lib/vsprintf.c: update documentation

%n is no longer just ignored; it results in early return from vsnprintf. Also add a request to add test cases for future %p extensions. Signed-off-by: Rasmus Villemoes <[email protected]> Reviewed-by: Martin Kletzander <[email protected]> Reviewed-by: Andy Shevchenko <[email protected]> Cc: Jonathan Corbet <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
iegor · Nov 7, 2015 · d7ec9a0 · d7ec9a0
1 parent 317dc34
commit d7ec9a0
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diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
@@ -23,6 +23,10 @@ Example:
 
 Reminder: sizeof() result is of type size_t.
 
+The kernel's printf does not support %n. For obvious reasons, floating
+point formats (%e, %f, %g, %a) are also not recognized. Use of any
+unsupported specifier or length qualifier results in a WARN and early
+return from vsnprintf.
 
 Raw pointer value SHOULD be printed with %p. The kernel supports
 the following extended format specifiers for pointer types:
@@ -305,13 +309,9 @@ Command from struct task_struct
 
 	Passed by reference.
 
-Ignored argument:
+If you add other %p extensions, please extend lib/test_printf.c with
+one or more test cases, if at all feasible.
 
-	%n	%n
-
-	The argument passed will be ignored.  In other words, literal "%n" will
-	be in the output and the argument will be considered for next format
-	specifier.
 
 Thank you for your cooperation and attention.
 

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
@@ -1448,7 +1448,6 @@ int kptr_restrict __read_mostly;
  * - 'Cn' For a clock, it prints the name (Common Clock Framework) or address
  *        (legacy clock framework) of the clock
  * - 'Cr' For a clock, it prints the current rate of the clock
- * - 'n' For ignored argument
  *
  * ** Please update also Documentation/printk-formats.txt when making changes **
  *
@@ -1814,8 +1813,10 @@ int format_decode(const char *fmt, struct printf_spec *spec)
  * @fmt: The format string to use
  * @args: Arguments for the format string
  *
- * This function follows C99 vsnprintf, but has some extensions:
- * %n is ignored
+ * This function generally follows C99 vsnprintf, but has some
+ * extensions and a few limitations:
+ *
+ * %n is unsupported
  * %p* is handled by pointer()
  *
  * See pointer() or Documentation/printk-formats.txt for more