[PATCH v2] C, ObjC: Add -Wunterminated-string-initialization

[Alejandro Colomar <alx.manpages@gmail.com>]

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.

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>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---

Hi,

I sent v1 to the wrong list.  This time I've made sure to write to
gcc-patches@.

v2 adds some draft of a test, as suggested by Martin.  However, I don't
know yet how to write those, so the test is just a draft.  But I did
test the feature, by compiling GCC and compiling some small program with
it.

Cheers,
Alex


Range-diff against v1:
1:  61ddf1eb816 ! 1:  e40d8f54942 C, ObjC: Add -Wunterminated-string-initialization
    @@ Commit message
         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>
         Signed-off-by: Alejandro Colomar <alx@kernel.org>
     
      ## gcc/c-family/c.opt ##
    @@ gcc/c/c-typeck.cc: digest_init (location_t init_loc, tree type, tree init, tree
      	      if (compare_tree_int (TYPE_SIZE_UNIT (type), len) < 0)
      		{
      		  unsigned HOST_WIDE_INT size
    +
    + ## gcc/testsuite/gcc.dg/Wunterminated-string-initialization.c (new) ##
    +@@
    ++/* { dg-do compile } */
    ++/* { dg-options "-Wunterminated-string-initialization" } */
    ++
    ++char a1[] = "a";
    ++char a2[1] = "a";	/* { dg-warning "unterminated char sequence" } */
    ++char a3[2] = "a";

 gcc/c-family/c.opt                                         | 4 ++++
 gcc/c/c-typeck.cc                                          | 6 +++---
 gcc/testsuite/gcc.dg/Wunterminated-string-initialization.c | 6 ++++++
 3 files changed, 13 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/Wunterminated-string-initialization.c

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 3333cddeece..7f1fccfe02b 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -1382,6 +1382,10 @@ Wunsuffixed-float-constants
 C ObjC Var(warn_unsuffixed_float_constants) Warning
 Warn about unsuffixed float constants.
 
+Wunterminated-string-initialization
+C ObjC Var(warn_unterminated_string_initialization) Warning LangEnabledBy(C ObjC,Wextra || Wc++-compat)
+Warn about character arrays initialized as unterminated character sequences by a string literal.
+
 Wunused
 C ObjC C++ ObjC++ LangEnabledBy(C ObjC C++ ObjC++,Wall)
 ; documented in common.opt
diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index 45bacc06c47..ce2750f98bb 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -8420,11 +8420,11 @@ digest_init (location_t init_loc, tree type, tree init, tree origtype,
 		pedwarn_init (init_loc, 0,
 			      ("initializer-string for array of %qT "
 			       "is too long"), typ1);
-	      else if (warn_cxx_compat
+	      else if (warn_unterminated_string_initialization
 		       && compare_tree_int (TYPE_SIZE_UNIT (type), len) < 0)
-		warning_at (init_loc, OPT_Wc___compat,
+		warning_at (init_loc, OPT_Wunterminated_string_initialization,
 			    ("initializer-string for array of %qT "
-			     "is too long for C++"), typ1);
+			     "is too long"), typ1);
 	      if (compare_tree_int (TYPE_SIZE_UNIT (type), len) < 0)
 		{
 		  unsigned HOST_WIDE_INT size
diff --git a/gcc/testsuite/gcc.dg/Wunterminated-string-initialization.c b/gcc/testsuite/gcc.dg/Wunterminated-string-initialization.c
new file mode 100644
index 00000000000..c6517702d51
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wunterminated-string-initialization.c
@@ -0,0 +1,6 @@
+/* { dg-do compile } */
+/* { dg-options "-Wunterminated-string-initialization" } */
+
+char a1[] = "a";
+char a2[1] = "a";	/* { dg-warning "unterminated char sequence" } */
+char a3[2] = "a";
-- 
2.39.2