Given

parser a
parser b

stream a
stream b
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I would like to give parser a data from stream a, and parser b data from stream b, simultaneously, in the same process (no threads).

You can just use a select loop, and read nonblocking on your own, to get data from multiple streams at the same time.

What I need to get this done is parsers that can be told 'here is more data', and have them return even if that's not a complete XML document. They parse as much as they can given the current buffer which they keep. When more data will become available, they continue parsing.

Basically, instead of reading more from a handle, i'd like the parsers to return control to me, so I can give them more strings.