This programming problebm is not specific to Linux; it can be reproduced under Unix (e.g Solaris). Here is the scenario: I have a C++ server (a sort of logger, a bit like syslog) and many Java clients. Each Java client sends the server a text header record which I'll describe in a moment, and thereafter, user generated text "log" records. Every client write() (using the output stream obtained from the socket), is followed by a flush() call, to ensure that each write() is indeed carried out. The header record, a comma delimited list, contains a file name, where I want the server to throw out the subseqent stuff on a per client basis. It contains other fields not pertinent to this query. The server is built in "classic" fashion: A listener thread, assigns each client a file descriptor, and a multiplexed read thread (using select()) processes reads from the clients. I use standard pthreads (Write once, run many). When a Java client comes "on board", it first sends the header, upon which basis a log file is opened by the server on the client's behalf. All subsequent message from that client are recognized as log messages and written to the client's log file. When a client disconnects, the server detects it (as a zero length read buffer) and cleanly closes the file and the socket connection. I maintain a table of online client records, which uses standard pthread mutexes to avoid a few simple potential clashes. Now for the problem: When a client comes up, for some reason or other, the header and the first first few log records are aggregated by TCP, and appear on select() as a single read(). All subsequent client writes are picked up correctly. I can vary the number of initially aggregated records by reducing the select() timeout (but not setting it to zero!). The worst results are obtained, when I allow select() to block. If for example, I set the select() timeout to say, 1 sec and force the client to sleep 2 secs after issueing the header, I can make the problem "disappear", if for an obvious reason. 1. Why does TCP aggregate the first few records, and then "settle down" and behave as expected? 2. Any suggested "fix"? If it is felt that this is somewhat off-topic, please reply directly to me. References: Stevens, UNP Vol 1, and APUE, syslog sources (for an example).
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