Mutliple threads reading single disk file? fpurge() workaround.

[noleander]

I've got an application with multiple threads. They are all reading from a single disk file. The following code works fine on Windows, but produces bad data restults on the Mac:

threadFunction() {
FILE* f = ...;
for (..) {
pthread_mutex_lock(); // ensure threads dont interfere with each other
fseek ( .... , f);
fread ( .... , f);
pthread_mutex_unlock();
} // end for
} // end thread function

On Windows, the above code works fine. On the Mac, it runs, but the data returned from fread() is from the wrong part of the disk file: namely, a part of the file being read by antoher thread. Clearly, the FILE* buffers are getting mixed up between the threads. Yet I am carefule to ensure that each thread has its own dedicated FILE* object.

I can get it to work on the Mac by placing a call to fpurge() as follows ... but this is not really satisfactory because it slows down the threads:


threadFunction() {
FILE* f = ...;
for (..) {
pthread_mutex_lock(); // ensure threads dont interfere with each other
fseek ( .... , f);
fread ( .... , f);
fpurge ( f ); // slows down read rate
pthread_mutex_unlock();
} // end for
} // end thread function

Any ideas why the first pseudo code (without fpurge) produces bad results from fread()? Is the stdio library thread safe?

Thanks in advance for any help,
neal