Hello! Couldn't you say, how to add data at beginning of a file effectively?
(in C)

do you have the file created and with existing content??
i.e.:
is your file something like:
and you need to insert a string before the "some bla bla bla..."??

Yes

go here and look for the seekg function:
http://www.cplusplus.com/doc/tutorial/tut6-1.html

it explains it very well there

This is C++

There's no portable way to open a file for writing and writing to the start of the file (or in the middle) without overwriting previous content. When one wants to add data to another location than the end of the file or modify existing data, you usually open the existing file for reading and a new file for writing. Then you write the data, new and old, in the order you want to new file. When you're done you close both files, delete the old one and rename the new one. This can be done using functions in the C or C++ standard library, so it's portable.

Thanks. I knew about this way, but I thougt that there is some specific FS lowlevel functions

Maybe it can be done using memory-mapped files, but I don't have a working example to show you and it would also make the program unportable, of course. Even if memory-mapped files doesn't buy you anything compared to the method I described above, it will be a good exercise!

Couldn't you give me POSIX (or libc) functions for memory mapped files?
(I've done this in windows, but I don't know UNIX analogues).
BTW, I want to have approximately terabyte files