Getting hardware access as normal user

Zotty · 02-01-2005, 08:14 AM

I'm writing a library which accesses the parallel port to output data to an external device. To do this, I use ioperm(). However, ioperm() needs root privileges and thus programs using the library must be run as root.

How can the library be used by a normal user. Is there a function in C++ which allows me to code the library in a way that it get the access by itself without the need to start the program that uses the library as root?

I've tried setting it with setregid() and setuid(), but this doesn't work (error "Operation not permitted"):

Code:

 uid_t uid = 0;
 gid_t gid = 0;

 setregid( gid, gid );
 if (getgid() != 0) {
  perror("Could not get GID root");
  return false;
 }

 setuid( 0 );
 if (getuid() != 0 ) {
  perror("Could not get UID root");
  return false;
 }

On the other hand, this does not work for obvious security related reasons.

Or how to solve this problem using a other approach? I'm not interrested in having root privileges, just direct access to the parallel port. So if there's another way, I'd be happy to hear it.

jim mcnamara · 02-01-2005, 10:44 AM

setuid()

Hko · 02-01-2005, 11:13 AM

On my (debian) computer I get:

Code:

ls -l /dev/lp0
crw-rw----  1 root lp 6, 0 Mar 14  2002 /dev/lp0

As you can see group permissions are "rw". And the group of the parallel port device file (/dev/lp0) is "lp".

Now say you want user "hko" to be able to access this device, so he can run your program. To do this, you make "hko" a member of group "lp" with this (as root):

Code:

adduser hko lp

When "hko" logs in again after this he will be able to acces the parallel ports.

If this is not what you were looking for, try reading about the "sudo" program.

Or, make you executable SUID root (not recommended, security issues).