Knowledge Base

There are several SCADA BASIC functions that use buffers (e.g. SVTREND and SELECTOR).

Before using any of these functions, it is necessary to allocate a buffer (i.e. a block of memory) with ALLOC_BUFFER.

ALLOC_BUFFER returns a handle that is used subsequently for referencing the reserved memory area.

Problem

An important issue in programming is that of freeing all the memory that has been reserved. Otherwise, the application occupies excess memory which may cause erroneous operation of the operating system.

You must check that the logic of your program includes provision to free its allocated buffers in all execution conditions.

The verb FREE_BUFFER is used to free an allocated buffer.

Sometimes buffers are created directly by a SCADA BASIC function (e.g. FILETOBUF). It is still necessary then to free those buffers.

You must not allocate a handle before executing the function FILETOBUF otherwise two buffers are created by PcVue.

Example

This example allocates a buffer, then frees it after use if it is still allocated:

Buf = FILETOBUF( file );
…
if (Buf != 0) then

FREE_BUFFER( Buf );
Buf = 0;

End if

In the example, if the file does not exist then buf is null (= 0). When FREE_BUFFER is used on a null handle, it generates an error.

You must check that the handle is not null before trying to free the buffer and setting it to 0 as shown above.

To check whether your application is dealing correctly with buffers, you can examine the PcVue log files called Audit.Counters.xxxx.txt located in the PcVue\Bin\Log Files\ folder.

In those files, towards the end of file, a line shows:

[ ALLOC_BUFFER count ] Current = xxx Cumulative = yyy Current allocated memory = zzz

The number of current buffers xxx should be kept below 20, depending on the application. If the number is increasing it means that buffers are being allocated but not being released.