[TUHS] Excise process from a pipe

Doug McIlroy doug at cs.dartmouth.edu
Tue Jul 15 00:13:06 AEST 2014

Larry wrote in separate emails

> If you really think that this could be done I'd suggest trying to
> write the man page for the call.

> I already claimed splice(2) back in 1998; the Linux guys did
> implement part of it ...

I began to write the following spec without knowing that Linux had
appropriated the name "splice" for a capability that was in DTSS
over 40 years ago under a more accurate name, "copy". The spec
below isn't hard: just hook two buffer chains together and twiddle
a couple of file desciptors.  For stdio, of course, one would need
fsplice(3), which must flush the in-process buffers--penance for
stdio's original sin of said buffering.

Incidentally, the question is not abstract. I have code that takes
quadratic time because it grows a pipeline of length proportional
to the input, though only a bounded number of the processes are
usefully active at any one time; the rest are cats. Splicing out
the cats would make it linear. Linear approaches that don't need
splice are not nearly as clean.




int splice(int fd0, int fd1);


Splice connects the source for a reading file descriptor fd0
directly to the destination for a writing file descriptor fd1
and closes both fd0 and fd1. Either the source or the destination
must be another process (via a pipe). Data buffered for fd0 at
the time of splicing follows such data for fd1. If both source
and destination are processes, they become connected by a pipe. If
the source (destination) is a process, the file descriptor
in that process becomes write-only (read-only).

If file descriptor fd0 is associated with a pipe and fd1 is not,
then fd1 is updated to reflect the effect of buffered data for fd0,
and the pipe's other descriptor is replaced with a duplicate of fd1.
The same statement holds when "fd0" is exchanged with "fd1" and
"write" is exchanged with "read".

Splice's effect on any file descriptor propagates to shared file
descriptors in all processes.


One file must be a pipe lest the spliced data stream have no
controlling process. It might seem that a socket would suffice,
ceding control to a remote system; but that would allow the
uncontrolled connection file-socket-socket-file.

The provision about a file descriptor becoming either write-only or
read-only sidesteps complications due to read-write file descriptors.

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