ACSE/Presentation and ROSE Interface Programmer's Guide

56 Chapter 4

Programming Guide

4. Using Synchronous vs. Asynchronous Mode

4. Using Synchronous vs. Asynchronous

Mode

ACSE/Presentation supports both synchronous and asynchronous mode

execution of calls. In synchronous mode, an ACSE/Presentation call

blocks until the call can be completed. While blocked, no other tasks can

be performed. Synchronous mode is the default.

In asynchronous mode, an ACSE/Presentation call is not blocked (except

under kernel resource shortages). The function completes as much of its

task as it can, then returns to the user.

Synchronous Mode

Synchronous mode is useful for processes maintaining a single

connection, and for processes that can wait for events to occur.

When synchronous mode is used, ap_snd() blocks until resources are

available to send the entire primitive. Similarly, ap_rcv() blocks until

either an entire primitive is received, or the user buffer is ﬁlled by the

library. In the latter case, ap_rcv() returns with the AP_MORE bit set in

the ﬂags argument. To receive the remaining data, the application must

continue to call ap_rcv() until it returns with the AP_MORE bit reset.

Asynchronous Mode

Asynchronous mode is used for applications in which long delays are

expected between events, and for processes that can perform other tasks

while waiting for events to happen. Asynchronous mode is also useful for

managing multiple connections concurrently.

Asynchronous mode is speciﬁed in ap_open() by setting the O_NDELAY

bit in the oﬂags parameter. When used asynchronously, ap_snd() and

ap_rcv() do not block in most cases.

If there are insufﬁcient resources from OTS to send the entire primitive

for an ap_snd(), a partial send can occur. A return code of -1 is received

and ap_errno is set to AP_AGAIN. To complete sending the primitive,

ap_snd() must be invoked again with the same set of buffers and

arguments until the call returns successfully.