Does lseek work on file redirection?

Question

Usually lseek doesn't work on stdin.

But, what if I run my program like this:

./a.out < filename

Now stdin is a file and not keyboard.

Will lseek work there?

Can I do

lseek(0, -1, SEEK_CUR)

and get the same effect like

ungetc() ?

Answer 1

Yes, lseek will change the seek pointer. No, it is not equivalent to ungetc().

fseek(stdin, -1, SEEK_CUR) comes closer to ungetc(), but still isn't identical.