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The SAC creates two environment variables for each port monitor it starts:
The SAC communicates with port monitors through FIFOs. A port monitor should open _pmpipe, in the current directory, to receive messages from the SAC and ../_sacpipe to send return messages to the SAC.
``SAC messages'' describes the messages that may be sent from the SAC to a port monitor (SAC messages), and from a port monitor to the SAC (port monitor messages). These messages are sent through FIFOs and are in the form of C structures (see ``The sac.h header file'').
The format of messages from the SAC is defined by the structure sacmsg:
struct sacmsg { int sc_size; /* size of optional data portion */ char sc_type; /* type of message */ };The SAC may send four types of messages to port monitors. The type of message is indicated by setting the
sc_type
field of the sacmsg structure to one of the following:
sc_size
indicates the size of the optional data part of the message.
For System V Release 4, sc_size
should always be set to 0.
It is discussed in
``Message classes''.
A port monitor must respond to every message sent by the SAC.
The format of messages from a port monitor to the SAC is defined by the structure pmmsg:
struct pmmsg { char pm_type; /* type of message */ unchar pm_state; /* current state of port monitor */ char pm_maxclass; /* maximum message class this port monitor understands */ char pm_tag[PMTAGSIZE + 1]; /* port monitor's tag */ int pm_size; /* size of optional data portion */ };Port monitors may send two types of messages to the SAC. The type of message is indicated by setting the
pm_type
field
of the pmmsg structure
to one of the following:
pm_tag
field is set to the port monitor's tag
and the pm_state
field is set to the port monitor's
current state.
Valid states are:
The status message is the normal return message. The negative acknowledgment should be sent only when the message received is not understood.
pm_size
indicates the size of the optional data part of the message.
pm_maxclass
is used to specify a message class.
Both are discussed in
``Message classes''.
For System V Release 4, pm_maxclass
should always be set to 1 and
sc_size
should always be set to 0.
Port monitors may never initiate messages; they may only respond to messages that they receive.
The concept of a message class has been included to accommodate possible SAF extensions. The messages described above are all class 1 messages. None of these messages contains a variable data portion; all pertinent information is contained in the message header.
If new messages are added to the protocol, they
will be defined as new message classes (for example, class 2).
The first message the SAC sends to a port monitor will
always be a class 1 message.
Since all port monitors, by definition, understand class 1 messages,
the first message the SAC sends is guaranteed to be understood.
In its response to the SAC,
the port monitor sets the pm_maxclass
field
to the maximum message class number for that
port monitor.
The SAC will not send messages to a port monitor
from a class with a larger number than the value of pm_maxclass
.
Requests that require messages of a higher class than the port
monitor can understand will fail.
For System V Release 4, pm_maxclass
should always be set to 1.
pm_maxclass
and messages of all classes with values lower than
pm_maxclass
are valid.
Thus, if the pm_maxclass
field is set to 3, the port
monitor understands messages of classes 1, 2, and 3.
Port monitors may not generate messages; they may only respond to messages. A port monitor's response must be of the same class as the originating message.
Since only the SAC can generate messages, this protocol will function even if the port monitor is capable of dealing with messages of a higher class than the SAC can generate.
pm_size
(an element of the pmmsg structure) and sc_size
(an element of the sacmsg structure)
show the size of the optional data part of the message.
The format of this part of the message is undefined.
Its definition is inherent in the type of message.
For System V Release 4, both sc_size
and pm_size
should always be set to 0.