From cc32f967605178358d7b0d3e6fc1d36d036e6a8f Mon Sep 17 00:00:00 2001
From: Rical Jasan <ricaljasan@pacific.net>
Date: Tue, 6 Feb 2018 21:22:27 -0800
Subject: [PATCH] manual: Document the standardized scanf flag, "m". [BZ
#16376]
POSIX.1-2008 introduced the optional assignment-allocation modifier,
"m", whose functionality was previously provided by the GNU extension
"a" (and still is).
[BZ #16376]
* manual/stdio.texi (Input Conversion Syntax)
(String Input Conversions, Dynamic String Input): Document the
"m" flag.
---
manual/stdio.texi | 34 +++++++++++++++++++++-------------
1 file changed, 21 insertions(+), 13 deletions(-)
@@ -3440,10 +3440,9 @@ 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}.
+(There is also a GNU extension, @samp{a}.) @xref{Dynamic String Input}.
@item
An optional decimal integer that specifies the @dfn{maximum field
@@ -3720,8 +3719,9 @@ 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
+Ask @code{scanf} to allocate a big-enough buffer, by specifying the
+@samp{m} flag character (or the GNU extension, @samp{a}).
+You should provide
an argument of type @code{char **} for the buffer address to be stored
in. @xref{Dynamic String Input}.
@end itemize
@@ -3825,7 +3825,7 @@ 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} or @samp{a} flags, 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 +3833,26 @@ 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.
+
+As a GNU extension, @samp{a} is available in place of @samp{m}.
-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 +3860,7 @@ 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 ();
--
2.14.3