linux-kernel - splice() on two pipes

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Message-ID: <20090429103308.GA23684@squirrel.roonstrasse.net>
Date:	Wed, 29 Apr 2009 12:33:08 +0200
From:	Max Kellermann <max@...mpel.org>
To:	linux-kernel@...r.kernel.org
Subject: splice() on two pipes

Hi,

when I read about the splice() system call, I thought it was obvious
that it could copy data between two pipes.  I was surprised that this
assumption is wrong, it's not possible on 2.6.29, I get EINVAL.  Can
anybody please explain this limitation?

Background: I want to forward data between two subprocesses, which are
connected to me with a pipe().

I have attached a small test program which prints a table of supported
splice operations.  Here's the output on 2.6.29.1:

 in\out  pipe    sock    reg     chr
 pipe    no      yes     yes     yes
 sock    no      no      no      no
 reg     yes     no      no      no
 chr     no      no      no      no

Max

View attachment "test_splice.c" of type "text/x-csrc" (2972 bytes)