How does vim writes a read-only file?

Question

Sorry for the newbie question. I would like to know, how vim manages to write a read-only file. I've 555 permissions on a text file. But, when I open & write something to it and does :w! , the changes I made to file are saved. I wonder how vim is doing this in background!!. Is it like changing permissions temporarily to 755 and writing to it and reverting the permissions back? Please enlighten.

Answer 1

EDIT: I originally answered with correct, but ultimately irrelevant information on how UNIX permissions work: that was not what Vim was doing.

Indeed, you're right: when you issue :w!, and you're on UNIX, Vim will add the write permission if it needs to:

/* When using ":w!" and the file was read-only: make it writable */
if (forceit && perm >= 0 && !(perm & 0200) && st_old.st_uid == getuid()
                 && vim_strchr(p_cpo, CPO_FWRITE) == NULL)
{
    perm |= 0200;
    (void)mch_setperm(fname, perm);
    made_writable = TRUE;
}

and subsequently reset it back:

if (made_writable)
    perm &= ~0200;      /* reset 'w' bit for security reasons */

It is also reflected in the help:

Note: This may change the permission and ownership of
the file and break (symbolic) links. Add the 'W' flag to 'cpoptions' to avoid this.