File I/O across processes in Linux?

Question

So, I've got a Linux process where I'm trying to manage some files on a tape. I have the following code that attempts to extract a file, catalog.xml, from the current archive on the tape and copy it to a fixed location (eventually, I'll parse the file and do some work with the results). But, my code is intermittently failing to work correctly. The tar command always succeeds, (and if I check the file system, I see the catalog.xml file), but sometimes my followup check to see if the file exists returns false. Am I doing something obviously wrong? It seems like I'm probably encountering a race condition between the fork()ed process returning and the results of that process being visible on the file system - is there some call I need to make?

  pid_t tPid = vfork();
  if (0 == tPid)
  {
    int tChildRet;
    tChildRet = execlp("tar", "tar", "-xvf", "/dev/nst0", "-C", "/tmp", "catalog.xml", (char *) 0);
    _exit(-1 == tChildRet ? EXIT_FAILURE : EXIT_SUCCESS);
  }
  else
  {
    wait(&ret);
  }
std::ifstream tCatalogFile("/tmp/catalog.xml");
if (tCatalogFile)
{
   cout << "File exists!" << endl;
} else {
   cout << "File does not exist!" << endl;
}

And I'm getting either "file exists!" or "file does not exist!", seemingly at random.

Other notes: On the failure cases:

• if I do a stat ("/tmp/catalog.xml"), I get a return of -1 with errno set to ENOENT.
• the tar command (run with the -v flag) produces the expected one line of output ("catalog.xml")

/tmp is a local tmpfs filesystem; the tape drive is a local device. I'm using 2.6.30.9 Linux kernel with g++ 4.1.2 on a x86_64 box.

Thanks in advance!

Answer 1

If execlp succeeds, it will never get to the line where you call _exit. You aren't checking the return value (ret) from wait. It's not clear why you should be using vfork. Etc.

Since the parent is not doing anything else besides waiting for the child to complete, why not make your life easier and just use system()?

Answer 2

Try calling sync after the wait call in the parent.

If that doesn't work, you may need to loop and/or sleep until the parent can open the file, since you know it's there.