fifo.9

7号信令功能代码,为开源代码

.\"
.\" Copyright (C) 2000  John A. Boyd Jr.  protologos, LLC
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.\" Library General Public License for more details.
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.TH FIFO 9 "24 Apr 2000" "LiS 2.9" "Linux STREAMS"
.SH NAME
.B fifo
\- pseudo-driver for
.BR STREAMS "\-based"
.I fifos
.SH DESCRIPTION
The
.B fifo
pseudo-driver
(which is internal to LiS) provides
.BR STREAMS "\-based" 
.I fifos
as single character special files, and
.BR STREAMS "\-based" 
.I pipes
as pairs of character special files which are interconnected (see
.BR pipe "(3)).  "
.sp
.BR STREAMS "\-based"
.I fifos
differ from typical
.BR STREAMS "\-based"
character special files in that there are not separate
.I "stream head"
and
.I driver
queue pair within the
.BR STREAMS "\-based"
file.  Instead, a
.I fifo
is created with only a single queue pair for the
.IR "stream head" .
Moreover, in a typical
.I driver
queue pair, the write queue is not connected to a next queue.
In a
.IR fifo ,
the write queue is directed to the read queue of the pair.
A
.I pipe
comprises a pair of
.IR fifos ", "
with the write queue of each pair directed to the read queue of the other.
The two
.B fifos
comprising a
.B pipe
are referred to as
.IR peers ", "
and each somewhat represents a
.I driver
to the other.
As a degenerate case, a
.B fifo
is its own peer.
.sp
.B STREAMS
.I modules
may be pushed onto
.B fifos
and
.BR pipes ", "
but should not expect a
.I driver
below them; instead, the
.BR SAMESTR "()"
function should be used from the write queue of a pair to determine if
the module is the lowest in the
.BR STREAMS "\-based"
file (this is called the
.IR midpoint "). "
The structure of a
.B fifo
or
.B pipe
is preserved when modules are pushed (and popped); i.e., the write
queue at the
.I midpoint
will always be directed at the read queue of the
.IR peer "."
.sp
Input and output are handled at a
.B fifo
stream head as they would normally be handled
at a stream head.  In LiS, an
.BR "fifo open" "() "
entry point exists to assign minor device numbers to new opens under the
.B fifo
major device number, and a
.BR close "() "
entry point is used correspondingly to release them.  These functions
are kept in a
.I streamtab
data structure (as they would normally be for any
.B STREAMS
driver or module) which is private to the LiS implementation.
.SH "APPLICATION USAGE"
In the current Linux kernels, character special major numbers are limited
to 16 bits, and major and minor device numbers to 8 bis each.  This limits
a system to 256 total major device numbers and 256 total minor devices
per major device number.  This is a rather severe limitation where
mechanisms like
.B fifos
and
.B pipes
are concerned.
.sp
However, a driver may handle more than one major device number.  The
.B fifo
pseudo-driver uses this to overcome this limitation, by supporting the
automatic allocation and use of multiple major device numbers for
.B fifos
and
.BR pipes .
Specifying more than 256 minor devices is done in the usual manner, i.e.,
by specifying the number of "units" in the appropriate
.B Config
file.  Enough major device numbers will be allocated to cover the
requested number of minor devices (if available, else an error will
occur in 
.BR strconf "(8)). "
The number allocated will include one minor device per major number
to be used as a
.BR fifo "\-specific"
.I clone
minor device (specifically, minor number 0), which exhibits special behavior.
.sp
Normally, when cloning is done via the
.B clone
pseudo-driver, the
.B clone
major device number is used, along with the desired actual major number
as the minor device number.  When an
.BR open "() "
is performed on such a device, the
.BR "clone open" "() "
routine in turn calls the appropriate driver's
.BR open "(), "
with the
.I sflag
parameter set to
.BR CLONEOPEN .
The driver's
.BR open "() "
is expected in this case to allocate an unused minor device number,
and return it via an entirely new device number in the
.I "devp" 
parameter.  In this way, a driver can change the device number to be
used for a
.BR STREAMS "\-based"
file.
When minor device 0 for a specified for a
.B fifo
major device, the driver will also clone a new minor device number.
However, LiS opens
.B fifo
devices differently; specifically, when an already-opened
.BR fifo "\-specific"
clone minor device is reopened, the new and subsequent opens will use
the already-opened clone.
Thus, using minor device 0 for a
.B fifo
when creating a file system node will ensure that all concurrent
opens of the associated path name will use the same
.BR STREAMS "\-based"
file; at the same time, opens of different file system nodes via
different paths will open their respectively different
.BR STREAMS "\-based"
files.
This is essentially how kernel-based
.B fifos
behave -applications and users of
.BR STREAMS "\-based"
.B fifos
don't have to keep track of minor numbers to achieve this same behavior
when it is desired.
.sp
It is in fact recommended that only two forms of file system nodes be used for
.BR STREAMS "\-based " fifos ": "
the
.B clone
major number as major number with a
.B fifo
major number as minor number, to be used when
.I every
open of the associated path must clone a new
.BR fifo ", "
and a
.B fifo
major number as major number with 0 as the minor number, to be used
when new opens are to clone a new
.B fifo
but subsequent concurrent opens are to use the already opened
.BR fifo ". "
These are represented by two device special file paths created when LiS
is installed:
.RB "/" dev "/" fifo
for the former, and
.RB "/" dev "/" fifo.0
for the latter.  It is recommended that these be used, possibly
along with the equivalent of
.BR stat "(2) "
to determine appropriate major device numbers for the
.B clone
and
.B fifo
pseudo-drivers, which are also determined when LiS is installed.
It can be noted that
.B pipes
are actually created as instances of the former, after which the write
queues are peer-connected.
.sp
The
.B fifo
pseudo-driver allocates minor devices in
.I round-robin
fashion; i.e., a list of available minor devices is kept, and once a
minor number is finally closed, it is put at the end of this list.
Thus, a
.B fifo
minor device which is opened and closed will not be immediately reused.
.SH WARNINGS
Because
.BR STREAMS "\-based"
.B fifos
and
.B pipes
are implemented as character special devices, they do not appear as
.B pipe
devices when examined with
.BR stat "(2) "
or the equivalent (e.g., 
.BR ls "(1)); "
i.e. the
.B S_IFIFO
indication is not set in the mode -
.B S_IFCHR
is set instead, and the actual device number is indicated in the
.I st_rdev
field of the
.I stat
data structure.
.sp
Because of the potential use of multiple major numbers, applications
should not depend on a
.B fifo
or
.B pipe
having a specific major device number, nor should an application depend on
all
.B fifos
and
.B pipes
having the same major device number.
.SH "SEE ALSO"
.BR clone "(9), "
.BR connld "(9), "
.BR fifo "(4), "
.BR ls "(1), "
.BR pipe "(3), "
.BR pipemod "(9), "
.BR STREAMS "(4), "
.BR stat "(2), "
.BR strconf "(8)"
.SH HISTORY
Unix System V Release 4 (SVR4)
.SH BUGS
Please report bugs (and/or comments and suggestions) to the LiS mailing list.
.SH AUTHOR
John Boyd, protologos LLC.