I was looking at the permission bits used in various functions like stat() and chmod(), and I wanted a description of what the macros defined actually are. For instance S_IRUSR says it's represented by 00400 (GNU/Linux). My question is, could someone describe what the 00400 actually is? Is it a number, what? I understand how to OR the macros, I just don't get what the macro actually is.
Best Answer
I am going to describe the left most three numbers in permission and that would also explain about S_IRUSR,
So each of the numbers is an octal number. Each number could be from 0 to 7. Each octal number could be converted to 3-bit binary number. Each bit represents a permission.
Left most bit = Read permissionMiddle bit = Write permissionRight most bit= Execute permissionLets write 0 to 7 into binary and see the permission bits:
Octal Binary 0 0 0 0 (No Read, No Write, No Execute) -- No permission1 0 0 1 (No Read, No Write, Yes you can execute)2 0 1 0 (No Read, Can Write, No execute)3 0 1 1 (No Read, Can Write, Can execute)4 1 0 0 (Can Read, No Write, No Execute)5 1 0 1 (Can Read, No Write, Can execute)6 1 1 0 (Can Read, Can Write, No execute)7 1 1 1 (Can Read, Can Write, Can execute)So each number represents permissions. Now next part is for who these pemmissions are.Let the left most three number be XYZ:Now,
X means permission given to the owner of the file.Y means permission given to the group of the owner.Z means permission given to all other users in system , outside of user's group.Given that, Z_ISUSR = 00400, now 4 means readable by user IRUSR = Is Readable by user.
These three are the important numbers in permissions, and these only specify the permissions given to the file.
Wiki has very good explanations here
From the linked page:
0 --- no permission1 --x execute2 -w- write3 -wx write and execute4 r-- read5 r-x read and execute6 rw- read and write7 rwx read, write, and execute