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-ec66160cf...@gmail.com/T/> Acked-by: Doug McIlroy <douglas.mcil...@dartmouth.edu> Cc: "G. Branden Robinson" <g.branden.robin...@gmail.com> Cc: Ralph Corderoy <ra...@inputplus.co.uk> Cc: Dave Kemper <saint.s...@gmail.com> Cc: Larry McVoy <l...@mcvoy.com> Cc: Andrew Pinski <pins...@gmail.com> Cc: Jonathan Wakely <jwakely....@gmail.com> Cc: Andrew Clayton <and...@digital-domain.net> Signed-off-by: Alejandro Colomar <a...@kernel.org> --- Hi! I finally have a working patch for this warning :-) Tested with the following code: $ cat str.c int main(void) { char a[2] = "foo"; char b[3] = "bar"; char c[4] = "baz"; char d[5] = "qwe"; char log_levels[][N] = { // -DN=7 "info", "warning", "err" }; return *a + *b + *c + *d + log_levels[0][0]; } One thing which doesn't make me fully happy about this warning is that the message is a bit worse than the one in C++. See: $ /opt/local/gnu/gcc/wusi/1/bin/gcc str.c \ -Wall -Wunterminated-string-initialization -DN=8 str.c: In function ‘main’: str.c:4:21: warning: initializer-string for array of ‘char’ is too long 4 | char a[2] = "foo"; | ^~~~~ str.c:5:21: warning: initializer-string for array of ‘char’ is too long for C++ [-Wunterminated-string-initialization] 5 | char b[3] = "bar"; | ^~~~~ $ /opt/local/gnu/gcc/wusi/1/bin/g++ str.c \ -Wall -Wunterminated-string-initialization -DN=8 str.c: In function ‘int main()’: str.c:4:21: error: initializer-string for ‘char [2]’ is too long [-fpermissive] 4 | char a[2] = "foo"; | ^~~~~ str.c:5:21: error: initializer-string for ‘char [3]’ is too long [-fpermissive] 5 | char b[3] = "bar"; | ^~~~~ In C++ we see the complete type in the error message, which is more informative than "array of 'char'". This is especially relevant for multiline definitions, where the shown line may not contain the type, but only the string. However, that was already the case previously with -Wc++-compat, so a fix for that might be better as a different patch. $ /opt/local/gnu/gcc/wusi/1/bin/gcc str.c \ -Wall -Wunterminated-string-initialization -DN=7 str.c: In function ‘main’: str.c:4:21: warning: initializer-string for array of ‘char’ is too long 4 | char a[2] = "foo"; | ^~~~~ str.c:5:21: warning: initializer-string for array of ‘char’ is too long for C++ [-Wunterminated-string-initialization] 5 | char b[3] = "bar"; | ^~~~~ str.c:10:17: warning: initializer-string for array of ‘char’ is too long for C++ [-Wunterminated-string-initialization] 10 | "warning", | ^~~~~~~~~ $ /opt/local/gnu/gcc/wusi/1/bin/g++ str.c \ -Wall -Wunterminated-string-initialization -DN=7 str.c: In function ‘int main()’: str.c:4:21: error: initializer-string for ‘char [2]’ is too long [-fpermissive] 4 | char a[2] = "foo"; | ^~~~~ str.c:5:21: error: initializer-string for ‘char [3]’ is too long [-fpermissive] 5 | char b[3] = "bar"; | ^~~~~ str.c:10:17: error: initializer-string for ‘char [7]’ is too long [-fpermissive] 10 | "warning", | ^~~~~~~~~ BTW, I only tested C; not ObjC. I never in my life used Objective C, so I don't even know how relevant this is for that language. I just found that it has -Wc++-compat, and so I guessed that this warning would also trigger in that language, so I did the same as for C. I hope that's correct. Cheers, Alex gcc/c-family/c.opt | 4 ++++ gcc/c/c-typeck.cc | 6 +++--- 2 files changed, 7 insertions(+), 3 deletions(-) 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 -- 2.39.2