## 36.9. Preprocessor Directives

[36.9.1. Including Files](ecpg-preproc.html#ECPG-INCLUDE)

[36.9.2. The define and undef Directives](ecpg-preproc.html#ECPG-DEFINE)

[36.9.3. ifdef, ifndef, elif, else, and endif Directives](ecpg-preproc.html#ECPG-IFDEF)

 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;

```