How to truncate a file from the end? (cross platform)

Question

I am trying to find a cross-platform method to delete X bytes from the end of a file.

Currently I have found:

Platform specific solutions (such as truncate for posix) : This is what I don't want, because I want the C++ program to run on multiple platforms.
Read in the whole file, and write out the file again minus the bytes I want to delete: I would like to avoid this as much as I can since I want the program to be as efficient and fast as possible

Any ideas?

If there is a "go to end of file stream" method/function, could I then rewind X bytes and cut the remainder of the file off or something similar?

You can write a cross-platform wrapper for the platform-specific methods. — Ben Voigt, Mar 15 '12 at 20:06
duplicate eof http://stackoverflow.com/questions/873454/how-to-truncate-a-file-in-c — CapelliC, Mar 15 '12 at 20:12
"I want the C++ program to run on multiple platforms" Could you tell us which platforms you want to run on? — Robᵩ, Mar 15 '12 at 20:20
Well frankly, I'm using just unix, but this is an assignment so ideally, any platform that supports C++. — midnightBlue, Mar 15 '12 at 20:24
@user1255592: That's potentially infinite. I could implement a C++ interpreter in my chest hair... should you support that? — Lightness Races in Orbit, Mar 15 '12 at 20:26
By any platform I mean code that doesn't require platform specific functions. — midnightBlue, Mar 15 '12 at 20:29
May be try use boost::filesystem. It contains function resize_file — mikithskegg, Mar 15 '12 at 20:40
@mikithskegg: Boost is not available on "any platform that supports C++" — Lightness Races in Orbit, Mar 15 '12 at 20:58
(a) I'm not "worked up" (b) Don't tell me to chill (c) I'm trying to train you to think in precise terms, which will make you a better programmer. There is no "mocking" going on. Jumping to that conclusion is just a waste of your time and mine, not to mention ungrateful. — Lightness Races in Orbit, Mar 16 '12 at 09:56

score 4 · Answer 1 · edited Oct 18 '13 at 11:05

4

How do you think cross platform functions work? Just make your own function like this:

int truncate(int fd, long size)
{
#ifdef _WIN32 || _WIN64 
    return _chsize(fd, size);
#else
  #ifdef POSIX
    return ftruncate(fd, size);
  #else
    // code for other OSes
  #endif
#endif
}

edited Oct 18 '13 at 11:05

David Sykes

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answered Mar 15 '12 at 20:39

GreenScape

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score 1 · Answer 2 · answered Mar 15 '12 at 20:31

1

There is no such thing in the Standard library. They support streams- but streams don't have ends, files happen to have ends- at the current time.

All you can do is write a cross-platform wrapper on the function.

answered Mar 15 '12 at 20:31

Puppy

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Ok... a slightly different but related question; For a given OS, say windows. How exactly does the OS know when its reached the end of a file? Is there an EOF byte or something similar? If so, what is the implementation of this in other platforms? (e.g. linux) – midnightBlue Mar 16 '12 at 01:45
I asked this because I was wondering if it was possible to simply put that byte when I want to truncate it off. – midnightBlue Mar 16 '12 at 01:45

score 0 · Answer 3 · answered Jul 23 '12 at 15:06

0

The Boost.Filesystem library (version 1.44 and above) provides a resize_file function.

answered Jul 23 '12 at 15:06

Josh Kelley

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How to truncate a file from the end? (cross platform)

3 Answers3

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