gpioset: extend help text

There's a common misconception about the way the character device
works. Some users expect line states to be driven as desired even
after gpioset exits. Add a note about that to the help text to avoid
further confusion.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>

diff --git a/src/tools/gpioset.c b/src/tools/gpioset.c
index 0678373fc41a52db1a221e9b601cb237c983b812..bbd9fd1bcc5b31abb0a56c3f9f951d20219418fd 100644 (file)
--- a/src/tools/gpioset.c
+++ b/src/tools/gpioset.c
@@ -36,7 +36,7 @@ static void print_help(void)
 {
        printf("Usage: %s [OPTIONS] <chip name/number> <offset1>=<value1> <offset2>=<value2> ...\n",
               get_progname());
-       printf("Set GPIO line values of a GPIO chip\n");
+       printf("Set GPIO line values of a GPIO chip and maintain the state until the process exits\n");
        printf("\n");
        printf("Options:\n");
        printf("  -h, --help:\t\tdisplay this message and exit\n");
@@ -53,6 +53,12 @@ static void print_help(void)
        printf("  wait:\t\tset values and wait for user to press ENTER\n");
        printf("  time:\t\tset values and sleep for a specified amount of time\n");
        printf("  signal:\tset values and wait for SIGINT or SIGTERM\n");
+       printf("\n");
+       printf("Note: the state of a GPIO line controlled over the character device reverts to default\n");
+       printf("when the last process referencing the file descriptor representing the device file exits.\n");
+       printf("This means that it's wrong to run gpioset, have it exit and expect the line to continue\n");
+       printf("being driven high or low. It may happen if given pin is floating but it must be interpreted\n");
+       printf("as undefined behavior.\n");
 }
 
 struct callback_data {