attrib: qualify device with an option.

I learned that the in-kernel FAT implementation in Linux supports two
ioctls, FAT_IOCTL_GET_ATTRIBUTES and FAT_IOCTL_SET_ATTRIBUTES, to get
and set file attributes on a mounted file system.

If this FUSE exFAT implementation (and/or the new in-kernel
implementation on Linux) were to implement such an ioctl in future, it
would be nice to be able to adjust this tool to support that usage
without breaking its command-line arguments in a backwards-incompatible
way. To future-proof against that, require a '-d' option to specify the
target device.

This would also allow the command to accept more that one path, like
chmod.

Author: wjt

attrib/exfatattrib.8

@@ -23,6 +23,7 @@
 |
 .B \-A
 ]
+.B \-d
 .I device
 .I file
 .SH DESCRIPTION
@@ -44,6 +45,10 @@ error to set and clear the same flag.
 .SH COMMAND LINE OPTIONS
 Command line options available:
 
+.TP
+.BI \-d " device"
+The path to an unmounted disk partition or disk image file containing an exFAT
+file system. This option is required.
 .TP
 .BI -r
 Set read\-only flag

attrib/main.c

@@ -29,10 +29,10 @@ static void usage(const char* prog)
 {
 	fprintf(stderr,
 		"Display current attributes:\n"
-		"  %1$s <device> <file>\n"
+		"  %1$s -d <device> <file>\n"
 		"\n"
 		"Set attributes:\n"
-		"  %1$s [FLAGS] <device> <file>\n"
+		"  %1$s [FLAGS] -d <device> <file>\n"
 		"\n"
 		"Flags:\n"
 		"  -r    Set read-only flag\n"
@@ -111,7 +111,7 @@ int main(int argc, char* argv[])
 	uint16_t add_flags = 0;
 	uint16_t clear_flags = 0;
 
-	while ((opt = getopt(argc, argv, "rRiIsSaAhV")) != -1)
+	while ((opt = getopt(argc, argv, "d:rRiIsSaAhV")) != -1)
 	{
 		switch (opt)
 		{
@@ -120,6 +120,15 @@ int main(int argc, char* argv[])
 			puts("Copyright (C) 2011-2018  Andrew Nayenko");
 			puts("Copyright (C) 2020       Endless OS Foundation LLC");
 			return 0;
+		/*
+			The path to the unmounted exFAT partition is a (mandatory) named
+			option rather than a positional parameter. If the FUSE file system
+			ever gains an ioctl to get and set attributes, this option could be
+			made optional, and this tool taught to use the ioctl.
+		*/
+		case 'd':
+			spec = optarg;
+			break;
 		case 'r':
 			add_flags |= EXFAT_ATTRIB_RO;
 			break;
@@ -157,11 +166,10 @@ int main(int argc, char* argv[])
 		return 1;
 	}
 
-	if (argc - optind != 2)
+	if (spec == NULL || argc - optind != 1)
 		usage(argv[0]);
 
-	spec = argv[optind];
-	file_path = argv[optind + 1];
+	file_path = argv[optind];
 
 	if ((add_flags | clear_flags) == 0)
 		options = "ro";