly just because it was long and hard to read.</p>
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>
163 t.c:15:11: <font color="red">error:</font> can't convert between vector values of different size ('__m128' and 'int const *')
164 <font color="darkgreen"> myvec[1]/P;</font>
165 <font color="blue"> ~~~~~~~~^~</font>
168 <p>The following example shows where it is useful for the compiler to expose
169 underlying details of a typedef. If the user was somehow confused about how the
170 system "pid_t" typedef is defined, Clang helpfully displays it with "aka".</p>
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>
176 t.c:13:9: <font color="red">error:</font> member reference base type 'pid_t' (aka 'int') is not a structure or union
177 <font color="darkgreen"> myvar = myvar.x;</font>
178 <font color="blue"> ~~~~~ ^</font>
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:
186 struct WebService { };
194 using namespace myapp;
195 void addHTTPService(servers::Server const &;amp;server, ::services::WebService const *http) {
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"):
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>
207 t.cpp:9:10: <font color="red">error:</font> invalid operands to binary expression ('servers::Server const' and '::services::WebService const *')
208 <font color="darkgreen">server += http;</font>
209 <font color="blue">~~~~~~ ^ ~~~~</font>
212 <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>
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>
218 t.cpp:12:7: <font color="red">error:</font> incompatible type assigning 'vector<Real>', expected 'std::string' (aka 'class std::basic_string<char>')
219 <font color="darkgreen">str = vec</font>;
220 <font color="blue">^ ~~~</font>
223 <h2>Fix-it Hints</h2>
225 <p>"Fix-it" hints provide advice for fixing small, localized problems
226 in source code. When Clang produces a diagnostic about a particular
227 problem that it can work around (e.g., non-standard or redundant
228 syntax, missing keywords, common mistakes, etc.), it may also provide
229 specific guidance in the form of a code transformation to correct the
230 problem. In the following example, Clang warns about the use of a GCC
231 extension that has been considered obsolete since 1993. The underlined
232 code should be removed, then replaced with the code below the
233 caret line (".x =" or ".y =", respectively).</p>
237 t.c:5:28: <font color="magenta">warning:</font> use of GNU old-style field designator extension
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>
241 t.c:5:36: <font color="magenta">warning:</font> use of GNU old-style field designator extension
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>
247 <p>"Fix-it" hints are most useful for
248 working around common user errors and misconceptions. For example, C++ users
249 commonly forget the syntax for explicit specialization of class templates,
250 as in the error in the following example. Again, after describing the problem,
251 Clang provides the fix--add <code>template<></code>--as part of the
256 t.cpp:9:3: <font color="red">error:</font> template specialization requires 'template<>'
257 struct iterator_traits<file_iterator> {
258 <font color="blue">^</font>
259 <font color="darkgreen">template<> </font>
262 <h2>Automatic Macro Expansion</h2>
264 <p>Many errors happen in macros that are sometimes deeply nested. With
265 traditional compilers, you need to dig deep into the definition of the macro to
266 understand how you got into trouble. The following simple example shows how
267 Clang helps you out by automatically printing instantiation information and
268 nested range information for diagnostics as they are instantiated through macros
269 and also shows how some of the other pieces work in a bigger example.</p>
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 < (have 'struct mystruct' and 'float')
275 $ <b>clang -fsyntax-only t.c</b>
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'))
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 < __b ? __b : __a; })</font>
281 <font color="blue"> ~~~ ^ ~~~</font>
284 <p>Here's another real world warning that occurs in the "window" Unix package (which
285 implements the "wwopen" class of APIs):</p>
288 $ <b>clang -fsyntax-only t.c</b>
289 t.c:22:2: <font color="magenta">warning:</font> type specifier missing, defaults to 'int'
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>
300 <p>In practice, we've found that Clang's treatment of macros is actually more useful in multiply nested
301 macros that in simple ones.</p>
303 <h2>Quality of Implementation and Attention to Detail</h2>
305 <p>Finally, we have put a lot of work polishing the little things, because
306 little things add up over time and contribute to a great user experience.</p>
308 <p>The following example shows a trivial little tweak, where we tell you to put the semicolon at
309 the end of the line that is missing it (line 4) instead of at the beginning of
310 the following line (line 5). This is particularly important with fixit hints
311 and caret diagnostics, because otherwise you don't get the important context.
316 t.c: In function 'foo':
317 t.c:5: error: expected ';' before '}' token
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>
325 <p>The following example shows much better error recovery than GCC. The message coming out
326 of GCC is completely useless for diagnosing the problem. Clang tries much harder
327 and produces a much more useful diagnosis of the problem.</p>
331 t.c:3: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
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>
338 <p>The following example shows that we recover from the simple case of
339 forgetting a ; after a struct definition much better than GCC.</p>
343 template<class T>
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
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>
365 <p>While each of these details is minor, we feel that they all add up to provide
366 a much more polished experience.</p>