-Wformat= in make step at v4l2loopback-ctl compilation

[RobsonLana]

Sometimes, when I recompile (after a clean on the modules), the v4l2loopback-ctl fails at compilation; I had a lucky time when it not happened, but I am stuck again on this:

v4l2loopback-ctl.c:441:27: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘struct v4l2_loopback_config *’ [-Wformat=]
  441 |   printf("configuration: %s\n", cfg);
      |                          ~^     ~~~
      |                           |     |
      |                           |     struct v4l2_loopback_config *
      |                           char *
v4l2loopback-ctl.c: In function ‘parse_caps’:
v4l2loopback-ctl.c:660:24: warning: format ‘%c’ expects argument of type ‘char *’, but argument 3 has type ‘char (*)[5]’ [-Wformat=]
  660 |  if (sscanf(buffer, "%4c:%dx%d@%d/%d", &fourcc, &caps->width,
      |                      ~~^               ~~~~~~~
      |                        |               |
      |                        char *          char (*)[5]

I tried a few changes, but I got nothing; how could I correct this in the source file I have in my computer?

[umlaeute]

these are warnings, and as such they are not fatal to the build process.
unless you (or something else; but nothing on the side of this repository) marks warnings as errors with -Werror.

i'm closing this, as not actable upon.
if you can come up with actual errors, please don't hesitate to add them to this ticket and I will consider re-opening it.

[BenBE]

Looking at

v4l2loopback/utils/v4l2loopback-ctl.c

Lines 437 to 444 in f62fb90

	static void print_conf(struct v4l2_loopback_config *cfg)
	{
	MARK();
	if (!cfg) {
	printf("configuration: %s\n", cfg);
	return;
	}
	MARK();

I see a straight forward type confusion. You are reading memory straight from a structure, when you'd actually want to format the structure's content into something readable for the user. Oh, and on second glance I also see a NULL dereference, as cfg is guaranteed NULL by the line just before the printf.

For the second warning you'd probably want to use %4s instead of the %4c the compiler complains about. Also the &fourcc should likely just be fourcc.

Please take warnings seriously.

[umlaeute]

Oh, and on second glance I also see a NULL dereference

ah well, i figure that "%s", 0 would print as (null). but of course this nevertheless should read %p instead of %s.

For the second warning you'd probably want to use %4s instead of the %4c

most certainly not. the FOURCC string can contain (trailing) spaces (e.g. V4L2_PIX_FMT_Y16)

Also the &fourcc should likely just be fourcc.

yes.

[RobsonLana]

the %p for cfg at line 441 and fourcc instead of &fourcc at line 660 worked for compiling the code. Thank you.