C changing user

Question

Alright so my question is simple but I'm kinda new to C and I was hoping someone could help me. I want to change to a different user in C. For example, I know the user's password, and if the user types: ./change user1 It will change to user1. I know this may be a security issue right now, but I'm not worried about security at the moment.

Answer 1

You will want to use setuid which can be found in unistd.h so it is available on POSIX systems (so any linux/unix falvour should be ok)

From the man page

If the process has appropriate privileges, setuid() shall set the real user ID, effective user ID, and the saved set-user-ID of the calling process to uid.

If the process does not have appropriate privileges, but uid is equal to the real user ID or the saved set-user-ID, setuid() shall set the effective user ID to uid; the real user ID and saved set-user-ID shall remain unchanged.

The setuid() function shall not affect the supplementary group list in any way.

here is an example

Answer 2

Call the su binary (such as through system() or a combination of fork() and one of the exec functions), which knows about all the intricacies of changing credentials, both Unix-generically and on your particular OS. The terminal is shared and continues to belong to the original user.

Some of these intricacies not handled by a simple setuid() call are groups, account disabling and resource limits.

If this is just for fun, setuid() preceded by setgid() and initgroups() will probably be sufficient for you.

Answer 3

You have to find out the UID using getpwuid(). Afterwords you have to set the UID in your program with setuid() followed by the application/function you intent to run with these new rights, e.g. the shell. To start an application with the current UID use system().