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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
37information, and use this to print "caret diagnostics". This is a feature
38provided by many commercial compilers, but is generally missing from open source
39compilers. This is nice because it makes it very easy to understand exactly
40what is wrong in a particular piece of code, an example is:</p>
41
42<pre>
43 $ <b>gcc-4.2 -fsyntax-only -Wformat format-strings.c</b>
44 format-strings.c:91: warning: too few arguments for format
45 $ <b>clang -fsyntax-only format-strings.c</b>
46 format-strings.c:91:13: warning: '.*' specified field precision is missing a matching 'int' argument
47 <font color="darkgreen"> printf("%.*d");</font>
48 <font color="blue"> ^</font>
49</pre>
50
51<p>The caret (the blue "^" character) exactly shows where the problem is, even
52inside of the string. This makes it really easy to jump to the problem and
53helps when multiple instances of the same character occur on a line. We'll
54revisit this more in following examples.</p>
55
56<h2>Range Highlighting for Related Text</h2>
57
58<p>Clang captures and accurately tracks range information for expressions,
59statements, and other constructs in your program and uses this to make
60diagnostics highlight related information. For example, here's a somewhat
61nonsensical example to illustrate this:</p>
62
63<pre>
64 $ <b>gcc-4.2 -fsyntax-only t.c</b>
65 t.c:7: error: invalid operands to binary + (have 'int' and 'struct A')
66 $ <b>clang -fsyntax-only t.c</b>
67 t.c:7:39: error: invalid operands to binary expression ('int' and 'struct A')
68 <font color="darkgreen"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</font>
69 <font color="blue"> ~~~~~~~~~~~~~~ ^ ~~~~~</font>
70</pre>
71
72<p>Here you can see that you don't even need to see the original source code to
73understand what is wrong based on the Clang error: Because clang prints a
74caret, you know exactly <em>which</em> plus it is complaining about. The range
75information highlights the left and right side of the plus which makes it
76immediately obvious what the compiler is talking about, which is very useful for
77cases involving precedence issues and many other cases.</p>
78
79<h2>Precision in Wording</h2>
80
81<p>A detail is that we have tried really hard to make the diagnostics that come
82out of clang contain exactly the pertinent information about what is wrong and
83why. In the example above, we tell you what the inferred types are for
84the left and right hand sides, and we don't repeat what is obvious from the
85caret (that this is a "binary +"). Many other examples abound, here is a simple
86one:</p>
87
88<pre>
89 $ <b>gcc-4.2 -fsyntax-only t.c</b>
90 t.c:5: error: invalid type argument of 'unary *'
91 $ <b>clang -fsyntax-only t.c</b>
92 t.c:5:11: error: indirection requires pointer operand ('int' invalid)
93 <font color="darkgreen"> int y = *SomeA.X;</font>
94 <font color="blue"> ^~~~~~~~</font>
95</pre>
96
97<p>In this example, not only do we tell you that there is a problem with the *
98and point to it, we say exactly why and tell you what the type is (in case it is
99a complicated subexpression, such as a call to an overloaded function). This
100sort of attention to detail makes it much easier to understand and fix problems
101quickly.</p>
102
103<h2>No Pretty Printing of Expressions in Diagnostics</h2>
104
105<p>Since Clang has range highlighting, it never needs to pretty print your code
106back out to you. This is particularly bad in G++ (which often emits errors
107containing lowered vtable references), but even GCC can produce
108inscrutible error messages in some cases when it tries to do this. In this
109example P and Q have type "int*":</p>
110
111<pre>
112 $ <b>gcc-4.2 -fsyntax-only t.c</b>
113 #'exact_div_expr' not supported by pp_c_expression#'t.c:12: error: called object is not a function
114 $ <b>clang -fsyntax-only t.c</b>
115 t.c:12:8: error: called object type 'int' is not a function or function pointer
116 <font color="darkgreen"> (P-Q)();</font>
117 <font color="blue"> ~~~~~^</font>
118</pre>
119
120
121<h2>Typedef Preservation and Selective Unwrapping</h2>
122
123<p>Many programmers use high-level user defined types, typedefs, and other
124syntactic sugar to refer to types in their program. This is useful because they
125can abbreviate otherwise very long types and it is useful to preserve the
126typename in diagnostics. However, sometimes very simple typedefs can wrap
127trivial types and it is important to strip off the typedef to understand what
128is going on. Clang aims to handle both cases well.<p>
129
130<p>For example, here is an example that shows where it is important to preserve
131a typedef in C:</p>
132
133<pre>
134 $ <b>gcc-4.2 -fsyntax-only t.c</b>
135 t.c:15: error: invalid operands to binary / (have 'float __vector__' and 'const int *')
136 $ <b>clang -fsyntax-only t.c</b>
137 t.c:15:11: error: can't convert between vector values of different size ('__m128' and 'int const *')
138 <font color="darkgreen"> myvec[1]/P;</font>
139 <font color="blue"> ~~~~~~~~^~</font>
140</pre>
141
142<p>Here the type printed by GCC isn't even valid, but if the error were about a
143very long and complicated type (as often happens in C++) the error message would
144be ugly just because it was long and hard to read. Here's an example where it
145is useful for the compiler to expose underlying details of a typedef:</p>
146
147<pre>
148 $ <b>gcc-4.2 -fsyntax-only t.c</b>
149 t.c:13: error: request for member 'x' in something not a structure or union
150 $ <b>clang -fsyntax-only t.c</b>
151 t.c:13:9: error: member reference base type 'pid_t' (aka 'int') is not a structure or union
152 <font color="darkgreen"> myvar = myvar.x;</font>
153 <font color="blue"> ~~~~~ ^</font>
154</pre>
155
156<p>If the user was somehow confused about how the system "pid_t" typedef is
157defined, Clang helpfully displays it with "aka".</p>
158
159<h2>Fix-it Hints</h2>
160
161<p>simple example + template&lt;&gt; example</p>
162
163
164<h2>Automatic Macro Expansion</h2>
165
166<p>Many errors happen in macros that are sometimes deeply nested. With
167traditional compilers, you need to dig deep into the definition of the macro to
168understand how you got into trouble. Here's a simple example that shows how
169Clang helps you out:</p>
170
171<pre>
172 $ <b>gcc-4.2 -fsyntax-only t.c</b>
173 t.c: In function 'test':
174 t.c:80: error: invalid operands to binary &lt; (have 'struct mystruct' and 'float')
175 $ <b>clang -fsyntax-only t.c</b>
176 t.c:80:3: error: invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float'))
177 <font color="darkgreen"> X = MYMAX(P, F);</font>
178 <font color="blue"> ^~~~~~~~~~~</font>
179 t.c:76:94: note: instantiated from:
180 <font color="darkgreen">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a &lt; __b ? __b : __a; })</font>
181 <font color="blue"> ~~~ ^ ~~~</font>
182</pre>
183
184<p>This shows how clang automatically prints instantiation information and
185nested range information for diagnostics as they are instantiated through macros
186and also shows how some of the other pieces work in a bigger example. Here's
187another real world warning that occurs in the "window" Unix package (which
188implements the "wwopen" class of APIs):</p>
189
190<pre>
191 $ <b>clang -fsyntax-only t.c</b>
192 t.c:22:2: warning: type specifier missing, defaults to 'int'
193 <font color="darkgreen"> ILPAD();</font>
194 <font color="blue"> ^</font>
195 t.c:17:17: note: instantiated from:
196 <font color="darkgreen">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</font>
197 <font color="blue"> ^</font>
198 t.c:14:2: note: instantiated from:
199 <font color="darkgreen"> register i; \</font>
200 <font color="blue"> ^</font>
201</pre>
202
203<p>In practice, we've found that this is actually more useful in multiply nested
204macros that in simple ones.</p>
205
206<h2>C++ Fun Examples</h2>
207
208<p>...</p>
209
210
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