bug#32189: 27.0.50; GCC 7 warning due to -Wformat-truncation=2

[Ken Brown <kbrown@cornell.edu>]

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On 7/20/2018 10:17 AM, Eli Zaretskii wrote:
>> Cc: p.stephani2@gmail.com, 32189@debbugs.gnu.org
>> From: Ken Brown <kbrown@cornell.edu>
>> Date: Fri, 20 Jul 2018 09:49:39 -0400
>>
>> ../../master/src/w32cygwinx.c: In function ‘format_string’:
>> ../../master/src/w32cygwinx.c:32:3: warning: function ‘format_string’
>> might be a candidate for ‘gnu_printf’ format attribute
>> [-Wsuggest-attribute=format]
>>      Lisp_Object str = vformat_string (format, args);
>>      ^~~~~~~~~~~
> 
> We do this with vformat_string:
> 
>    extern Lisp_Object vformat_string (const char *, va_list)
>      ATTRIBUTE_FORMAT_PRINTF (1, 0);
> 
> So I guess this new function needs a similar treatment.

Amended patch attached.

Ken

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From 47bc76b73456c6b9ad02bb02123f2b1285434192 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Thu, 19 Jul 2018 16:12:17 -0700
Subject: [PATCH] Simplify w32cygwinx.c and pacify GCC (Bug#32189)

* src/w32cygwinx.c (format_string): New function.
(Fw32_battery_status): Use it.
---
 src/w32cygwinx.c | 37 ++++++++++++++++---------------------
 1 file changed, 16 insertions(+), 21 deletions(-)

diff --git a/src/w32cygwinx.c b/src/w32cygwinx.c
index 8d3ae164cf..5d48c3a9e1 100644
--- a/src/w32cygwinx.c
+++ b/src/w32cygwinx.c
@@ -24,6 +24,16 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include "lisp.h"
 #include "w32common.h"
 
+static Lisp_Object ATTRIBUTE_FORMAT_PRINTF (1, 2)
+format_string (char const *format, ...)
+{
+  va_list args;
+  va_start (args, format);
+  Lisp_Object str = vformat_string (format, args);
+  va_end (args);
+  return str;
+}
+
 DEFUN ("w32-battery-status", Fw32_battery_status, Sw32_battery_status, 0, 0, 0,
        doc: /* Get power status information from Windows system.
 
@@ -92,32 +102,17 @@ The following %-sequences are provided:
       if (system_status.BatteryLifePercent > 100)
 	load_percentage = build_string ("N/A");
       else
-	{
-	  char buffer[16];
-	  snprintf (buffer, 16, "%d", system_status.BatteryLifePercent);
-	  load_percentage = build_string (buffer);
-	}
+	load_percentage = format_string ("%d", system_status.BatteryLifePercent);
 
       if (seconds_left < 0)
 	seconds = minutes = hours = remain = build_string ("N/A");
       else
 	{
-	  long m;
-	  double h;
-	  char buffer[16];
-	  snprintf (buffer, 16, "%ld", seconds_left);
-	  seconds = build_string (buffer);
-
-	  m = seconds_left / 60;
-	  snprintf (buffer, 16, "%ld", m);
-	  minutes = build_string (buffer);
-
-	  h = seconds_left / 3600.0;
-	  snprintf (buffer, 16, "%3.1f", h);
-	  hours = build_string (buffer);
-
-	  snprintf (buffer, 16, "%ld:%02ld", m / 60, m % 60);
-	  remain = build_string (buffer);
+	  long m = seconds_left / 60;
+	  seconds = format_string ("%ld", seconds_left);
+	  minutes = format_string ("%ld", m);
+	  hours = format_string ("%3.1f", seconds_left / 3600.0);
+	  remain = format_string ("%ld:%02ld", m / 60, m % 60);
 	}
 
       status = listn (CONSTYPE_HEAP, 8,
-- 
2.17.0