Several preprocessor directives are available that modify how
the ecpg(Oracle Pro*c compatible) preprocessor parses and processes a
file.
To include an external file into your embedded SQL program, use:
EXEC SQL INCLUDEfilename; EXEC SQL INCLUDE <filename>; EXEC SQL INCLUDE "filename";
The embedded SQL preprocessor will look for a file named
filename.h
The ecpg(Oracle Pro*c compatible) preprocessor will search a file at
several directories in following order:
/usr/local/include/usr/local/lightdb-x/include)/usr/include
But when EXEC SQL INCLUDE
" is used, only the
current directory is searched.
filename"
In each directory, the preprocessor will first look for the file
name as given, and if not found will append .h
to the file name and try again (unless the specified file name
already has that suffix).
Note that EXEC SQL INCLUDE is not the same as:
#include <filename.h>
because this file would not be subject to SQL command preprocessing.
Naturally, you can continue to use the C
#include directive to include other header
files.
The include file name is case-sensitive, even though the rest of
the EXEC SQL INCLUDE command follows the normal
SQL case-sensitivity rules.
Similar to the directive #define that is known from C,
embedded SQL has a similar concept:
EXEC SQL DEFINEname; EXEC SQL DEFINEnamevalue;
So you can define a name:
EXEC SQL DEFINE HAVE_FEATURE;
And you can also define constants:
EXEC SQL DEFINE MYNUMBER 12; EXEC SQL DEFINE MYSTRING 'abc';
Use undef to remove a previous definition:
EXEC SQL UNDEF MYNUMBER;
Of course you can continue to use the C versions #define
and #undef in your embedded SQL program. The difference
is where your defined values get evaluated. If you use EXEC SQL
DEFINE then the ecpg(Oracle Pro*c compatible) preprocessor evaluates the defines and substitutes
the values. For example if you write:
EXEC SQL DEFINE MYNUMBER 12; ... EXEC SQL UPDATE Tbl SET col = MYNUMBER;
then ecpg(Oracle Pro*c compatible) will already do the substitution and your C compiler will never
see any name or identifier MYNUMBER. Note that you cannot use
#define for a constant that you are going to use in an
embedded SQL query because in this case the embedded SQL precompiler is not
able to see this declaration.
You can use the following directives to compile code sections conditionally:
EXEC SQL ifdef name;
Checks a name and processes subsequent lines if
name has been defined via EXEC SQL define
.
name
EXEC SQL ifndef name;
Checks a name and processes subsequent lines if
name has not been defined via
EXEC SQL define .
name
EXEC SQL elif name;
Begins an optional alternative section after an
EXEC SQL ifdef or
nameEXEC SQL ifndef
directive. Any number of nameelif sections can appear.
Lines following an elif will be processed
if name has been
defined and no previous section of the same
ifdef/ifndef...endif
construct has been processed.
EXEC SQL else;
Begins an optional, final alternative section after an
EXEC SQL ifdef or
nameEXEC SQL ifndef
directive. Subsequent lines will be processed if no previous section
of the same
nameifdef/ifndef...endif
construct has been processed.
EXEC SQL endif;
Ends an
ifdef/ifndef...endif
construct. Subsequent lines are processed normally.
ifdef/ifndef...endif
constructs can be nested, up to 127 levels deep.
This example will compile exactly one of the three SET
TIMEZONE commands:
EXEC SQL ifdef TZVAR; EXEC SQL SET TIMEZONE TO TZVAR; EXEC SQL elif TZNAME; EXEC SQL SET TIMEZONE TO TZNAME; EXEC SQL else; EXEC SQL SET TIMEZONE TO 'GMT'; EXEC SQL endif;
By the way, ecpg does not parse code in C preprocessor directive '#if 0' block.
#if 0 EXEC SQL I AM NOT PARSED BY ECPG #endif