Hello Cameron Simpson, Thank you for your reply. I now think the way I did (using fstat()) was a very bad idea (as you pointed out). I haven't decided how to fix this yet. I also considered attaching the message length to the head of the message. It will work, too. I need to a bit more experiment.
Thank you for providing me directions to solve this problem. Best regards, Aki Niimura On Apr 7, 3:26 pm, Cameron Simpson <c...@zip.com.au> wrote: > On 07Apr2009 10:08, akineko <akin...@gmail.com> wrote: > | I'm trying to use named pipes to fuse a Python program and a C > | program. > | One side creates pipes using os.mkfifo() and both sides use the same > | named pipes (one side reads, another side writes). The read side uses > | select.select() to wait for incoming messages and read the message > | when select.select() says it is ready. > | The length of the message is unknown to the read side. > > That's a serious flaw in the message protocol. > > | I cannot use file.read() because it will block waiting for an EOF. > | I cannot use file.readline() because how many lines have arrived is > | unknown. > | So, I needed to use os.read() with the exact number of characters to > | read. > > No! > > You should use os.read() with the maximum size of a message. > It _should_ return with the number of bytes in the message, provided the > C program writes messages with a single OS-level write() call. > > Forget all the fstat() stuff - it's racy. > > Personally, I'd use a thread to just do continuous blocking os.read()s of > the pipe, and putting the resulting messages on a Queue for collection > by your main program. If you're the only consumer of a Queue it's safe > to poll it for emptiness or not, or to use a no-wait get(). > > All the above is untested, but absent a size in the protocol or other > ways of parsing message boundaries in data stream, you can only rely on > the C program writing messages with a single write() and collect using a > large os.read(), which should return with what is there. > > Cheers, > -- > Cameron Simpson <c...@zip.com.au> DoD#743http://www.cskk.ezoshosting.com/cs/ > > Language... has created the word "loneliness" to express the pain of > being alone. And it has created the word "solitude" to express the glory > of being alone. - Paul Johannes Tillich -- http://mail.python.org/mailman/listinfo/python-list
