[Bug libstdc++/45628] std::fstream::tellg invalidates I/O buffer

potswa at mac dot com gcc-bugzilla@gcc.gnu.org
Fri Sep 17 17:52:00 GMT 2010

------- Comment #38 from potswa at mac dot com  2010-09-17 17:51 -------
Created an attachment (id=21822)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=21822&action=view)
Works with codecvt. Tested Tested x86_64-darwin, mainline

Ah, now I see the trick:

        if (__off == 0 && !(_M_mode & ios_base::out))

So if the file is open for writing, disable the optimization.

I had a problem with this condition for these testcases:


which contain code such as

    strmsz_2 = fb_01.sputn(", i wanna reach out and", 10);
    fb_01.pubseekoff(0, std::ios_base::cur); // if this doesn't flush
    c1 = fb_01.sgetc(); // this underflow is ignored
    c2 = fb_01.sputbackc('z'); // as well as this putback

Essentially, pubseekoff(0,cur) is being used as a sync(). I see nothing in the
Standard to support that, and indeed the sync() shouldn't be needed either, so
I was planning to open a new bug.

Anyway, if I apply your limitation to my patch, it passes the unmodified
testsuite, so here it is.



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