How does mmap() help read information at a specific offset versus regular Posix I/O

Question

I'm trying to understanding something a bit better about mmap. I recently read this portion of this accepted answer in the related stackoverflow question (quoted below):mmap and memory usage

Let's say you read a 100MB chunk of data, and according to the initial 1MB of header data, the information that you want is located at offset 75MB, so you don't need anything between 1~74.9MB! You have read it for nothing but to make your code simpler. With mmap, you will only read the data you have actually accessed (rounded 4kb, or the OS page size, which is mostly 4kb), so it would only read the first and the 75th MB.

I understand most of the benefits of mmap (no need for context-switches, no need to swap contents out, etc), but I don't quite understand this offset. If we don't mmap and we need information at the 75th MB offset, can't we do that with standard POSIX file I/O calls without having to use mmap? Why does mmap exactly help here?

Answer 1

Of course you could. You can always open a file and read just the portions you need.

mmap() can be convenient when you don't want to write said code or you need sparse access to the contents and don't want to have to write a bunch of caching logic.

With mmap(), you're "mapping" the entire contest of the file to offsets in memory. Most implementation of mmap() do this lazily, so each ~4K block of the file is read on-demand, as you access those memory locations.

All you have to do is access the data in your file like it was a huge array of chars (i.e. int* someInt = &map[750000000]; return *someInt;), and let the OS worry about what portions of the file have been read, when to read the file, how much, writing the dirty data blocks back to the file, and purging the memory to free up RAM.