Determine the size of a pipe without calling read()

Question

I need a function called SizeOfPipe() which should return the size of a pipe - I only want to know how much data is in the pipe and not actually read data off t

Answer 1

It's almost never necessary to know how many bytes are in the pipe: perhaps you just want to do a non-blocking read() on the pipe, ie. to check if there are any bytes ready, and if so, read them, but never stop and wait for the pipe to be ready.

You can do that in two steps. First, use the select() system call to find out whether data is available or not. An example is here: http://www.developerweb.net/forum/showthread.php?t=2933

Second, if select tells you data is available, call read() once, and only once, with a large block size. It will read only as many bytes are available, or up to the size of your block, whichever is smaller. If select() returns true, read() will always return right away.

Answer 2

I don't think it is possible, isn't the point of a pipe to provide interprocess communication between the two ends (in one direction). If I'm correct in that assertion, the send may not yet have finished pushing data into the pipe -- so it'd be impossible to determine the length.

What platform are you using?

Answer 3

I do not think it's possible. Pipes present stream-oriented protocol rather than packet-oriented one. IOW, if you write to a pipe twice, once with,say, 250 bytes and once with, say, 520 bytes, there is no way to tell how many bytes you'll get from the other end in one read request. You could get 256, 256, and then the rest.

If you need to impose packets on a pipe, you need to do it yourself by writing pre-determined (or delimited) number of bytes as packet length, and then the rest of teh packet. Use select() to find out if there is data to read, use read() to get a reasonably-sized buffer. When you have your buffer, it's your responsibility to determine the packet boundary.

Answer 4

Some UNIX implementations return the number of bytes that can be read in the st_size field after calling fstat(), but this is not portable.

Answer 5

You can wrap it in object with buffering that can be rewinded. This would be feasible only for small amounts of data.

One way to do this in C is to define stuct and wrap all functions operating on pipes for your struct.

Answer 6

On Windows you can always use PeekNamedPipe, but I doubt that's what you want to do anyway.