Weird bug with readdir, Windows and Chrome App Native Messaging

Question

I seem to have run in to a weird bug, which I have narrowed down to readdir() failing (and never returning) in Windows 7 & 8, but only when used with Native Messaging from a Chrome App, and only when a random but specific number of files is in the directory. The relevant C++ code is:

char            ac[10000];
std::string     sData   = "C:/TEST";
DIR *           pdir    = NULL;
struct dirent * pdirent = NULL;

if(   ( pdir = opendir(sData.c_str()) )   != NULL   ){
 while( pdirent = readdir(pdir) ){
  sReply += "|";
  sprintf(ac ,"%s" , pdirent->d_name);
  sReply += ac;
 }
 sReply += "|";
 closedir(pdir);
}

This works fine when part of a console application, and it works most of the time when part of an executable that receives a Native Message from a Chrome Packaged App - it gets and returns a text string with all file names in the directory, delimited by "|".

I wrote a test into my Chrome App, adding one file at a time between trials. It works multiple times, but then fails every time once some number of files has been added. For example, if the directory contains the files Tryc_3.txt, Tryc_4.txt, Tryc_5.txt ... Tryc_65.txt (each containing the text "test"), the directory listing works just fine. If I then add Tryc_66.txt it fails, at readdir(pdir).

I have tried different file names and directories with the same result, except the failure occurs at some other number of files. I have tried FindFirstFile, FindNextFile and FindClose with the exact same results. I have tried wrapping the whole thing with Wow64DisableWow64FsRedirection and Wow64RevertWow64FsRedirection with the exact same results. (I tried readdir_r but was unable to get it to compile, and some sites seem to say this is not available in Windows).

I am out of ideas, and am happy to hear any clues or speculation that anyone has.

I can supply the complete code, including the Chrome App, but it fairly large even after I cut as much as I can.

Why would a directory listing fail only when used with Native Messaging?