Clarify that OpenBSD ignores the dangerous category LC_NUMERIC,

and explain best practice for portable programs below CAVEATS.
Triggered by a smaller diff from Jan Stary <hans at stare dot cz>.
Emphatic OKs from cheloha@ and tedu@.

1 files changed, 30 insertions, 16 deletions

diff --git a/lib/libc/stdio/wprintf.3 b/lib/libc/stdio/wprintf.3
index 98711c21cfb..2914716e692 100644
--- a/lib/libc/stdio/wprintf.3
+++ b/lib/libc/stdio/wprintf.3
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: wprintf.3,v 1.5 2017/12/01 10:56:07 schwarze Exp $
+.\"	$OpenBSD: wprintf.3,v 1.6 2019/01/11 09:53:26 schwarze Exp $
 .\"
 .\" Copyright (c) 1990, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -33,7 +33,7 @@
 .\"
 .\"     @(#)printf.3	8.1 (Berkeley) 6/4/93
 .\"
-.Dd $Mdocdate: December 1 2017 $
+.Dd $Mdocdate: January 11 2019 $
 .Dt WPRINTF 3
 .Os
 .Sh NAME
@@ -225,17 +225,17 @@ A
 .Cm +
 overrides a space if both are used.
 .It Sq Cm '
-Decimal conversions
-.Cm ( d , u ,
+On
+.Ox ,
+this flag has no effect.
+On other systems, it may cause the insertion of
+.Xr locale 1 Ns -dependent
+thousands separator characters into the integral parts of arguments
+of the
+.Cm d , i , u , f ,
 or
-.Cm i )
-or the integral portion of a floating point conversion
-.Cm ( f
-or
-.Cm F )
-should be grouped and separated by thousands using
-the non-monetary separator returned by
-.Xr localeconv 3 .
+.Cm F
+conversions.
 .El
 .It
 An optional decimal digit string specifying a minimum field width.
@@ -578,10 +578,6 @@ is
 .Ql %% .
 .El
 .Pp
-The decimal point
-character is defined in the program's locale (category
-.Dv LC_NUMERIC ) .
-.Pp
 In no case does a non-existent or small field width cause truncation of
 a numeric field; if the result of a conversion is wider than the field
 width, the
@@ -605,3 +601,21 @@ and
 functions
 conform to
 .St -isoC-99 .
+.Sh CAVEATS
+On systems other than
+.Ox ,
+the
+.Dv LC_NUMERIC
+.Xr locale 1
+category can cause erratic output, for example different characters
+substituted for the decimal point or thousands separator characters
+inserted into numbers.
+For security reasons, make sure that portable programs calling these
+functions never call
+.Xr setlocale 3
+with the
+.Dv LC_ALL
+or
+.Dv LC_NUMERIC
+categories but only use
+.Dv LC_CTYPE .