Is it allowed to open a file for both reading and writing?

I opened the same file for reading and writing in 2 seperate
channels. Some anomalies happened, e.g. reading from a position
after overwriting some old data doesn't get the newly written
value.

So if I want read/write file access, is there a simple way
to do so besides using UNIX module? I prefer the core library
because Marshalling works better with standard channel library.

Thanks.

sincerely,
Shin-Cheng Mu