Bo Berglund wrote:
Notice that the file used is /dev/mem
I don't know if there exists a user group named mem, but in any case the
user pi is not member of any such group.
Could this be the cause for root to be required?
No, if you do
$ ls -l /dev/mem
crw-r----T 1 root kmem 1, 1 Jan 1 1970 /dev/mem
I'll step through those bits in stages. First, I've prefixed the ls
command with $ since this is the prompt that was displayed. You don't
enter that, it's just "unixspeak" shorthand showing that the command was
entered by an unprivileged user.
Looking at the response, c shows that the thing you're looking at
represents a character device. After that you get permissions: the user
that owns it has rw- (i.e. read, write, no execute), the group has r--
(read only) and others have --- (i.e. no access at all). Skipping a bit,
the owner is root and the group that has limited access is kmem.
If you look in /etc/group, you'll probably see that no users are in the
kmem group. The RPi probably has something in the GUI to add/remove
users from groups and that should be used if possible, but otherwise you
can do it by editing BOTH /etc/group and /etc/gshadow CAREFULLY so that
you see something like
kmem:x:15:pi
and
kmem:*::pi
respectively. DO NOT GET THIS WRONG, A SCREWED FILE HERE WILL LOCK YOU OUT.
Look, I've not got much time but going back to another point. The
example at
http://wiki.freepascal.org/Lazarus_on_Raspberry_Pi#1._Native_hardware_access
shows the /sys tree being manipulated to get access to GPIO pins, and
this sort of thing can be the easiest way to start off since you can
twiddle things by hand. So initially in /sys/class/gpio we've got
$ ls -l
total 0
-rwxrwx--- 1 root gpio 4096 Jan 1 1970 export
lrwxrwxrwx 1 root gpio 0 Jan 1 1970 gpiochip0 -> ...
-rwxrwx--- 1 root gpio 4096 Jan 1 1970 unexport
If I now do
$ echo '17' >export
it creates a new entry
lrwxrwxrwx 1 root gpio 0 Oct 10 13:18 gpio17 -> ...
If I cd into gpio17 there are various things exposed, /but/ you'll see
that they're owned by root... that's going to be a problem for
production code but is still OK for getting things working by hand.
Switch to root using su [root password here] or sudo su [your password
here] as appropriate. Note password prompt now ends in #
# cat direction
in
# echo 'out' >direction
# cat direction
out
and so on.
I'll try to keep an eye on this ML, but our gateway rejects quite a lot
of stuff /particularly/ if it's formatted as HTML or has an attachment.
--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk
[Opinions above are the author's, not those of his employers or colleagues]
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