[Lab 1] shared memory synchronization

[tobias.hamann]

Hi,

We are currently experiencing strange behaviour regarding Exercise 4 of the 1st lab assignment:

After opening the shared memory and truncating it, we fork a second process. The child mmaps the shared memory, writes to it and munmaps it again. The parent waits for the child to exit, mmaps the shared mem itself, reads from it and munmaps it again. This approach, however, succeeds only in some cases. The reading process fails to obtain any content from shared mem in many cases.

Are we misssing some important step here? Or is the write operation to the mmaped shared mem an asynchronous write? Adding a sleep() in the parent seems to solve the problem, after 100 msec the reading always succeeds.

Thanks in advance.

[z33ky]

I'm guessing that something akin to fflush/fsync is missing. Do you close the SMO on the child?

[tobias.hamann]

We open the shm before forking and unlink it in the parent after read access. In the child process, we only mmap and shm and unmap it after writing to it. What exactly do you mean by closing it? There is no function like shm_close(), or at least I did'nt find one...

[JanH]

Use the same close() function that was used to close the pipe file descriptors.

[tobias.hamann]

Oh, good point, we did not think of that initially. However, that does not seem to make a difference either...

[tobias.hamann]

With assistance by the teaching staff, we found the problem: there was a wait() missing in one of our functions. Thanks a lot for the help everyone.