HP-UX SNAplus2 CPI-C Programmer's Guide (June 2003)

Writing CPI-C Applications

HP-UX Considerations

Chapter 2 97

NOTE

1. In general, SNA_GET_FD() returns the same ﬁle descriptor each time

you call it. However, you should call it before each call to select() or

poll(), because there are some situations (such as when your

application has issued the fork() call, or in error cases) when the

ﬁle descriptor may change.

2. The SNA event handler processes internal control messages between

SNAplus2 components, in addition to processing the completion of

CPI-C functions. In addition, if your application uses multiple

conversations and there are multiple functions outstanding on

different conversations, the SNA event handler may report the

completion of any of these functions (because the same ﬁle descriptor

is used for all conversations).

Because of this, you cannot assume that an event on the SNA ﬁle

descriptor corresponds to the completion of a particular CPI-C

function. Check_For_Completion is the correct method of checking

which function, if any, has completed. If Check_For_Completion

indicates that a function has completed, the application can then call

Wait_For_Conversation to obtain the results of the function.

3. If you are familiar with using application scheduling mode on other

SNAplus2 APIs, where the application calls the SNA event handler

using SNA_EVENT_FD(), note that the Check_For_Completion

function includes a call to SNA_EVENT_FD() and so you do not need to

call SNA_EVENT_FD() directly from a CPI-C application.

Signal-Based Scheduling Mode

In signal-based scheduling mode, the CPI-C library uses the SIGPOLL

signal to detect work arriving using the ﬁle descriptor.

Applications can require the SIGPOLL() signal for other purposes.

Therefore, the library daisy-chains to any existing signal catchers. The

ﬁrst verb issued by the application initializes the library's signal catcher.

When the SIGPOLL() signal arrives, either it indicates work for

SNAplus2 or it was generated for some other reason. The CPI-C library's

signal catcher processes the signal and then calls the previous signal

catcher to enable the signal to be used for other purposes.