ifndef Statement-- Process a Set of Statements without Variables or Logicals
Processes a set of statements if a $variable or $$logical is not defined.
The statements within the ifndef-endif block are executed only if $$logical or $variable has not been defined. Statements in the optional else block are executed if the $$logical or $variable has been defined. A $variable can be defined without a value. In this case, the statements in the ifndef block are not executed, because the $variable is defined.
This statement has the following syntax:
##ifndef $$logical | $variable {statement}
[## else {statement}]
## endif
Example--ifndef statement:
##ifndef $mode
/* Caution: '$mode' has not been defined */
##else
/* The value of '$mode' is $mode */
##endif