[Bug libstdc++/45574] cin.getline() is extremely slow

["tstarling at wikimedia dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #19 from tstarling at wikimedia dot org  2010-09-09 14:28 -------
(In reply to comment #16)
> The *_unlocked versions are faster a lot actually, at least for the one
> character ops, because no locking is performed and the calls are inlined.
> But the question is whether libstdc++ can use them, unless there is some
> restriction that would disallow several threads from using the same FILE *
> (including using STL APIs in one thread and C stdio APIs in another thread).

My current idea is to do:

flockfile(stdin);
while (!eof) {
    c = getc_unlocked(stdin);
    ...
}
funlockfile(stdin);

This is not only much faster, it's an improvement to the current behaviour in
terms of locking and thread safety. The current behaviour, as I said in comment
#4, could cause data to be badly mangled if one thread uses stdio while another
uses cin.getline(). Using getc() in preference to getc_unlocked() does not
help.

And unlike getdelim(), the unlocked I/O functions are in POSIX.1-2001, says the
man page, so it's relatively portable.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45574