C linux internal difference between O_WRONLY and O_RDWR if only writting

Question

I know open offers these mutually exclusive flags: O_RDONLY, O_WRONLY and O_RDWR.

I want to know: Are there any performance issues (even of it's just a fraction of a ms) or different ways of treating the file if the file is opened as O_RDWR and

• I only write to the file. (Versus opening as O_WRONLY)
• I only read data from the file. (Versus opening as O_RDONLY)

Answer 1

First, you seem to have mistyped (inverted) in the two cases of your description the write/read tags. As to what you ask, the VFS, in its various structures, keeps track of desired access rights by flags. The read/write flag is typically a different bit in the same flag (multi) byte. When a process request access as read or write, the kernel checks if it has the requested access rights, and proceeds accordingly. As setting 2 bits doesn't increase your execution time, you should see no difference as later access is the same. Using the proper tag is simply good style and part of file protection.