Getting Started with QNX Neutrino - QNX Software Systems

Writing a resource manager
© 2009, QNX Software Systems GmbH & Co. KG.
Unfortunately, if someone forgot to obtain a mutex for each and every file descriptor
operation, there’d be a possibility that such an “unprotected” access would cause a
thread to read or write data to the wrong location.
Let’s look at the C library call readblock() (from ):
int
readblock (int fd,
size_t blksize,
unsigned block,
int numblks,
void *buff );
(The writeblock() function is similar.)
You can imagine a fairly “simplistic” implementation for readblock():
int
readblock (int fd, size_t blksize, unsigned block,
int numblks, void *buff)
{
lseek (fd, blksize * block, SEEK_SET); // get to the block
read (fd, buff, blksize * numblks);
}
Obviously, this implementation isn’t useful in a multi-threaded environment. We’d
have to at least put a mutex around the calls:
int
readblock (int fd, size_t blksize, unsigned block,
int numblks, void *buff)
{
pthread_mutex_lock (&block_mutex);
lseek (fd, blksize * block, SEEK_SET); // get to the block
read (fd, buff, blksize * numblks);
pthread_mutex_unlock (&block_mutex);
}
(We’re assuming the mutex is already initialized.)
This code is still vulnerable to “unprotected” access; if some other thread in the
process does a simple non-mutexed lseek() on the file descriptor, we’ve got a bug.
The solution to this is to use a combine message, as we discussed above for the
chown() function. In this case, the C library implementation of readblock() puts both
the lseek() and the read() operations into a single message and sends that off to the
resource manager:
_IO_LSEEK
_IO_READ
The readblock() function’s combine message.
The reason that this works is because message passing is atomic. From the client’s
point of view, either the entire message has gone to the resource manager, or none of it
has. Therefore, an intervening “unprotected” lseek() is irrelevant — when the
readblock() operation is received by the resource manager, it’s done in one shot.
218 Chapter 5 • Resource Managers April 30, 2009