select() doesn't respect SO_RCVLOWAT ?

From: Felix Matathias
Date: Thu Mar 10 2005 - 17:16:44 EST

Dear all,

I am running a 2.4.21-9.0.3.ELsmp #1 kernel and I can setsockopt and getsockopt correctly the SO_RCVLOWAT option, but select() seems to mark a socket readable even if a single byte is ready to be read. Then, a read() blocks until the specified number of bytes in SO_RCVLOWAT makes it to the socket buffer.

This is the exact opposite behaviour of what I yould have expected/desired. Our application receives data at many Khz rate and we want to avoid reading the socket until a predetermined amount of data is sent, to avoid partial reads. SO_RCVLOWAT seemed to be a nice way to implement that.

An earlier message by Alan Cox was a bit cryptic:

"But is the cost of all those special case checks and all the handling
for it such as select computing if enough tcp packets together accumulated
worth the cost on every app not using LOWAT for the microscopic gain given
that essentially nobody uses it."

Does this mean that select() in Linux will wake up no matter what SO_RCVLOWAT is set to ?

Best Regards,
Felix Matathias

P.S. I would appreciate if you could also cc your response to me.


Felix Matathias of Columbia University, Nevis Labs

Brookhaven National Lab cell : 631-988-3694
Bldg 1005, 3-304 web :
Upton, NY, 11973 photo:
tel/fax :631-344-7622/3253 email: felix@xxxxxxxxxxxxxxxxxx

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