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wood · 09-14-2003, 04:19 PM

This is an easy one... However, since it looked like a GRE CS questions, I wanted to share with you. I'll post the answer after a few posts.

6.23 On Unix, each file is associated with one user and one group. A limited access list is implemented by assigning a set of permissions for the owner of the file, those in the same group as the file but who are not the owner, and for everyone else. The operations controlled are read, write, and execute access. Assume the system may have up to 2^16 users and 2^8 groups. How many bits are needed to store each file's access data?

rafi_dery · 09-14-2003, 07:25 PM

Not sure what exactly access data contains... I assume it must contain the owner, and the operations for the owner, the group and the others.
I guess the group of the owner is stored elsewhere.
So: 16 bits (Owner) + 9bits (3*3 for operations) total of 25bits.

wood · 09-14-2003, 08:50 PM

Answer:

The number of users and groups is irrelevant. Each set of three permissions requires three bits to indicate whether or not the permission is granted. There are three sets of permissions. Nine bits total are needed per file.

Vinay · 09-15-2003, 02:27 AM

i thought 9 bits but never though it wud b so easy, got confused in users.
rwx rwx rwx themselves denote permissions for owner,group and others. seems like i have to buy schaum's OS.

but is UNix thr in GRE CS?

wood · 09-15-2003, 10:37 AM

Yes, they do ask questions like this... In fact you don't need to know Unix to answer the question, but if you do, you can be reassured of the answer.

Wood

max_on · 11-06-2003, 01:55 PM

I think one has to know Unix to answer this question, since he has to know that "users and groups is irrelevant"

wood · 11-06-2003, 02:03 PM

I didn't say that users and groups are irrelevant. I said that the number of user/groups is irrelevant. That's a big difference. The question says "assigning a set of permissions for the owner of the file (just one can be the owner of course), those in the same group as the file but who are not the owner (everyone else in the group except me! doesn't matter how much in the group there are), and for everyone else.", which pretty much tells you that the number of them is irrelevant.

Wood

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09-14-2003, 04:19 PM	#1 (permalink)
wood TestMagic Guru Join Date: Jul 2003 Location: Brazil Posts: 1,360	This is an easy one... However, since it looked like a GRE CS questions, I wanted to share with you. I'll post the answer after a few posts. 6.23 On Unix, each file is associated with one user and one group. A limited access list is implemented by assigning a set of permissions for the owner of the file, those in the same group as the file but who are not the owner, and for everyone else. The operations controlled are read, write, and execute access. Assume the system may have up to 2^16 users and 2^8 groups. How many bits are needed to store each file's access data?

09-14-2003, 07:25 PM	#2 (permalink)
rafi_dery Within my grasp! Join Date: Jun 2003 Location: Israel Posts: 111	Not sure what exactly access data contains... I assume it must contain the owner, and the operations for the owner, the group and the others. I guess the group of the owner is stored elsewhere. So: 16 bits (Owner) + 9bits (3*3 for operations) total of 25bits.

09-14-2003, 08:50 PM	#3 (permalink)
wood TestMagic Guru Join Date: Jul 2003 Location: Brazil Posts: 1,360	Answer: The number of users and groups is irrelevant. Each set of three permissions requires three bits to indicate whether or not the permission is granted. There are three sets of permissions. Nine bits total are needed per file.

09-15-2003, 02:27 AM	#4 (permalink)
Vinay TestMagic Guru-in-Training Join Date: Jan 2003 Location: India Posts: 766	i thought 9 bits but never though it wud b so easy, got confused in users. rwx rwx rwx themselves denote permissions for owner,group and others. seems like i have to buy schaum's OS. but is UNix thr in GRE CS?

09-15-2003, 10:37 AM	#5 (permalink)
wood TestMagic Guru Join Date: Jul 2003 Location: Brazil Posts: 1,360	Yes, they do ask questions like this... In fact you don't need to know Unix to answer the question, but if you do, you can be reassured of the answer. Wood

11-06-2003, 01:55 PM	#6 (permalink)
max_on Trying to make mom and pop proud Join Date: Oct 2003 Location: Russia Posts: 1	I think one has to know Unix to answer this question, since he has to know that "users and groups is irrelevant"

11-06-2003, 02:03 PM	#7 (permalink)
wood TestMagic Guru Join Date: Jul 2003 Location: Brazil Posts: 1,360	I didn't say that users and groups are irrelevant. I said that the number of user/groups is irrelevant. That's a big difference. The question says "assigning a set of permissions for the owner of the file (just one can be the owner of course), those in the same group as the file but who are not the owner (everyone else in the group except me! doesn't matter how much in the group there are), and for everyone else.", which pretty much tells you that the number of them is irrelevant. Wood

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