Preprocessor Directives
| PostgreSQL 9.6.13 Documentation | |||
|---|---|---|---|
| Prev | Up | Chapter 34. ECPG - Embedded SQL in C | Next |
Several preprocessor directives are available that modify how the ecpg preprocessor parses and processes a file.
34.9.1. 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.
34.9.2. 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.
34.9.3. 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;