1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <meta http-equiv="content-type" content="text/html; charset=iso-8859-1"> 6 <title>Clang - Expressive Diagnostics</title> 7 <link type="text/css" rel="stylesheet" href="menu.css" /> 8 <link type="text/css" rel="stylesheet" href="content.css" /> 9 <style type="text/css"> 10 </style> 11 </head> 12 <body> 13 14 <!--#include virtual="menu.html.incl"--> 15 16 <div id="content"> 17 18 19 <!--=======================================================================--> 20 <h1>Expressive Diagnostics</h1> 21 <!--=======================================================================--> 22 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. 29 <!-- 30 Other clients 31 that 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 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> 44 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> 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> 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> 57 </pre> 58 59 <h2>Range Highlighting for Related Text</h2> 60 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> 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> 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> 79 </pre> 80 81 <h2>Precision in Wording</h2> 82 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> 88 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 93 quickly.</p> 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> 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> 102 </pre> 103 104 <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 107 back out to you. This is particularly bad in G++ (which often emits errors 108 containing lowered vtable references), but even GCC can produce 109 inscrutible error messages in some cases when it tries to do this. In this 110 example P and Q have type "int*":</p> 111 112 <pre> 113 $ <b>gcc-4.2 -fsyntax-only t.c</b> 114 #'exact_div_expr' not supported by pp_c_expression#'t.c:12: error: called object is not a function 115 $ <b>clang -fsyntax-only t.c</b> 116 t.c:12:8: <font color="red">error:</font> called object type 'int' is not a function or function pointer 117 <font color="darkgreen"> (P-Q)();</font> 118 <font color="blue"> ~~~~~^</font> 119 </pre> 120 121 122 <h2>Typedef Preservation and Selective Unwrapping</h2> 123 124 <p>Many programmers use high-level user defined types, typedefs, and other 125 syntactic sugar to refer to types in their program. This is useful because they 126 can abbreviate otherwise very long types and it is useful to preserve the 127 typename in diagnostics. However, sometimes very simple typedefs can wrap 128 trivial types and it is important to strip off the typedef to understand what 129 is going on. Clang aims to handle both cases well.<p> 130 131 <p>The following example shows where it is important to preserve 132 a typedef in C. Here the type printed by GCC isn't even valid, but if the error 133 were about a very long and complicated type (as often happens in C++) the error 134 message would be ugly just because it was long and hard to read.</p> 135 136 <pre> 137 $ <b>gcc-4.2 -fsyntax-only t.c</b> 138 t.c:15: error: invalid operands to binary / (have 'float __vector__' and 'const int *') 139 $ <b>clang -fsyntax-only t.c</b> 140 t.c:15:11: <font color="red">error:</font> can't convert between vector values of different size ('__m128' and 'int const *') 141 <font color="darkgreen"> myvec[1]/P;</font> 142 <font color="blue"> ~~~~~~~~^~</font> 143 </pre> 144 145 <p>The following example shows where it is useful for the compiler to expose 146 underlying details of a typedef. If the user was somehow confused about how the 147 system "pid_t" typedef is defined, Clang helpfully displays it with "aka".</p> 148 149 <pre> 150 $ <b>gcc-4.2 -fsyntax-only t.c</b> 151 t.c:13: error: request for member 'x' in something not a structure or union 152 $ <b>clang -fsyntax-only t.c</b> 153 t.c:13:9: <font color="red">error:</font> member reference base type 'pid_t' (aka 'int') is not a structure or union 154 <font color="darkgreen"> myvar = myvar.x;</font> 155 <font color="blue"> ~~~~~ ^</font> 156 </pre> 157 158 <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: 159 160 <blockquote> 161 <pre> 162 namespace services { 163 struct WebService { }; 164 } 165 namespace myapp { 166 namespace servers { 167 struct Server { }; 168 } 169 } 170 171 using namespace myapp; 172 void addHTTPService(servers::Server const &server, ::services::WebService const *http) { 173 server += http; 174 } 175 </pre> 176 </blockquote> 177 178 <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"): 179 180 <pre> 181 $ <b>g++-4.2 -fsyntax-only t.cpp</b> 182 t.cpp:9: error: no match for 'operator+=' in 'server += http' 183 $ <b>clang -fsyntax-only t.cpp</b> 184 t.cpp:9:10: <font color="red">error:</font> invalid operands to binary expression ('servers::Server const' and '::services::WebService const *') 185 <font color="darkgreen">server += http;</font> 186 <font color="blue">~~~~~~ ^ ~~~~</font> 187 </pre> 188 189 <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> 190 191 <pre> 192 $ <b>g++-4.2 -fsyntax-only t.cpp</b> 193 t.cpp:12: error: no match for 'operator=' in 'str = vec' 194 $ <b>clang -fsyntax-only t.cpp</b> 195 t.cpp:12:7: <font color="red">error:</font> incompatible type assigning 'vector<Real>', expected 'std::string' (aka 'class std::basic_string<char>') 196 <font color="darkgreen">str = vec</font>; 197 <font color="blue">^ ~~~</font> 198 </pre> 199 200 <h2>Fix-it Hints</h2> 201 202 <p>"Fix-it" hints provide advice for fixing small, localized problems 203 in source code. When Clang produces a diagnostic about a particular 204 problem that it can work around (e.g., non-standard or redundant 205 syntax, missing keywords, common mistakes, etc.), it may also provide 206 specific guidance in the form of a code transformation to correct the 207 problem. In the following example, Clang warns about the use of a GCC 208 extension that has been considered obsolete since 1993. The underlined 209 code should be removed, then replaced with the code below the 210 caret line (".x =" or ".y =", respectively).</p> 211 212 <pre> 213 $ <b>clang t.c</b> 214 t.c:5:28: <font color="magenta">warning:</font> use of GNU old-style field designator extension 215 <font color="darkgreen">struct point origin = { x: 0.0, y: 0.0 };</font> 216 <font color="red">~~</font> <font color="blue">^</font> 217 <font color="darkgreen">.x = </font> 218 t.c:5:36: <font color="magenta">warning:</font> use of GNU old-style field designator extension 219 <font color="darkgreen">struct point origin = { x: 0.0, y: 0.0 };</font> 220 <font color="red">~~</font> <font color="blue">^</font> 221 <font color="darkgreen">.y = </font> 222 </pre> 223 224 <p>"Fix-it" hints are most useful for 225 working around common user errors and misconceptions. For example, C++ users 226 commonly forget the syntax for explicit specialization of class templates, 227 as in the error in the following example. Again, after describing the problem, 228 Clang provides the fix--add <code>template<></code>--as part of the 229 diagnostic.<p> 230 231 <pre> 232 $ <b>clang t.cpp</b> 233 t.cpp:9:3: <font color="red">error:</font> template specialization requires 'template<>' 234 struct iterator_traits<file_iterator> { 235 <font color="blue">^</font> 236 <font color="darkgreen">template<> </font> 237 </pre> 238 239 <h2>Automatic Macro Expansion</h2> 240 241 <p>Many errors happen in macros that are sometimes deeply nested. With 242 traditional compilers, you need to dig deep into the definition of the macro to 243 understand how you got into trouble. The following simple example shows how 244 Clang helps you out by automatically printing instantiation information and 245 nested range information for diagnostics as they are instantiated through macros 246 and also shows how some of the other pieces work in a bigger example.</p> 247 248 <pre> 249 $ <b>gcc-4.2 -fsyntax-only t.c</b> 250 t.c: In function 'test': 251 t.c:80: error: invalid operands to binary < (have 'struct mystruct' and 'float') 252 $ <b>clang -fsyntax-only t.c</b> 253 t.c:80:3: <font color="red">error:</font> invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float')) 254 <font color="darkgreen"> X = MYMAX(P, F);</font> 255 <font color="blue"> ^~~~~~~~~~~</font> 256 t.c:76:94: note: instantiated from: 257 <font color="darkgreen">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })</font> 258 <font color="blue"> ~~~ ^ ~~~</font> 259 </pre> 260 261 <p>Here's another real world warning that occurs in the "window" Unix package (which 262 implements the "wwopen" class of APIs):</p> 263 264 <pre> 265 $ <b>clang -fsyntax-only t.c</b> 266 t.c:22:2: <font color="magenta">warning:</font> type specifier missing, defaults to 'int' 267 <font color="darkgreen"> ILPAD();</font> 268 <font color="blue"> ^</font> 269 t.c:17:17: note: instantiated from: 270 <font color="darkgreen">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</font> 271 <font color="blue"> ^</font> 272 t.c:14:2: note: instantiated from: 273 <font color="darkgreen"> register i; \</font> 274 <font color="blue"> ^</font> 275 </pre> 276 277 <p>In practice, we've found that Clang's treatment of macros is actually more useful in multiply nested 278 macros that in simple ones.</p> 279 280 <h2>Quality of Implementation and Attention to Detail</h2> 281 282 <p>Finally, we have put a lot of work polishing the little things, because 283 little things add up over time and contribute to a great user experience.</p> 284 285 <p>The following example shows a trivial little tweak, where we tell you to put the semicolon at 286 the end of the line that is missing it (line 4) instead of at the beginning of 287 the following line (line 5). This is particularly important with fixit hints 288 and caret diagnostics, because otherwise you don't get the important context. 289 </p> 290 291 <pre> 292 $ <b>gcc-4.2 t.c</b> 293 t.c: In function 'foo': 294 t.c:5: error: expected ';' before '}' token 295 $ <b>clang t.c</b> 296 t.c:4:8: <font color="red">error:</font> expected ';' after expression 297 <font color="darkgreen"> bar()</font> 298 <font color="blue"> ^</font> 299 <font color="blue"> ;</font> 300 </pre> 301 302 <p>The following example shows much better error recovery than GCC. The message coming out 303 of GCC is completely useless for diagnosing the problem. Clang tries much harder 304 and produces a much more useful diagnosis of the problem.</p> 305 306 <pre> 307 $ <b>gcc-4.2 t.c</b> 308 t.c:3: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token 309 $ <b>clang t.c</b> 310 t.c:3:1: <font color="red">error:</font> unknown type name 'foo_t' 311 <font color="darkgreen">foo_t *P = 0;</font> 312 <font color="blue">^</font> 313 </pre> 314 315 <p>The following example shows that we recover from the simple case of 316 forgetting a ; after a struct definition much better than GCC.</p> 317 318 <pre> 319 $ <b>cat t.cc</b> 320 template<class T> 321 class a {} 322 class temp {}; 323 a<temp> b; 324 struct b { 325 } 326 $ <b>gcc-4.2 t.cc</b> 327 t.cc:3: error: multiple types in one declaration 328 t.cc:4: error: non-template type 'a' used as a template 329 t.cc:4: error: invalid type in declaration before ';' token 330 t.cc:6: error: expected unqualified-id at end of input 331 $ <b>clang t.cc</b> 332 t.cc:2:11: <font color="red">error:</font> expected ';' after class 333 <font color="darkgreen">class a {}</font> 334 <font color="blue"> ^</font> 335 <font color="blue"> ;</font> 336 t.cc:6:2: <font color="red">error:</font> expected ';' after struct 337 <font color="darkgreen">}</font> 338 <font color="blue"> ^</font> 339 <font color="blue"> ;</font> 340 </pre> 341 342 <p>While each of these details is minor, we feel that they all add up to provide 343 a much more polished experience.</p> 344 345 </div> 346 </body> 347 </html> 348