Hands-On System Programming with Linux

Real and effective IDs

We understand from the preceding section that both the shared object that is being worked upon (here, the file myfile) and the process that is performing some access (rwx) on the object (here, the echo process) matter in terms of permissions.

Let's zoom deeper into the process attributes with respect to the permissions model. So far, we have learned that each process is associated with a UID and a GID, thereby allowing the kernel to run its internal algorithms and determine whether access to a resource (or object) should be allowed.

If we look deeper, we find that each process UID is actually not a single integer value, but two values:

• The Real User ID (RUID)
• The Effective User ID (EUID)

Similarly, the group information ...