The ifdef, ifndef, elif, else, and endif directives

The Informix® ESQL/C preprocessor supports the following directives for conditional compilation:

ifdef: Tests a name and executes subsequent statements if define has created the name.
ifndef: Tests a name and executes subsequent statements if define has not created the name.
elif: Begins an alternative section to an ifdef or ifndef condition and checks for the presence of another define. It is shorthand for “else if define ”.
else: Begins an alternative section to an ifdef or ifndef condition.
endif: Closes an ifdef or ifndef condition.

In the following example, the BEGIN WORK statement compiles only if you previously defined the name USETRANSACTIONS with a define directive:

EXEC SQL ifdef USETRANSACTIONS;
EXEC SQL begin work;
EXEC SQL endif;

The following example illustrates the use of the elif statement. This sample code will print “USETRANSACTIONS defined”.

EXEC SQL define USETRANSACTIONS;
   
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   EXEC SQL ifndef USETRANSACTIONS;
      printf(“USETRANSACTIONS not defined”);
   EXEC SQL elif USETRANSACTIONS;
      printf(“USETRANSACTIONS defined”);
   EXEC SQL endif;

The Informix ESQL/C preprocessor does not support a general if directive; it supports only the ifdef and ifndef statements that test whether a name was defined with define.

The Informix ESQL/C preprocessor processes conditional compilation definitions in stage 1 of the preprocessing.