Document behavior of -Wformat-nonliteral, it is different from GCC

llvm-svn: 172362

clang/docs/LanguageExtensions.rst

@@ -1861,3 +1861,61 @@ expression is compared to the type tag.  There are two supported flags:
                                                         // was specified but buffer
                                                         // is not a null pointer
 
+Format String Checking
+======================
+
+Clang supports the ``format`` attribute, which indicates that the function
+accepts a ``printf`` or ``scanf``-like format string and corresponding
+arguments or a ``va_list`` that contains these arguments.
+
+Please see `GCC documentation about format attribute
+<http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_ to find details
+about attribute syntax.
+
+Clang implements two kinds of checks with this attribute.
+
+#. Clang checks that the function with the ``format`` attribute is called with
+   a format string that uses format specifiers that are allowed, and that
+   arguments match the format string.  This is the ``-Wformat`` warning, it is
+   on by default.
+
+#. Clang checks that the format string argument is a literal string.  This is
+   the ``-Wformat-nonliteral`` warning, it is off by default.
+
+   Clang implements this mostly the same way as GCC, but there is a difference
+   for functions that accept a ``va_list`` argument (for example, ``vprintf``).
+   GCC does not emit ``-Wformat-nonliteral`` warning for calls to such
+   fuctions.  Clang does not warn if the format string comes from a function
+   parameter, where function is annotated with a compatible attribute,
+   otherwise it warns.  For example:
+
+   .. code-block:: c
+
+     __attribute__((__format__ (__scanf__, 1, 3)))
+     void foo(const char* s, char *buf, ...) {
+       va_list ap;
+       va_start(ap, buf);
+
+       vprintf(s, ap); // warning: format string is not a string literal
+     }
+
+   In this case we warn because ``s`` contains a format string for a
+   ``scanf``-like function, but it is passed it to a ``printf``-like function.
+
+   If the attribute is removed, clang still warns, because the format string is
+   not a string literal.
+
+   But in this case Clang does not warn because the format string ``s`` and
+   corresponding arguments are annotated.  If the arguments are incorrect,
+   caller of ``foo`` will get a warning.
+
+   .. code-block: c
+
+     __attribute__((__format__ (__printf__, 1, 3)))
+     void foo(const char* s, char *buf, ...) {
+       va_list ap;
+       va_start(ap, buf);
+
+       vprintf(s, ap); // warning
+     }
+