There's no specific functions for that, the f* functions don't deal with what's in the files, just read the data in. There is a str* function that searches for one string in another, you could read in the whole file and then use that, but tge file couldn't be very big for that to be practical
There are various algorithms, like Boyer-Moore, but really if it's on disk then your bottleneck is most likely disk IO.
I'd most certainly just start off with the basic obvious solution. Those other algorithms are all targeted at very specific situations. We'd need to more about the string you're searching for etc:
Number of distinct characters in the string
Number of distinct characters in the file (if known)