[v7] C, ObjC: Add -Wunterminated-string-initialization

Warn about the following:

    char  s[3] = "foo";

Initializing a char array with a string literal of the same length as
the size of the array is usually a mistake.  Rarely is the case where
one wants to create a non-terminated character sequence from a string
literal.

In some cases, for writing faster code, one may want to use arrays
instead of pointers, since that removes the need for storing an array of
pointers apart from the strings themselves.

    char  *log_levels[]   = { "info", "warning", "err" };
vs.
    char  log_levels[][7] = { "info", "warning", "err" };

This forces the programmer to specify a size, which might change if a
new entry is later added.  Having no way to enforce null termination is
very dangerous, however, so it is useful to have a warning for this, so
that the compiler can make sure that the programmer didn't make any
mistakes.  This warning catches the bug above, so that the programmer
will be able to fix it and write:

    char  log_levels[][8] = { "info", "warning", "err" };

This warning already existed as part of -Wc++-compat, but this patch
allows enabling it separately.  It is also included in -Wextra, since
it may not always be desired (when unterminated character sequences are
wanted), but it's likely to be desired in most cases.

Since Wc++-compat now includes this warning, the test has to be modified
to expect the text of the new warning too, in <gcc.dg/Wcxx-compat-14.c>.

Link: <https://lists.gnu.org/archive/html/groff/2022-11/msg00059.html>
Link: <https://lists.gnu.org/archive/html/groff/2022-11/msg00063.html>
Link: <https://inbox.sourceware.org/gcc/36da94eb-1cac-5ae8-7fea-ec66160cf413@gmail.com/T/>
Acked-by: Doug McIlroy <douglas.mcilroy@dartmouth.edu>
Cc: "G. Branden Robinson" <g.branden.robinson@gmail.com>
Cc: Ralph Corderoy <ralph@inputplus.co.uk>
Cc: Dave Kemper <saint.snit@gmail.com>
Cc: Larry McVoy <lm@mcvoy.com>
Cc: Andrew Pinski <pinskia@gmail.com>
Cc: Jonathan Wakely <jwakely.gcc@gmail.com>
Cc: Andrew Clayton <andrew@digital-domain.net>
Cc: Martin Uecker <muecker@gwdg.de>
Cc: David Malcolm <dmalcolm@redhat.com>
Cc: Mike Stump <mikestump@comcast.net>
Cc: Joseph Myers <josmyers@redhat.com>
Cc: Sandra Loosemore <sloosemore@baylibre.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---
Range-diff against v6:
1:  e8fd975bde7 ! 1:  c0f3ffcca7a C, ObjC: Add -Wunterminated-string-initialization
    @@ gcc/doc/invoke.texi: arithmetic that may yield out of bounds values. This warnin
      
     +@opindex Wunterminated-string-initialization
     +@opindex Wno-unterminated-string-initialization
    -+@item -Wunterminated-string-initialization
    ++@item -Wunterminated-string-initialization @r{(C and Objective-C only)}
     +Warn about character arrays
     +initialized as unterminated character sequences
     +with a string literal.
    @@ gcc/doc/invoke.texi: arithmetic that may yield out of bounds values. This warnin
     +char arr[3] = "foo";
     +@end smallexample
     +
    -+@option{-Wunterminated-string-initialization} is enabled by @option{-Wextra}.
    ++This warning is enabled by @option{-Wextra} and @option{-Wc++-compat}.
    ++In C++, such initializations are an error.
     +
      @opindex Warray-compare
      @opindex Wno-array-compare

 gcc/c-family/c.opt                            |  4 ++++
 gcc/c/c-typeck.cc                             |  6 +++---
 gcc/doc/invoke.texi                           | 20 ++++++++++++++++++-
 gcc/testsuite/gcc.dg/Wcxx-compat-14.c         |  2 +-
 .../Wunterminated-string-initialization.c     |  6 ++++++
 5 files changed, 33 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/Wunterminated-string-initialization.c

On 3/5/24 13:33, Alejandro Colomar wrote:
> Warn about the following:
> 
>      char  s[3] = "foo";
> 
> Initializing a char array with a string literal of the same length as
> the size of the array is usually a mistake.  Rarely is the case where
> one wants to create a non-terminated character sequence from a string
> literal.
> 
> In some cases, for writing faster code, one may want to use arrays
> instead of pointers, since that removes the need for storing an array of
> pointers apart from the strings themselves.
> 
>      char  *log_levels[]   = { "info", "warning", "err" };
> vs.
>      char  log_levels[][7] = { "info", "warning", "err" };
> 
> This forces the programmer to specify a size, which might change if a
> new entry is later added.  Having no way to enforce null termination is
> very dangerous, however, so it is useful to have a warning for this, so
> that the compiler can make sure that the programmer didn't make any
> mistakes.  This warning catches the bug above, so that the programmer
> will be able to fix it and write:
> 
>      char  log_levels[][8] = { "info", "warning", "err" };
> 
> This warning already existed as part of -Wc++-compat, but this patch
> allows enabling it separately.  It is also included in -Wextra, since
> it may not always be desired (when unterminated character sequences are
> wanted), but it's likely to be desired in most cases.
> 
> Since Wc++-compat now includes this warning, the test has to be modified
> to expect the text of the new warning too, in <gcc.dg/Wcxx-compat-14.c>.

The documentation parts of the patch are OK.

-Sandra