On Oct 29, 2009, at 04:31 , Kazu Yamamoto (山本和彦) wrote:
Since the handle is associated with UDP, the entire data size should be known when a UDP packet arrived. But I cannot find a way to know data size. (I want a function like hFileSize.) Note that hGetContents blocks.
The short answer is: you can't. You need to find a way to make recv() work.
Longer: read() is intended for streams. If you read() a datagram socket you may get a single packet or the system may combine packets; which one happens depends on the kernel implementation, and you are not guaranteed to have anything sane happen. The best case for you would be if the system returns a single packet per system call, in which case you read() into a maximally sized buffer and the returned length is that of the actual packet. However, many systems choose to retain the stream nature of read() and will return any information that comes in, losing the packet boundaries.
In the case of DNS, you can almost get away with this anyway because the protocol specifies a maximum UDP request/response length of 255 octets sent as a single packet. If this is insufficient, you need to switch to TCP. In the case of a response, you will get a truncated response and will need to recognize it by parsing the data and retry the query over TCP.
-- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH
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