STREAMS Modules bufmod(7M)
NAME
bufmod - STREAMS Buffer Module
SYNOPSIS
#include
ioctl(fd, I_PUSH, "bufmod");
DESCRIPTION
bufmod is a STREAMS module that buffers incoming messages,
reducing the number of system calls and the associated over-
head required to read and process them. Although bufmod was
originally designed to be used in conjunction with STREAMS-
based networking device drivers, the version described here is general purpose so that it can be used anywhere STREAMS input buffering is required.Read-side Behavior
The behavior of bufmod depends on various parameters and
flags that can be set and queried as described below underIOCTLS. bufmod collects incoming M_DATA messages into
chunks, passing each chunk upstream when the chunk becomesfull or the current read timeout expires. It optionally con-
verts M_PROTO messages to M_DATA and adds them to chunks as
well. It also optionally adds to each message a header con-
taining a timestamp, and a cumulative count of messages dropped on the stream read side due to resource exhaustionor flow control. Thedefault settings of bufmod allow it to
drop messages when flow control sets in or resources are exhausted; disabling headers and explicitly requesting nodrops makes bufmod pass all messages through. Finally, buf-
mod is capable of truncating upstream messages to a fixed, programmable length.When a message arrives, bufmod processes it in several
steps. The following paragraphs discuss each step in turn.Upon receiving a message from below, if the SB_NO_HEADER
flag is not set, bufmod immediately timestamps it and saves
the current time value for later insertion in the header described below.Next, if SB_NO_PROTO_CVT is not set, bufmod converts all
leading M_PROTO blocks in the message to M_DATA blocks,
altering only the message type field and leaving the con-
tents alone.SunOS 5.11 Last change: 11 Nov 1997 1
STREAMS Modules bufmod(7M)
It then truncates the message to the current snapshot length, which is set with the SBIOCSSNAP ioctl described below.Afterwards, if SB_NO_HEADER is not set, bufmod prepends a
header to the converted message. This header is defined as follows.struct sb_hdr {
uint_t sbh_origlen;
uint_t sbh_msglen;
uint_t sbh_totlen;
uint_t sbh_drops;
#if defined(_LP64) || defined(_I32LPx)
struct timeval32 sbh_timestamp;
#else
struct timeval sbh_timestamp;
#endif /* !_LP64 */
};The sbh_origlen field gives the message's original length
before truncation in bytes. The sbh_msglen field gives the
length in bytes of the message after the truncation has beendone. sbh_totlen gives the distance in bytes from the start
of the truncated message in the current chunk (described below) to the start of the next message in the chunk; the value reflects any padding necessary to insure correct data alignment for the host machine and includes the length ofthe header itself. sbh_drops reports the cumulative number
of input messages that this instance of bufmod has dropped
due to flow control or resource exhaustion. In the current implementation message dropping due to flow control canoccur only if the SB_NO_DROPS flag is not set. (Note: this
accounts only for events occurring within bufmod, and does
not count messages dropped by downstream or by upstreammodules.) The sbh_timestamp field contains the message
arrival time expressed as a struct timeval.After preparing a message, bufmod attempts to add it to the
end of the current chunk, using the chunk size and timeout values to govern the addition. The chunk size and timeout values are set and inspected using the ioctl() calls described below. If adding the new message would make thecurrent chunk grow larger than the chunk size, bufmod closes
off the current chunk, passing it up to the next module in line, and starts a new chunk. If adding the message would still make the new chunk overflow, the module passes itupward in an over-size chunk of its own. Otherwise, the
SunOS 5.11 Last change: 11 Nov 1997 2
STREAMS Modules bufmod(7M)
module concatenates the message to the end of the current chunk.To ensure that messages do not languish forever in an accu-
mulating chunk, bufmod maintains a read timeout. Whenever
this timeout expires, the module closes off the current chunk and passes it upward. The module restarts the timeout period when it receives a read side data message and a timeout is not currently active. These two rules insure thatbufmod minimizes the number of chunks it produces during
periods of intense message activity and that it periodically disposes of all messages during slack intervals, but avoids any timeout overhead when there is no activity.bufmod handles other message types as follows. Upon receiv-
ing an M_FLUSH message specifying that the read queue be
flushed, the module clears the currently accumulating chunk and passes the message on to the module or driver above.(Note: bufmod uses zero length M_CTL messages for internal
synchronization and does not pass them through.) bufmod
passes all other messages through unaltered to its upper neighbor, maintaining message order for non high priority messages by passing up any accumulated chunk first.If the SB_DEFER_CHUNK flag is set, buffering does not begin
until the second message is received within the timeout win-
dow.If the SB_SEND_ON_WRITE flag is set, bufmod passes up the
read side any buffered data when a message is received onthe write side. SB_SEND_ON_WRITE and SB_DEFER_CHUNK are
often used together.Write-side Behavior
bufmod intercepts M_IOCTL messages for the ioctls described
below. The module passes all other messages through unal-
tered to its lower neighbor. If SB_SEND_ON_WRITE is set,
message arrival on the writer side suffices to close and transmit the current read side chunk. IOCTLSbufmod responds to the following ioctls.
SBIOCSTIME Set the read timeout value to the value referred to by the struct timeval pointer given as argument. Setting the timeout valueto zero has the side-effect of forcing the
chunk size to zero as well, so that theSunOS 5.11 Last change: 11 Nov 1997 3
STREAMS Modules bufmod(7M)
module will pass all incoming messages upward immediately upon arrival. Negative values are rejected with an EINVAL error. SBIOCGTIME Return the read timeout in the struct timeval pointed to by the argument. If the timeout has been cleared with the SBIOCCTIME ioctl, return with an ERANGE error. SBIOCCTIME Clear the read timeout, effectively setting its value to infinity. This results in no timeouts being active and the chunk being delivered when it is full. SBIOCSCHUNK Set the chunk size to the value referred toby the uint_t pointer given as argument. See
Notes for a description of effect on stream head high water mark.SBIOCGCHUNK Return the chunk size in the uint_t pointed
to by the argument. SBIOCSSNAP Set the current snapshot length to the valuegiven in the uint_t pointed to by the
ioctl's final argument. bufmod interprets a
snapshot length value of zero as meaning infinity, so it will not alter the message. See Notes for a description of effect on stream head high water mark. SBIOCGSNAP Returns the current snapshot length in theuint_t pointed to by the ioctl's final argu-
ment. SBIOCSFLAGS Set the current flags to the value given inthe uint_t pointed to by the ioctl's final
argument. Possible values are a combination of the following.SB_SEND_ON_WRITE Transmit the read side
chunk on arrival of a message on the write side.SunOS 5.11 Last change: 11 Nov 1997 4
STREAMS Modules bufmod(7M)
SB_NO_HEADER Do not add headers to
read side messages.SB_NO_DROPS Do not drop messages
due to flow control upstream.SB_NO_PROTO_CVT Do not convert M_PROTO
messages into M_DATA.
SB_DEFER_CHUNK Begin buffering on
arrival of the second read side message in a timeout interval.SBIOCGFLAGS Returns the current flags in the uint_t
pointed to by the ioctl's final argument.SEE ALSO
dlpi(7P), pfmod(7M) NOTESOlder versions of bufmod did not support the behavioral
flexibility controlled by the SBIOCSFLAGS ioctl. Applica-
tions that wish to take advantage of this flexibility can guard themselves against old versions of the module by invoking the SBIOCGFLAGS ioctl and checking for an EINVAL error return. When buffering is enabled by issuing an SBIOCSCHUNK ioctl toset the chunk size to a non zero value, bufmod sends a
SETOPTS message to adjust the stream head high and low water marks to accommodate the chunked messages. When buffering is disabled by setting the chunk size to zero, message truncation can have a significant influence on data traffic at the stream head and therefore the stream head high and low water marks are adjusted to new values appropriate for the smaller truncated message sizes.BUGS
bufmod does not defend itself against allocation failures,
so that it is possible, although very unlikely, for the stream head to use inappropriate high and low water marksSunOS 5.11 Last change: 11 Nov 1997 5
STREAMS Modules bufmod(7M)
after the chunk size or snapshot length have changed.SunOS 5.11 Last change: 11 Nov 1997 6