[PATCH v3] manual: Document the standardized scanf flag, "m". [BZ #16376]

[Rical Jasan <ricaljasan at pacific dot net>]

This commit documents the optional assignment-allocation modifier,
"m", introduced in POSIX.1-2008, whose functionality was previously
provided by the GNU extension "a".  Use of the GNU extension is
discouraged due to ambiguity when operating in any conformance mode
equivalent to C99/POSIX.1-2001, which introduced the "a", "A", and "F"
conversion specifiers.  (Consider a format string containing "%as".)
The additional conversion specifiers are also documented.

	[BZ #16376]
	* manual/stdio.texi (Input Conversion Syntax)
	(String Input Conversions, Dynamic String Input): Document the
	"m" flag.
	(Table of Input Conversions, Numeric Input Conversions):
	Document the "a", "A", and "F" conversion specifiers.
---
Changes in v3:

	* Discourage use of the "a" GNU extension.
	* Document the "a", "A", and "F" conversion specifiers.
---
 manual/stdio.texi | 41 +++++++++++++++++++++++++----------------
 1 file changed, 25 insertions(+), 16 deletions(-)

diff --git a/manual/stdio.texi b/manual/stdio.texi
index 38be236991..b00911620f 100644
--- a/manual/stdio.texi
+++ b/manual/stdio.texi
@@ -3440,9 +3440,8 @@ successful assignments.
 @cindex flag character (@code{scanf})
 
 @item
-An optional flag character @samp{a} (valid with string conversions only)
+An optional flag character @samp{m} (valid with string conversions only)
 which requests allocation of a buffer long enough to store the string in.
-(This is a GNU extension.)
 @xref{Dynamic String Input}.
 
 @item
@@ -3508,7 +3507,7 @@ Matches an unsigned integer written in decimal radix.
 Matches an unsigned integer written in hexadecimal radix.
 @xref{Numeric Input Conversions}.
 
-@item @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, @samp{%G}
+@item @samp{%a}, @samp{%e}, @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}
 Matches an optionally signed floating-point number.  @xref{Numeric Input
 Conversions}.
 
@@ -3664,7 +3663,8 @@ Specifies that the argument is a @code{size_t *}.
 This modifier was introduced in @w{ISO C99}.
 @end table
 
-All of the @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, and @samp{%G}
+All of the @samp{%a}, @samp{%e}, @samp{%f}, @samp{%g}, @samp{%A},
+@samp{%E}, @samp{%F}, and @samp{%G}
 input conversions are interchangeable.  They all match an optionally
 signed floating point number, in the same syntax as for the
 @code{strtod} function (@pxref{Parsing of Floats}).
@@ -3720,10 +3720,8 @@ provide the buffer, always specify a maximum field width to prevent
 overflow.}
 
 @item
-Ask @code{scanf} to allocate a big enough buffer, by specifying the
-@samp{a} flag character.  This is a GNU extension.  You should provide
-an argument of type @code{char **} for the buffer address to be stored
-in.  @xref{Dynamic String Input}.
+Ask @code{scanf} to allocate a large-enough buffer, by specifying the
+@samp{m} flag character.  @xref{Dynamic String Input}.
 @end itemize
 
 The @samp{%c} conversion is the simplest: it matches a fixed number of
@@ -3825,7 +3823,7 @@ is said about @samp{%ls} above is true for @samp{%l[}.
 
 One more reminder: the @samp{%s} and @samp{%[} conversions are
 @strong{dangerous} if you don't specify a maximum width or use the
-@samp{a} flag, because input too long would overflow whatever buffer you
+@samp{m} flag, because too-long input would overflow whatever buffer you
 have provided for it.  No matter how long your buffer is, a user could
 supply input that is longer.  A well-written program reports invalid
 input with a comprehensible error message, not with a crash.
@@ -3833,18 +3831,29 @@ input with a comprehensible error message, not with a crash.
 @node Dynamic String Input
 @subsection Dynamically Allocating String Conversions
 
-A GNU extension to formatted input lets you safely read a string with no
+POSIX.1-2008 specifies an @dfn{assignment-allocation character}
+@samp{m}, valid for use with the string conversion specifiers
+@samp{s}, @samp{S}, @samp{[}, @samp{c}, and @samp{C}, which
+lets you safely read a string with no
 maximum size.  Using this feature, you don't supply a buffer; instead,
 @code{scanf} allocates a buffer big enough to hold the data and gives
-you its address.  To use this feature, write @samp{a} as a flag
-character, as in @samp{%as} or @samp{%a[0-9a-z]}.
+you its address.  To use this feature, write @samp{m} as a flag
+character; e.g., @samp{%ms} or @samp{%m[0-9a-z]}.
 
 The pointer argument you supply for where to store the input should have
 type @code{char **}.  The @code{scanf} function allocates a buffer and
-stores its address in the word that the argument points to.  You should
-free the buffer with @code{free} when you no longer need it.
+stores its address in the word that the argument points to.  When
+using the @samp{l} modifier (or equivalently, @samp{S} or @samp{C}),
+the pointer argument should have the type @code{wchar_t **}.
+
+You should free the buffer with @code{free} when you no longer need it.
+
+@strong{Warning:} There was a GNU extension, @samp{a}, that predated
+@samp{m}, but its use conflicts with the @samp{a} conversion specifier
+introduced in C99 (and adopted in POSIX.1-2001), so its use is highly
+discouraged.
 
-Here is an example of using the @samp{a} flag with the @samp{%[@dots{}]}
+Here is an example of using the @samp{m} flag with the @samp{%[@dots{}]}
 conversion specification to read a ``variable assignment'' of the form
 @samp{@var{variable} = @var{value}}.
 
@@ -3852,7 +3861,7 @@ conversion specification to read a ``variable assignment'' of the form
 @{
   char *variable, *value;
 
-  if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
+  if (2 > scanf ("%m[a-zA-Z0-9] = %m[^\n]\n",
 		 &variable, &value))
     @{
       invalid_input_error ();
-- 
2.16.1