B.8 Declaring an OpenVMS Condition Handler

This section discusses a restriction on declaring an OpenVMS condition handler while using DECthreads exceptions and DECthreads behavior when a condition is signalled.

The following are three ways to declare an OpenVMS condition handler:

• Calling VAXC$ESTABLISH (from a program written in C)

• Calling LIB$ESTABLISH

• Placing the address of the condition handler directly into the stack frame (from a program written in VAX MACRO or VAX BLISS)

Do not declare an OpenVMS condition handler within a DECthreads TRY/ENDTRY exception block. Doing so deletes without notification any handler that exists for the current procedure. If your code declares a condition handler within the TRY/ENDTRY block, DECthreads exceptions will not be handled correctly until the next TRY statement is executed. The TRY statement restores the DECthreads condition handler.

On OpenVMS VAX, you can declare a condition handler outside of a TRY/ENDTRY block with no restrictions. If a condition handler has already been declared when you execute a TRY statement, DECthreads saves the previous handler address. When DECthreads receives a condition it does not handle (including SS$_UNWIND, SS$_DEBUG or a condition code that does not have a SEVERE severity), DECthreads invokes the saved condition handler. The condition handler will be re-established when the TRY block exits.