[PATCH v2 1/3] blkpr: prepare for _IOR() ioctls

[Stefan Hajnoczi <stefanha@redhat.com>]

parse_pr_command() returns the ioctl command constant for a given
command or -1 when the command is unknown. Up until now all ioctl
command constants were positive, so the following check worked:

  if (command < 0)
      err(EXIT_FAILURE, _("unknown command"));

The top two bits of ioctl command constants encode the direction (_IO,
_IOR, _IOW, _IOWR). ioctl commands defined with _IOR have negative ioctl
command constants.

Explicitly check for -1 to differentiate "unknown command" from valid
ioctls commands. Later commits will add ioctl commands that use _IOR.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 sys-utils/blkpr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sys-utils/blkpr.c b/sys-utils/blkpr.c
index 03ca9f7e5..c6b030def 100644
--- a/sys-utils/blkpr.c
+++ b/sys-utils/blkpr.c
@@ -276,7 +276,7 @@ int main(int argc, char **argv)
 			break;
 		case 'c':
 			command = parse_pr_command(optarg);
-			if (command < 0)
+			if (command == -1)
 				err(EXIT_FAILURE, _("unknown command"));
 			break;
 		case 't':
-- 
2.52.0