PIPE(3) PIPE(3)
NAME
pipe - two-way interprocess communication
SYNOPSIS
bind '#|'dir
dir/data
dir/data1
DESCRIPTION
A pipe provides a mechanism for interprocess I/O by reading
and writing file descriptors (see sys-read(2)). An
attach(5), typically via sys-pipe(2) or sys-bind(2), allo-
cates two files that are cross-connected: data written to
one can be read back from the other, in the same order.
Write boundaries are preserved: each read terminates when
the read buffer is full or after reading the last byte of a
write, whichever comes first. In particular, a write of
zero bytes will result in a zero-length read, which is usu-
ally interpreted by readers as end-of-file, but could be
used to delimit the data stream for other purposes.
Written data is buffered by the kernel and stored on inter-
nal queues (see qio(10.2)). The maximum block size is 128k
bytes; larger writes will be split across several blocks,
which are queued separately. Each read will return data
from at most one block. Concurrent writers are therefore
guaranteed that their data will not be interleaved with data
from other writers (ie, will be written atomically) only
when each write is less than the maximum buffer size. Writ-
ers to pipe interfaces on remotely mounted portions of the
namespace have their guarantee of atomicity lowered to
Sys->ATOMICIO bytes by mnt(3).
The system mediates between producer and consumer. Writers
will block when buffered data reaches a high-water mark,
currently 32k bytes, until a reader has reduced it by half.
The length returned by sys-stat(2) on each name gives the
number of bytes waiting to be read on the corresponding end
of the pipe.
When all file descriptors on one side of the pipe have been
closed, and after any remaining data has been read, a reader
on the other side sees end-of-file (count of zero) on a sub-
sequent read. Once both ends are closed, the pipe can be
reused.
A pipe persists until it is unmounted and no processes have
either end open.
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PIPE(3) PIPE(3)
SOURCE
/emu/port/devpipe.c
/os/port/devpipe.c
SEE ALSO
sys-file2chan(2), sys-pipe(2)
DIAGNOSTICS
Writes to a closed pipe generate an exception `write on
closed pipe'. Persistently reading a closed pipe after
reading end-of-file will result in a read error.
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