[Bug middle-end/78452] New: -Wformat-length=2 wrong length in %s directive with an array argument

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78452

            Bug ID: 78452
           Summary: -Wformat-length=2 wrong length in %s directive with an
                    array argument
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

When the -Wformat-length=2 option is used, in a call to sprintf with the %s
directive and an argument that refers to two or more arrays of different sizes
at least one of which is greater than the space in the destination, GCC issues
a warning as expected but indicates the wrong number of bytes is being written.
 The test case below shows the problem.  The second warning should say
something like

  ‘%-s’ directive writing up to 4 bytes into a region of size 3

$ cat x.c && /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -O2 -S
-Wall -Wextra -Wpedantic -Wformat-length=2 x.c
char d[3];

const char s3[] = "123";
const char s4[] = "1234";

void f (int i)
{
  const char *s = i < 0 ? s3 : s4;
  __builtin_sprintf (d, "%-s", s);   // warning (expected), bytes correct
}

char a3[3];
char a4[4];

void g (int i)
{
  const char *s = i < 0 ? a3 : a4;
  __builtin_sprintf (d, "%-s", s);   // warning (expected), bytes wrong
}

x.c: In function ‘f’:
x.c:9:26: warning: ‘%-s’ directive writing between 3 and 4 bytes into a region
of size 3 [-Wformat-length=]
   __builtin_sprintf (d, "%-s", s);   // warning (expected), bytes correct
                          ^~~
x.c:9:3: note: format output between 4 and 5 bytes into a destination of size 3
   __builtin_sprintf (d, "%-s", s);   // warning (expected), bytes correct
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
x.c: In function ‘g’:
x.c:18:26: warning: ‘%-s’ directive writing 1 or more bytes into a region of
size 3 [-Wformat-length=]
   __builtin_sprintf (d, "%-s", s);   // warning (expected), bytes wrong
                          ^~~
x.c:18:3: note: format output 2 bytes into a destination of size 3
   __builtin_sprintf (d, "%-s", s);   // warning (expected), bytes wrong
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~