std::ofstream::open will it read the entire file into memory?

Question

I'm writing things from my memory to the disk in order to free my memory.

I wonder each time I call open(), and appendix new elements to the end of the file, will it read the entire file into memory? or it is just a pointer to the end of the file?

Answer 1

The fstream implementation doesn't specify exactly what happens if you use the ofstream::app, ios::app, ofstream::ate or ios::ate mode to open the file.

But in any sane implementation, the file is not read into memory, all that happens is that the fstream implementation positions the "current position" to the end of the file.

To read the entire file into memoiry would be rather terrible if you have a system with 2GB of RAM and you wanted to append to a file that is bigger than 2GB.

Being very pedantic, when writing something to a text-file, it is likely that the filesystem that is part of the operating system will read the last few (kilo)bytes of the file, as most hard-disks and similar storage requires that the data is being written to a "block", which is a fixed size (e.g. 512 bytes or 4 kilobytes). So, unless the current filesize is exactly at a boundary of such a block, the filesystem must read the last block of the file and write it back with the additional data that you asked to write.

If you are worried about appending to a log-file that gets very large, no, it's not an issue. If you are worried about memory safety because your file has secret data that you won't want stored in memory, then may be a problem, because a portion of that will probably be loaded into memory, and there is nothing you can do to control that.