PostgreSQL
35.9. Preprocessor Directives
Several preprocessor directives are available that modify how the ecpg
preprocessor parses and processes a file.
35.9.1. Including Files
To include an external file into your embedded SQL program, use:
EXEC SQL INCLUDE filename;
EXEC SQL INCLUDE <filename>;
EXEC SQL INCLUDE "filename";
The embedded SQL preprocessor will look for a file named `filename.h`, preprocess it, and include it in the resulting C output. Thus, embedded SQL statements in the included file are handled correctly.
The ecpg
preprocessor will search a file at several directories in following order:
-
current directory
-
/usr/local/include
-
PostgreSQL include directory, defined at build time (e.g.,
/usr/local/pgsql/include
) -
/usr/include
But when EXEC SQL INCLUDE "`
filename"` is used, only the current directory is searched.
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.
Note
The include file name is case-sensitive, even though the rest of the EXEC SQL INCLUDE
command follows the normal SQL case-sensitivity rules.
35.9.2. The define and undef Directives
Similar to the directive #define
that is known from C, embedded SQL has a similar concept:
EXEC SQL DEFINE name;
EXEC SQL DEFINE name value;
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
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
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.
35.9.3. ifdef, ifndef, else, elif, and endif Directives
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 created with `EXEC SQL define +`_`+name`_.
EXEC SQL ifndef +`_`+name`_
;`-
Checks a `name
and processes subsequent lines if name` has not been created with `EXEC SQL define +`_`+name`_.
EXEC SQL else;
-
Starts processing an alternative section to a section introduced by either `EXEC SQL ifdef +`_`+name`_ or `EXEC SQL ifndef +`_`+name`_.
EXEC SQL elif +`_`+name`_
;`-
Checks `name
and starts an alternative section if name` has been created with `EXEC SQL define +`_`+name`_.
EXEC SQL endif;
-
Ends an alternative section.
Example:
EXEC SQL ifndef TZVAR;
EXEC SQL SET TIMEZONE TO 'GMT';
EXEC SQL elif TZNAME;
EXEC SQL SET TIMEZONE TO TZNAME;
EXEC SQL else;
EXEC SQL SET TIMEZONE TO TZVAR;
EXEC SQL endif;
Prev | Up | Next |
---|---|---|
35.8. Error Handling |
35.10. Processing Embedded SQL Programs |
Copyright © 1996-2023 The PostgreSQL Global Development Group