clang/www/diagnostics.html

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  27 <!--=======================================================================-->
  28 <h1>Expressive Diagnostics</h1>
  29 <!--=======================================================================-->
  30
  31 <p>In addition to being fast and functional, we aim to make Clang extremely user
  32 friendly.  As far as a command-line compiler goes, this basically boils down to
  33 making the diagnostics (error and warning messages) generated by the compiler
  34 be as useful as possible.  There are several ways that we do this.  This section
  35 talks about the experience provided by the command line compiler, contrasting
  36 Clang output to GCC 4.9's output in some cases.
  37 </p>
  38
  39 <h2>Column Numbers and Caret Diagnostics</h2>
  40
  41 <p>First, all diagnostics produced by clang include full column number
  42 information. The clang command-line compiler driver uses this information
  43 to print "point diagnostics".
  44 (IDEs can use the information to display in-line error markup.)
  45 This is nice because it makes it very easy to understand exactly
  46 what is wrong in a particular piece of code.</p>
  47
  48 <p>The point (the green "^" character) exactly shows where the problem is, even
  49 inside of a string.  This makes it really easy to jump to the problem and
  50 helps when multiple instances of the same character occur on a line. (We'll
  51 revisit this more in following examples.)</p>
  52
  53 <pre>
  54   $ <span class="cmd">clang -fsyntax-only format-strings.c</span>
  55   <span class="loc">format-strings.c:91:13:</span> <span class="warn">warning:</span> <span class="msg">'.*' specified field precision is missing a matching 'int' argument</span>
  56   <span class="snip" >  printf("%.*d");</span>
  57   <span class="point">            ^</span>
  58 </pre>
  59
  60 <p>Note that modern versions of GCC have followed Clang's lead, and are
  61 now able to give a column for a diagnostic, and include a snippet of source
  62 text in the result. However, Clang's column number is much more accurate,
  63 pointing at the problematic format specifier, rather than the <tt>)</tt>
  64 character the parser had reached when the problem was detected.
  65 Also, Clang's diagnostic is colored by default, making it easier to
  66 distinguish from nearby text.</p>
  67
  68 <h2>Range Highlighting for Related Text</h2>
  69
  70 <p>Clang captures and accurately tracks range information for expressions,
  71 statements, and other constructs in your program and uses this to make
  72 diagnostics highlight related information.  In the following somewhat
  73 nonsensical example you can see that you don't even need to see the original source code to
  74 understand what is wrong based on the Clang error. Because clang prints a
  75 point, you know exactly <em>which</em> plus it is complaining about.  The range
  76 information highlights the left and right side of the plus which makes it
  77 immediately obvious what the compiler is talking about.
  78 Range information is very useful for
  79 cases involving precedence issues and many other cases.</p>
  80
  81 <pre>
  82   $ <span class="cmd">gcc-4.9 -fsyntax-only t.c</span>
  83   t.c: In function 'int f(int, int)':
  84   t.c:7:39: error: invalid operands to binary + (have 'int' and 'struct A')
  85      return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);
  86                                          ^
  87   $ <span class="cmd">clang -fsyntax-only t.c</span>
  88   <span class="loc">t.c:7:39:</span> <span class="err">error:</span> <span class="msg">invalid operands to binary expression ('int' and 'struct A')</span>
  89   <span class="snip" >  return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</span>
  90   <span class="point">                       ~~~~~~~~~~~~~~ ^ ~~~~~</span>
  91 </pre>
  92
  93 <h2>Precision in Wording</h2>
  94
  95 <p>A detail is that we have tried really hard to make the diagnostics that come
  96 out of clang contain exactly the pertinent information about what is wrong and
  97 why.  In the example above, we tell you what the inferred types are for
  98 the left and right hand sides, and we don't repeat what is obvious from the
  99 point (e.g., that this is a "binary +").</p>
 100
 101 <p>Many other examples abound. In the following example, not only do we tell you
 102 that there is a problem with the <tt>*</tt>
 103 and point to it, we say exactly why and tell you what the type is (in case it is
 104 a complicated subexpression, such as a call to an overloaded function).  This
 105 sort of attention to detail makes it much easier to understand and fix problems
 106 quickly.</p>
 107
 108 <pre>
 109   $ <span class="cmd">gcc-4.9 -fsyntax-only t.c</span>
 110   t.c:5:11: error: invalid type argument of unary '*' (have 'int')
 111     return *SomeA.X;
 112            ^
 113   $ <span class="cmd">clang -fsyntax-only t.c</span>
 114   <span class="loc">t.c:5:11:</span> <span class="err">error:</span> <span class="msg">indirection requires pointer operand ('int' invalid)</span>
 115   <span class="snip" >  int y = *SomeA.X;</span>
 116   <span class="point">          ^~~~~~~~</span>
 117 </pre>
 118
 119 <h2>Typedef Preservation and Selective Unwrapping</h2>
 120
 121 <p>Many programmers use high-level user defined types, typedefs, and other
 122 syntactic sugar to refer to types in their program.  This is useful because they
 123 can abbreviate otherwise very long types and it is useful to preserve the
 124 typename in diagnostics.  However, sometimes very simple typedefs can wrap
 125 trivial types and it is important to strip off the typedef to understand what
 126 is going on.  Clang aims to handle both cases well.<p>
 127
 128 <p>The following example shows where it is important to preserve
 129 a typedef in C.</p>
 130
 131 <pre>
 132   $ <span class="cmd">clang -fsyntax-only t.c</span>
 133   <span class="loc">t.c:15:11:</span> <span class="err">error:</span> <span class="msg">can't convert between vector values of different size ('__m128' and 'int const *')</span>
 134   <span class="snip">  myvec[1]/P;</span>
 135   <span class="point">  ~~~~~~~~^~</span>
 136 </pre>
 137
 138 <p>The following example shows where it is useful for the compiler to expose
 139 underlying details of a typedef. If the user was somehow confused about how the
 140 system "pid_t" typedef is defined, Clang helpfully displays it with "aka".</p>
 141
 142 <pre>
 143   $ <span class="cmd">clang -fsyntax-only t.c</span>
 144   <span class="loc">t.c:13:9:</span> <span class="err">error:</span> <span class="msg">member reference base type 'pid_t' (aka 'int') is not a structure or union</span>
 145   <span class="snip">  myvar = myvar.x;</span>
 146   <span class="point">          ~~~~~ ^</span>
 147 </pre>
 148
 149 <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:
 150
 151 <blockquote>
 152 <pre>
 153 namespace services {
 154   struct WebService {  };
 155 }
 156 namespace myapp {
 157   namespace servers {
 158     struct Server {  };
 159   }
 160 }
 161
 162 using namespace myapp;
 163 void addHTTPService(servers::Server const &amp;server, ::services::WebService const *http) {
 164   server += http;
 165 }
 166 </pre>
 167 </blockquote>
 168
 169 <p>and then compile it, we see that Clang is both providing accurate information and is retaining the types as written by the user (e.g., "servers::Server", "::services::WebService"):
 170
 171 <pre>
 172   $ <span class="cmd">clang -fsyntax-only t.cpp</span>
 173   <span class="loc">t.cpp:9:10:</span> <span class="err">error:</span> <span class="msg">invalid operands to binary expression ('servers::Server const' and '::services::WebService const *')</span>
 174     <span class="snip">server += http;</span>
 175     <span class="point">~~~~~~ ^  ~~~~</span>
 176 </pre>
 177
 178 <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>
 179
 180 <pre>
 181   $ <span class="cmd">clang -fsyntax-only t.cpp</span>
 182   <span class="loc">t.cpp:12:7:</span> <span class="err">error:</span> <span class="msg">incompatible type assigning 'vector&lt;Real&gt;', expected 'std::string' (aka 'class std::basic_string&lt;char&gt;')</span>
 183     <span class="snip">str = vec</span>;
 184         <span class="point">^ ~~~</span>
 185 </pre>
 186
 187 <h2>Fix-it Hints</h2>
 188
 189 <p>"Fix-it" hints provide advice for fixing small, localized problems
 190 in source code. When Clang produces a diagnostic about a particular
 191 problem that it can work around (e.g., non-standard or redundant
 192 syntax, missing keywords, common mistakes, etc.), it may also provide
 193 specific guidance in the form of a code transformation to correct the
 194 problem. In the following example, Clang warns about the use of a GCC
 195 extension that has been considered obsolete since 1993. The underlined
 196 code should be removed, then replaced with the code below the
 197 point line (".x =" or ".y =", respectively).</p>
 198
 199 <pre>
 200   $ <span class="cmd">clang t.c</span>
 201   <span class="loc">t.c:5:28:</span> <span class="warn">warning:</span> <span class="msg">use of GNU old-style field designator extension</span>
 202   <span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span>
 203                           <span class="err">~~</span> <span class="msg"><span class="point">^</span></span>
 204                           <span class="snip">.x = </span>
 205   <span class="loc">t.c:5:36:</span> <span class="warn">warning:</span> <span class="msg">use of GNU old-style field designator extension</span>
 206   <span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span>
 207                                   <span class="err">~~</span> <span class="msg"><span class="point">^</span></span>
 208                                   <span class="snip">.y = </span>
 209 </pre>
 210
 211 <p>"Fix-it" hints are most useful for
 212 working around common user errors and misconceptions. For example, C++ users
 213 commonly forget the syntax for explicit specialization of class templates,
 214 as in the error in the following example. Again, after describing the problem,
 215 Clang provides the fix--add <code>template&lt;&gt;</code>--as part of the
 216 diagnostic.<p>
 217
 218 <pre>
 219   $ <span class="cmd">clang t.cpp</span>
 220   <span class="loc">t.cpp:9:3:</span> <span class="err">error:</span> <span class="msg">template specialization requires 'template&lt;&gt;'</span>
 221     struct iterator_traits&lt;file_iterator&gt; {
 222     <span class="point">^</span>
 223     <span class="snip">template&lt;&gt; </span>
 224 </pre>
 225
 226 <h2>Template Type Diffing</h2>
 227
 228 <p>Templates types can be long and difficult to read.  More so when part of an
 229 error message.  Instead of just printing out the type name, Clang has enough
 230 information to remove the common elements and highlight the differences.  To
 231 show the template structure more clearly, the templated type can also be
 232 printed as an indented text tree.</p>
 233
 234 Default: template diff with type elision
 235 <pre>
 236 <span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion from 'vector&lt;map&lt;[...], <span class="template-highlight">float</span>&gt;&gt;' to 'vector&lt;map&lt;[...], <span class="template-highlight">double</span>&gt;&gt;' for 1st argument;
 237 </pre>
 238 -fno-elide-type: template diff without elision
 239 <pre>
 240 <span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion from 'vector&lt;map&lt;int, <span class="template-highlight">float</span>&gt;&gt;' to 'vector&lt;map&lt;int, <span class="template-highlight">double</span>&gt;&gt;' for 1st argument;
 241 </pre>
 242 -fdiagnostics-show-template-tree: template tree printing with elision
 243 <pre>
 244 <span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion for 1st argument;
 245   vector&lt;
 246     map&lt;
 247       [...],
 248       [<span class="template-highlight">float</span> != <span class="template-highlight">double</span>]&gt;&gt;
 249 </pre>
 250 -fdiagnostics-show-template-tree -fno-elide-type: template tree printing with no elision
 251 <pre>
 252 <span class="loc">t.cc:4:5:</span> <span class="note">note:</span> candidate function not viable: no known conversion for 1st argument;
 253   vector&lt;
 254     map&lt;
 255       int,
 256       [<span class="template-highlight">float</span> != <span class="template-highlight">double</span>]&gt;&gt;
 257 </pre>
 258
 259 <h2>Automatic Macro Expansion</h2>
 260
 261 <p>Many errors happen in macros that are sometimes deeply nested.  With
 262 traditional compilers, you need to dig deep into the definition of the macro to
 263 understand how you got into trouble.  The following simple example shows how
 264 Clang helps you out by automatically printing instantiation information and
 265 nested range information for diagnostics as they are instantiated through macros
 266 and also shows how some of the other pieces work in a bigger example.</p>
 267
 268 <pre>
 269   $ <span class="cmd">clang -fsyntax-only t.c</span>
 270   <span class="loc">t.c:80:3:</span> <span class="err">error:</span> <span class="msg">invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float'))</span>
 271   <span class="snip">  X = MYMAX(P, F);</span>
 272   <span class="point">      ^~~~~~~~~~~</span>
 273   <span class="loc">t.c:76:94:</span> <span class="note">note:</span> expanded from:
 274   <span class="snip">#define MYMAX(A,B)    __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a &lt; __b ? __b : __a; })</span>
 275   <span class="point">                                                                                         ~~~ ^ ~~~</span>
 276 </pre>
 277
 278 <p>Here's another real world warning that occurs in the "window" Unix package (which
 279 implements the "wwopen" class of APIs):</p>
 280
 281 <pre>
 282   $ <span class="cmd">clang -fsyntax-only t.c</span>
 283   <span class="loc">t.c:22:2:</span> <span class="warn">warning:</span> <span class="msg">type specifier missing, defaults to 'int'</span>
 284   <span class="snip">        ILPAD();</span>
 285   <span class="point">        ^</span>
 286   <span class="loc">t.c:17:17:</span> <span class="note">note:</span> expanded from:
 287   <span class="snip">#define ILPAD() PAD((NROW - tt.tt_row) * 10)    /* 1 ms per char */</span>
 288   <span class="point">                ^</span>
 289   <span class="loc">t.c:14:2:</span> <span class="note">note:</span> expanded from:
 290   <span class="snip">        register i; \</span>
 291   <span class="point">        ^</span>
 292 </pre>
 293
 294 <p>In practice, we've found that Clang's treatment of macros is actually more useful in multiply nested
 295 macros than in simple ones.</p>
 296
 297 <h2>Quality of Implementation and Attention to Detail</h2>
 298
 299 <p>Finally, we have put a lot of work polishing the little things, because
 300 little things add up over time and contribute to a great user experience.</p>
 301
 302 <p>The following example shows that we recover from the simple case of
 303 forgetting a ; after a struct definition much better than GCC.</p>
 304
 305 <pre>
 306   $ <span class="cmd">cat t.cc</span>
 307   template&lt;class T&gt;
 308   class a {};
 309   struct b {}
 310   a&lt;int&gt; c;
 311   $ <span class="cmd">gcc-4.9 t.cc</span>
 312   t.cc:4:8: error: invalid declarator before 'c'
 313    a&lt;int&gt; c;
 314            ^
 315   $ <span class="cmd">clang t.cc</span>
 316   <span class="loc">t.cc:3:12:</span> <span class="err">error:</span> <span class="msg">expected ';' after struct</span>
 317   <span class="snip" >struct b {}</span>
 318   <span class="point">           ^</span>
 319   <span class="point">           ;</span>
 320 </pre>
 321
 322 <p>The following example shows that we diagnose and recover from a missing
 323 <tt>typename</tt> keyword well, even in complex circumstances where GCC
 324 cannot cope.</p>
 325
 326 <pre>
 327   $ <span class="cmd">cat t.cc</span>
 328   template&lt;class T&gt; void f(T::type) { }
 329   struct A { };
 330   void g()
 331   {
 332       A a;
 333       f&lt;A&gt;(a);
 334   }
 335   $ <span class="cmd">gcc-4.9 t.cc</span>
 336   t.cc:1:33: error: variable or field 'f' declared void
 337    template&lt;class T&gt; void f(T::type) { }
 338                                    ^
 339   t.cc: In function 'void g()':
 340   t.cc:6:5: error: 'f' was not declared in this scope
 341        f&lt;A&gt;(a);
 342        ^
 343   t.cc:6:8: error: expected primary-expression before '>' token
 344        f&lt;A&gt;(a);
 345           ^
 346   $ <span class="cmd">clang t.cc</span>
 347   <span class="loc">t.cc:1:26:</span> <span class="err">error:</span> <span class="msg">missing 'typename' prior to dependent type name 'T::type'</span>
 348   <span class="snip" >template&lt;class T&gt; void f(T::type) { }</span>
 349   <span class="point">                         ^~~~~~~</span>
 350   <span class="point">                         typename </span>
 351   <span class="loc">t.cc:6:5:</span> <span class="err">error:</span> <span class="msg">no matching function for call to 'f'</span>
 352   <span class="snip" >    f&lt;A&gt;(a);</span>
 353   <span class="point">    ^~~~</span>
 354   <span class="loc">t.cc:1:24:</span> <span class="note">note:</span> <span class="msg">candidate template ignored: substitution failure [with T = A]: no type named 'type' in 'A'</span>
 355   <span class="snip" >template&lt;class T&gt; void f(T::type) { }</span>
 356   <span class="point">                       ^    ~~~~</span>
 357 </pre>
 358
 359
 360
 361 <p>While each of these details is minor, we feel that they all add up to provide
 362 a much more polished experience.</p>
 363
 364 </div>
 365 </body>
 366 </html>