DosWaitNmPipe() Behavior (59951)
This article was previously published under Q59951
SUMMARY
The QuickHelp entry for DosWaitNmPipe() says that the longest waiting
client will receive the first go-ahead. Gordon Letwin's "Inside OS/2"
says that the highest priority process will receive the first
go-ahead.
The following information discusses how WaitNmPipe() works when the
clients are on separate workstations on a network.
MORE INFORMATION
Suppose a server program has only one instance of a pipe available and
it is currently used by one client. Suppose there are other N clients
waiting at DosWaitNmPipe(). As soon as the pipe instance becomes
available, the (client) process with highest priority will return
first from DosWaitNmPipe(). If all waiting (client) processes have the
same priority, all such (client) processes will return from
DosWaitNmPipe() together in the (chronological) order they called
DosWaitNmPipe().
Once a pipe instance becomes available, ALL (client) processes/threads
waiting on DosWaitNmPipe() will return; each such process/thread will
return from DosWaitNmPipe() in the order of its priority and the time
of blocking on DosWaitNmPipe(). Thus, when all processes have the same
priority, the process waiting the longest will return from
DosWaitNmPipe() first.
Once a (client) process has returned successfully from
DosWaitNmPipe(), it still does not have the pipe instance until it
does a DosOpen() again.
For example, suppose all N clients have the same priority. All of the
clients will then return from DosWaitNmPipe() one by one. Then, the
first (client) process to call a DosOpen() will get the instance of
the pipe; the rest will get an ERROR_PIPE_BUSY. The rest (N-1) of the
clients must then wait again for the next available instance of the
pipe.
Usually, in cases such as this, the first (client) process scheduled
to run by the OS/2 scheduler (which is completely independent of the
pipe contention scenario above) will get the pipe also.
For remote clients, the behavior is similar. The first DosOpen()
request to reach the server over the net will get the newly available
pipe instance; the rest will get an ERROR_PIPE_BUSY response.
| Modification Type: |
Major |
Last Reviewed: |
7/30/2001 |
| Keywords: |
KB59951 |
|