<p>In addition to being fast and functional, we aim to make Clang extremely user
friendly. As far as a command-line compiler goes, this basically boils down to
making the diagnostics (error and warning messages) generated by the compiler
-be as useful as possible. There are several ways that we do this. This section
-talks about the experience provided by the command line compiler, contrasting
-Clang output to GCC 4.2's output in several examples.
-<!--
-Other clients
-that embed Clang and extract equivalent information through internal APIs.-->
-</p>
-
-<h4>Column Numbers and Caret Diagnostics</h4>
-
-<p>First, all diagnostics produced by clang include full column number
-information, and use this to print "caret diagnostics". This is a feature
-provided by many commercial compilers, but is generally missing from open source
-compilers. This is nice because it makes it very easy to understand exactly
-what is wrong in a particular piece of code, an example is:</p>
-
-<pre>
- $ <b>gcc-4.2 -fsyntax-only -Wformat format-strings.c</b>
- format-strings.c:91: warning: too few arguments for format
- $ <b>clang -fsyntax-only format-strings.c</b>
- format-strings.c:91:13: warning: '.*' specified field precision is missing a matching 'int' argument
- <font color="darkgreen"> printf("%.*d");</font>
- <font color="blue"> ^</font>
-</pre>
-
-<p>The caret (the blue "^" character) exactly shows where the problem is, even
-inside of the string. This makes it really easy to jump to the problem and
-helps when multiple instances of the same character occur on a line. We'll
-revisit this more in following examples.</p>
+be as useful as possible. There are several ways that we do this, but it
+basically boils down to pinpointing exactly what is wrong in the program,
+highlighting related information so that it is easy to understand at a glance,
+and making the wording as clear as possible.</p>
-<h4>Range Highlighting for Related Text</h4>
-
-<p>Clang captures and accurately tracks range information for expressions,
-statements, and other constructs in your program and uses this to make
-diagnostics highlight related information. For example, here's a somewhat
-nonsensical example to illustrate this:</p>
+<p>Here is one simple example that illustrates the difference between a typical
+GCC and Clang diagnostic:</p>
<pre>
$ <b>gcc-4.2 -fsyntax-only t.c</b>
immediately obvious what the compiler is talking about, which is very useful for
cases involving precedence issues and many other cases.</p>
-<h4>Precision in Wording</h4>
-
-<p>A detail is that we have tried really hard to make the diagnostics that come
-out of clang contain exactly the pertinent information about what is wrong and
-why. In the example above, we tell you what the inferred types are for
-the left and right hand sides, and we don't repeat what is obvious from the
-caret (that this is a "binary +"). Many other examples abound, here is a simple
-one:</p>
-
-<pre>
- $ <b>gcc-4.2 -fsyntax-only t.c</b>
- t.c:5: error: invalid type argument of 'unary *'
- $ <b>clang -fsyntax-only t.c</b>
- t.c:5:11: error: indirection requires pointer operand ('int' invalid)
- <font color="darkgreen"> int y = *SomeA.X;</font>
- <font color="blue"> ^~~~~~~~</font>
-</pre>
-
-<p>In this example, not only do we tell you that there is a problem with the *
-and point to it, we say exactly why and tell you what the type is (in case it is
-a complicated subexpression, such as a call to an overloaded function). This
-sort of attention to detail makes it much easier to understand and fix problems
-quickly.</p>
-
-<h4>No Pretty Printing of Expressions in Diagnostics</h4>
-
-<p>Since Clang has range highlighting, it never needs to pretty print your code
-back out to you. This is particularly bad in G++ (which often emits errors
-containing lowered vtable references), but even GCC can produce
-inscrutible error messages in some cases when it tries to do this. In this
-example P and Q have type "int*":</p>
-
-<pre>
- $ <b>gcc-4.2 -fsyntax-only t.c</b>
- #'exact_div_expr' not supported by pp_c_expression#'t.c:12: error: called object is not a function
- $ <b>clang -fsyntax-only t.c</b>
- t.c:12:8: error: called object type 'int' is not a function or function pointer
- <font color="darkgreen"> (P-Q)();</font>
- <font color="blue"> ~~~~~^</font>
-</pre>
-
-
-<h4>Typedef Preservation and Selective Unwrapping</h4>
-
-<p>Many programmers use high-level user defined types, typedefs, and other
-syntactic sugar to refer to types in their program. This is useful because they
-can abbreviate otherwise very long types and it is useful to preserve the
-typename in diagnostics. However, sometimes very simple typedefs can wrap
-trivial types and it is important to strip off the typedef to understand what
-is going on. Clang aims to handle both cases well.<p>
-
-<p>For example, here is an example that shows where it is important to preserve
-a typedef in C:</p>
-
-<pre>
- $ <b>gcc-4.2 -fsyntax-only t.c</b>
- t.c:15: error: invalid operands to binary / (have 'float __vector__' and 'const int *')
- $ <b>clang -fsyntax-only t.c</b>
- t.c:15:11: error: can't convert between vector values of different size ('__m128' and 'int const *')
- <font color="darkgreen"> myvec[1]/P;</font>
- <font color="blue"> ~~~~~~~~^~</font>
-</pre>
-
-<p>Here the type printed by GCC isn't even valid, but if the error were about a
-very long and complicated type (as often happens in C++) the error message would
-be ugly just because it was long and hard to read. Here's an example where it
-is useful for the compiler to expose underlying details of a typedef:</p>
-
-<pre>
- $ <b>gcc-4.2 -fsyntax-only t.c</b>
- t.c:13: error: request for member 'x' in something not a structure or union
- $ <b>clang -fsyntax-only t.c</b>
- t.c:13:9: error: member reference base type 'pid_t' (aka 'int') is not a structure or union
- <font color="darkgreen"> myvar = myvar.x;</font>
- <font color="blue"> ~~~~~ ^</font>
-</pre>
-
-<p>If the user was somehow confused about how the system "pid_t" typedef is
-defined, Clang helpfully displays it with "aka".</p>
-
-<h4>Automatic Macro Expansion</h4>
-
-<p>Many errors happen in macros that are sometimes deeply nested. With
-traditional compilers, you need to dig deep into the definition of the macro to
-understand how you got into trouble. Here's a simple example that shows how
-Clang helps you out:</p>
-
-<pre>
- $ <b>gcc-4.2 -fsyntax-only t.c</b>
- t.c: In function 'test':
- t.c:80: error: invalid operands to binary < (have 'struct mystruct' and 'float')
- $ <b>clang -fsyntax-only t.c</b>
- t.c:80:3: error: invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float'))
- <font color="darkgreen"> X = MYMAX(P, F);</font>
- <font color="blue"> ^~~~~~~~~~~</font>
- t.c:76:94: note: instantiated from:
- <font color="darkgreen">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })</font>
- <font color="blue"> ~~~ ^ ~~~</font>
-</pre>
-
-<p>This shows how clang automatically prints instantiation information and
-nested range information for diagnostics as they are instantiated through macros
-and also shows how some of the other pieces work in a bigger example. Here's
-another real world warning that occurs in the "window" Unix package (which
-implements the "wwopen" class of APIs):</p>
-
-<pre>
- $ <b>clang -fsyntax-only t.c</b>
- t.c:22:2: warning: type specifier missing, defaults to 'int'
- <font color="darkgreen"> ILPAD();</font>
- <font color="blue"> ^</font>
- t.c:17:17: note: instantiated from:
- <font color="darkgreen">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</font>
- <font color="blue"> ^</font>
- t.c:14:2: note: instantiated from:
- <font color="darkgreen"> register i; \</font>
- <font color="blue"> ^</font>
-</pre>
-
-<p>In practice, we've found that this is actually more useful in multiply nested
-macros that in simple ones.</p>
-
-<h4>Fix-it Hints</h4>
-
-<p>simple example + template<> example</p>
-
-<h4>C++ Fun Examples</h4>
-
-<p>...</p>
+<p>Clang diagnostics are very right and have many features. For more
+information and examples, please see the <a href="diagnostics.html">Expressive
+Diagnostics</a> page.</p>
<!--=======================================================================-->
<h3><a name="gcccompat">GCC Compatibility</a></h3>
projects / clang / commitdiff