https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101397
Bug ID: 101397 Summary: spurious warning writing to the result of stpcpy minus 1 Product: gcc Version: 11.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: msebor at gcc dot gnu.org Target Milestone: --- This is reduced from a recent Glibc build with GCC 12 which shows the warning below: In function ‘nis_local_group’, inlined from ‘nis_local_group’ at nis_local_names.c:27:1: nis_local_names.c:38:13: error: array subscript -1 is outside array bounds of ‘char[1025]’ [-Werror=array-bounds] 38 | if (cp[-1] != '.') | ~~^~~~ nis_local_names.c: In function ‘nis_local_group’: nis_local_names.c:29:15: note: at offset -1 into object ‘__nisgroup’ of size 1025 29 | static char __nisgroup[NIS_MAXNAMELEN + 1]; | ^~~~~~~~~~ The following test case shows the warning is a false positive. Since stpcpy() returns a pointer to the terminating null it appends to the destination neither of the warnings below is appropriate since there's no indication that the copied string is empty. The output below is with GCC 11.1. In GCC 12 the second -Wstringop-overflow becomes a -Warray-bounds. $ cat t.c && gcc -O2 -S -Wall t.c void f (void*); void g (const char *s) { char d[8]; char *t = __builtin_stpcpy (d, s); __builtin_strcpy (t - 1, "x"); f (d); } void h (const char *s) { char d[8]; char *t = __builtin_stpcpy (d, s); t[-1] = 0; f (d); } t.c: In function ‘g’: t.c:7:3: warning: ‘__builtin_memcpy’ writing 2 bytes into a region of size 0 overflows the destination [-Wstringop-overflow=] 7 | __builtin_strcpy (t - 1, "x"); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~ t.c:5:8: note: at offset -1 into destination object ‘d’ of size 8 5 | char d[8]; | ^ t.c: In function ‘h’: t.c:15:9: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=] 15 | t[-1] = 0; | ~~~~~~^~~ t.c:13:8: note: at offset -1 into destination object ‘d’ of size 8 13 | char d[8]; | ^