re PR c/16833 (-fno-builtin prevents automatic format checks for standard functions)

	PR c/16833
	* doc/extend.texi, doc/invoke.texi: Document interaction of
	-fno-builtin with format checks.  Note that built-in functions
	have effects beyond generating code that avoids calls to those
	functions.

From-SVN: r87957

gcc/ChangeLog

+2004-09-23  Joseph S. Myers  <jsm@polyomino.org.uk>
+
+	PR c/16833
+	* doc/extend.texi, doc/invoke.texi: Document interaction of
+	-fno-builtin with format checks.  Note that built-in functions
+	have effects beyond generating code that avoids calls to those
+	functions.
+
 2004-09-23  Frank Ch. Eigler <fche@redhat.com>
 
 	PR tree-optimization/17533

gcc/doc/extend.texi

@@ -1764,10 +1764,11 @@ start with the third argument, so the correct parameters for the format
 attribute are 2 and 3.
 
 @opindex ffreestanding
+@opindex fno-builtin
 The @code{format} attribute allows you to identify your own functions
 which take format strings as arguments, so that GCC can check the
 calls to these functions for errors.  The compiler always (unless
-@option{-ffreestanding} is used) checks formats
+@option{-ffreestanding} or @option{-fno-builtin} is used) checks formats
 for the standard library functions @code{printf}, @code{fprintf},
 @code{sprintf}, @code{scanf}, @code{fscanf}, @code{sscanf}, @code{strftime},
 @code{vprintf}, @code{vfprintf} and @code{vsprintf} whenever such
@@ -1824,7 +1825,8 @@ type function whose operands are a call to one of your own function.
 The compiler always treats @code{gettext}, @code{dgettext}, and
 @code{dcgettext} in this manner except when strict ISO C support is
 requested by @option{-ansi} or an appropriate @option{-std} option, or
-@option{-ffreestanding} is used.  @xref{C Dialect Options,,Options
+@option{-ffreestanding} or @option{-fno-builtin}
+is used.  @xref{C Dialect Options,,Options
 Controlling C Dialect}.
 
 @item function_vector

gcc/doc/invoke.texi

@@ -1173,7 +1173,14 @@ instructions that adjust the stack directly, and calls to @code{memcpy}
 may become inline copy loops.  The resulting code is often both smaller
 and faster, but since the function calls no longer appear as such, you
 cannot set a breakpoint on those calls, nor can you change the behavior
-of the functions by linking with a different library.
+of the functions by linking with a different library.  In addition,
+when a function is recognized as a built-in function, GCC may use
+information about that function to warn about problems with calls to
+that function, or to generate more efficient code, even if the
+resulting code still contains calls to that function.  For example,
+warnings are given with @option{-Wformat} for bad calls to
+@code{printf}, when @code{printf} is built in, and @code{strlen} is
+known not to modify global memory.
 
 With the @option{-fno-builtin-@var{function}} option
 only the built-in function @var{function} is
@@ -2161,6 +2168,8 @@ messages.
 
 @item -Wformat
 @opindex Wformat
+@opindex ffreestanding
+@opindex fno-builtin
 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
 the arguments supplied have types appropriate to the format string
 specified, and that the conversions specified in the format string make
@@ -2168,6 +2177,10 @@ sense.  This includes standard functions, and others specified by format
 attributes (@pxref{Function Attributes}), in the @code{printf},
 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
 not in the C standard) families (or other target-specific families).
+Which functions are checked without format attributes having been
+specified depends on the standard version selected, and such checks of
+functions without the attribute specified are disabled by
+@option{-ffreestanding} or @option{-fno-builtin}.
 
 The formats are checked against the format features supported by GNU
 libc version 2.2.  These include all ISO C90 and C99 features, as well