GNU bug report logs - #57211
29.0.50; generate-new-buffer-name sprintf format overflow warning

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: "Basil L. Contovounesios" <contovob <at> tcd.ie>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.0.50; generate-new-buffer-name sprintf format overflow warning
Date: Sun, 14 Aug 2022 19:50:03 +0300

Severity: minor

Compiling with gcc (Debian 12.1.0-7) 12.1.0 and -Og, I get the following
-Wformat-overflow warning:

In file included from buffer.c:33:
buffer.c: In function ‘Fgenerate_new_buffer_name’:
buffer.c:1167:46: warning: ‘sprintf’ may write a terminating nul past the end of the destination [-Wformat-overflow=]
 1167 |       AUTO_STRING_WITH_LEN (lnumber, number, sprintf (number, "-%d", i));
      |                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~
lisp.h:5493:36: note: in definition of macro ‘AUTO_STRING_WITH_LEN’
 5493 |         ((&(struct Lisp_String) {{{len, -1, 0, (unsigned char *) (str)}}}), \
      |                                    ^~~
buffer.c:1167:46: note: ‘sprintf’ output between 3 and 9 bytes into a destination of size 8
 1167 |       AUTO_STRING_WITH_LEN (lnumber, number, sprintf (number, "-%d", i));
      |                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~
lisp.h:5493:36: note: in definition of macro ‘AUTO_STRING_WITH_LEN’
 5493 |         ((&(struct Lisp_String) {{{len, -1, 0, (unsigned char *) (str)}}}), \
      |                                    ^~~

Can the upper bound 9 ever be achieved?  If so, how?  If not, is this a
GCC bug?  Either way, is there a way to pacify the warning?

I tried

  snprintf (number, sizeof number, ...)

but got the same warning.

BTW, in the preceding

  int i = r % 1000000;

can the result of % ever exceed INT_MAX?  And do we care either way?

Thanks,

-- 
Basil

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 1.16.0, Xaw3d scroll bars)
 of 2022-08-14 built on tia
Repository revision: 1d3fe256907d5e78a4acedd194e55db8ab952952
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
System Description: Debian GNU/Linux bookworm/sid

Configured using:
 'configure CC=gcc-12 'CFLAGS=-Og -ggdb3' --config-cache
 --prefix=/home/blc/.local --enable-checking=structs
 --with-file-notification=yes --with-x-toolkit=lucid --with-x'

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  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

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