Partially truncating a stream (fstream or ofstream) in C++

Question

Hi,

I am trying to partially truncate (or shorten) an existing file, using fstream. I have tried writing an EOF character, but this seems to do nothing.

Any help would be appreciated...

Answer 1

It'll depend on the OS. Most OSes support this, but in different ways. On Windows, there's a SetEndOfFile(). On Unix and similar systems, you lseek to where you want the file to end, and do an lwrite of zero bytes there. Other OSes undoubtedly use other methods.

Answer 2

I don't think you can. There are many functions for moving "up and down" the wrapper hierarchy for HANDLE<->int<->FILE *, at least on Windows, but there is no "proper" to extract the FILE * from an iostreams object (if indeed it is even implemented with one).

You may find this question to be of assistance.

Personally I would strongly recommend steering clear of iostreams, they're poorly designed, heavily C++, and nasty to look at. Take a look at Boost's iostreams, or wrap stdio.h if you need to use classes.

The relevant function for stdio is ftruncate().

Answer 3

The Boost.Interprocess library defines a portable truncate function. For some reason it is not documented, but you can find it this header file.

Answer 4

I bit the bullet in the end and read the part of the file to be kept to an array then re-wrote it. It's not the best solution - but as the files will always be small I have decided to accept this method.