To explain shortly why I need this,
I am currently doing the detection by stat(2). I don't have control over the file descriptor (may get used up by some other thread as my code is getting injected to replace syscalls) , so i can't use fstat(2) (which is faster). I need to do this check a lot of times, so is there a faster way to do the same thing?
I am checking the same file in different processes which do not have a parent child relation.
You should probably benchmark it for yourself.
I've measured
//Real-time System-time
272.58 ns(R) 170.11 ns(S) //lseek
366.44 ns(R) 366.28 ns(S) //fstat
812.77 ns(R) 711.69 ns(S) //stat("/etc/profile",&sb)
on my Linux laptop. It fluctuates a little between runs but lseek is usually a bunch of ns faster than fstat, but you also need a fd for it and opening is quite expensive at about 1.6µs, so stat is probably the best choice for your case.
As tom-karzes has noted, stat should dependent on the number of directory components in the path. I tried it on a PATH_MAX long "/foo/foo/.../foo" directory and there I'm getting about 80µs.