What is the best way? To open them in block e.g. 64KB or more or less if this file is 4GB. Is this possible this with C functions like read, write, seek, etc.?

Thanks

Depends on what you do - but unless you're going to write for 64bit linux only, you do need to process the file in "chunks" - you can't mmap such large files on 32bit systems.

Iirc you need to use 64bit-size extensions to the standard read/write/seek functions (ie., not plain ansi or posix, but should be available on most systems) - notice that the 64bit here is size support, it's still available on 32bit systems.