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>Chapter 33. <SPAN
CLASS="APPLICATION"
>ECPG</SPAN
> - Embedded <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> in C</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ECPG-PREPROC"
>33.9. Preprocessor Directives</A
></H1
><P
> Several preprocessor directives are available that modify how
the <TT
CLASS="COMMAND"
>ecpg</TT
> preprocessor parses and processes a
file.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-INCLUDE"
>33.9.1. Including Files</A
></H2
><P
> To include an external file into your embedded SQL program, use:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL INCLUDE <TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>;
EXEC SQL INCLUDE <<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>>;
EXEC SQL INCLUDE "<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>";</PRE
><P>
The embedded SQL preprocessor will look for a file named
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>.h</TT
>,
preprocess it, and include it in the resulting C output. Thus,
embedded SQL statements in the included file are handled correctly.
</P
><P
> The <TT
CLASS="COMMAND"
>ecpg</TT
> preprocessor will search a file at
several directories in following order:
<P
></P
></P><UL
><LI
><P
>current directory</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>/usr/local/include</TT
></P
></LI
><LI
><P
>PostgreSQL include directory, defined at build time (e.g., <TT
CLASS="FILENAME"
>/usr/local/pgsql/include</TT
>)</P
></LI
><LI
><P
><TT
CLASS="FILENAME"
>/usr/include</TT
></P
></LI
></UL
><P>
But when <TT
CLASS="LITERAL"
>EXEC SQL INCLUDE
"<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>"</TT
> is used, only the
current directory is searched.
</P
><P
> In each directory, the preprocessor will first look for the file
name as given, and if not found will append <TT
CLASS="LITERAL"
>.h</TT
>
to the file name and try again (unless the specified file name
already has that suffix).
</P
><P
> Note that <TT
CLASS="COMMAND"
>EXEC SQL INCLUDE</TT
> is <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> the same as:
</P><PRE
CLASS="PROGRAMLISTING"
>#include <<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>.h></PRE
><P>
because this file would not be subject to SQL command preprocessing.
Naturally, you can continue to use the C
<TT
CLASS="LITERAL"
>#include</TT
> directive to include other header
files.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> The include file name is case-sensitive, even though the rest of
the <TT
CLASS="LITERAL"
>EXEC SQL INCLUDE</TT
> command follows the normal
SQL case-sensitivity rules.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-DEFINE"
>33.9.2. The define and undef Directives</A
></H2
><P
> Similar to the directive <TT
CLASS="LITERAL"
>#define</TT
> that is known from C,
embedded SQL has a similar concept:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DEFINE <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
>;
EXEC SQL DEFINE <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
>;</PRE
><P>
So you can define a name:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DEFINE HAVE_FEATURE;</PRE
><P>
And you can also define constants:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DEFINE MYNUMBER 12;
EXEC SQL DEFINE MYSTRING 'abc';</PRE
><P>
Use <TT
CLASS="LITERAL"
>undef</TT
> to remove a previous definition:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL UNDEF MYNUMBER;</PRE
><P>
</P
><P
> Of course you can continue to use the C versions <TT
CLASS="LITERAL"
>#define</TT
>
and <TT
CLASS="LITERAL"
>#undef</TT
> in your embedded SQL program. The difference
is where your defined values get evaluated. If you use <TT
CLASS="LITERAL"
>EXEC SQL
DEFINE</TT
> then the <TT
CLASS="COMMAND"
>ecpg</TT
> preprocessor evaluates the defines and substitutes
the values. For example if you write:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DEFINE MYNUMBER 12;
...
EXEC SQL UPDATE Tbl SET col = MYNUMBER;</PRE
><P>
then <TT
CLASS="COMMAND"
>ecpg</TT
> will already do the substitution and your C compiler will never
see any name or identifier <TT
CLASS="LITERAL"
>MYNUMBER</TT
>. Note that you cannot use
<TT
CLASS="LITERAL"
>#define</TT
> 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.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-IFDEF"
>33.9.3. ifdef, ifndef, else, elif, and endif Directives</A
></H2
><P
> You can use the following directives to compile code sections conditionally:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>EXEC SQL ifdef <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
>;</TT
></DT
><DD
><P
> Checks a <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> and processes subsequent lines if
<TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> has been created with <TT
CLASS="LITERAL"
>EXEC SQL define
<TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>EXEC SQL ifndef <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
>;</TT
></DT
><DD
><P
> Checks a <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> and processes subsequent lines if
<TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> has <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> been created with
<TT
CLASS="LITERAL"
>EXEC SQL define <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>EXEC SQL else;</TT
></DT
><DD
><P
> Starts processing an alternative section to a section introduced by
either <TT
CLASS="LITERAL"
>EXEC SQL ifdef <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></TT
> or
<TT
CLASS="LITERAL"
>EXEC SQL ifndef <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>EXEC SQL elif <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
>;</TT
></DT
><DD
><P
> Checks <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> and starts an alternative section if
<TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> has been created with <TT
CLASS="LITERAL"
>EXEC SQL define
<TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>EXEC SQL endif;</TT
></DT
><DD
><P
> Ends an alternative section.
</P
></DD
></DL
></DIV
><P>
</P
><P
> Example:
</P><PRE
CLASS="PROGRAMLISTING"
>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;</PRE
><P>
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