[Bug middle-end/93665] New: missing warning on strncmp reading past unterminated array

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93665

            Bug ID: 93665
           Summary: missing warning on strncmp reading past unterminated
                    array
           Product: gcc
           Version: 10.0
            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: ---

GCC 10 diagnoses the call to strcmp below because it attempts to read past the
end of the unterminated array a3 but not the call to strncmp even though it has
the same bug.

$ cat z.c && gcc -O2 -S -Wall -Wextra -Wpedantic z.c
const char a3[] = { 'a', 'b', 'c' };
const char s4[] = "abcd";

int f (void)
{
  return __builtin_strcmp (a3, s4);   // warning (good)
}

int g (void)
{
  return __builtin_strncmp (a3, s4, sizeof s4);   // missing warning
}
z.c: In function ‘f’:
z.c:6:10: warning: ‘__builtin_strcmp’ argument missing terminating nul
[-Wstringop-overflow=]
    6 |   return __builtin_strcmp (a3, s4);   // warning (good)
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~
z.c:1:12: note: referenced argument declared here
    1 | const char a3[] = { 'a', 'b', 'c' };
      |            ^~