36.9. Preprocessor Directives

36.9. Preprocessor Directives
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36.9. Preprocessor Directives #

36.9.1. Including Files
36.9.2. The define and undef Directives
36.9.3. ifdef, ifndef, elif, else, and endif Directives

+ Several preprocessor directives are available that modify how + the ecpg preprocessor parses and processes a + file. +

36.9.1. 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. +

36.9.2. 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. +

36.9.3. ifdef, ifndef, elif, else, 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 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 name or + EXEC SQL ifndef name + directive. Any number of elif 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 name or + EXEC SQL ifndef name + directive. Subsequent lines will be processed if no previous section + of the same + ifdef/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;
+

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