Increasing a file's size using mmap

Question

In Python on Windows I can create a large file by

    from mmap import mmap
    f = open('big.file', 'w')
    f.close()
    f = open('big.file', 'r+')
    m = mmap(f.fileno(), 10**9)

And now big.file is (about) 1 gigabyte. On Linux, though, this will return ValueError: mmap length is greater than file size.

Is there a way to get the same behavior on Linux as with Windows? That is, to be able to increase a file's size using mmap?

Answer 1

On POSIX systems at least, mmap() cannot be used to increase (or decrease) the size of a file. mmap()'s function is to memory map a portion of a file. It's logical that the thing you request to map should actually exist! Frankly, I'm really surprised that you would actually be able to do this under MS Windows.

If you want to grow a file, just ftruncate() it before you mmap() it.