ecpg

Preprocessing for Compilation

An embedded SQL source file must be preprocessed before compilation:

ecpg [ -d ] [ -o file ] file.pgc
    

where the optional -d flag turns on debugging. The .pgc extension is an arbitrary means of denoting ecpg source.

You may want to redirect the preprocessor output to a log file.

Compiling and Linking

Assuming the Postgres binaries are in /usr/local/pgsql, you will need to compile and link your preprocessed source file:

gcc -g -I /usr/local/pgsql/include [ -o file ] file.c -L /usr/local/pgsql/lib -lecpg -lpq
    

Libraries

The preprocessor will prepend two directives to the source:

#include <ecpgtype.h>
#include <ecpglib.h>
    

Variable Declaration

Variables declared within ecpg source code must be prepended with:

EXEC SQL BEGIN DECLARE SECTION;
    

Similarly, variable declaration sections must terminate with:

EXEC SQL END DECLARE SECTION;
    

Note: Prior to version 2.1.0, each variable had to be declared on a separate line. As of version 2.1.0 multiple variables may be declared on a single line:

char  foo(16), bar(16);
      

Error Handling

The SQL communication area is defined with:

EXEC SQL INCLUDE sqlca;
    

Note: The sqlca is in lowercase. While SQL convention may be followed, i.e., using uppercase to separate embedded SQL from C statements, sqlca (which includes the sqlca.h header file) MUST be lowercase. This is because the EXEC SQL prefix indicates that this INCLUDE will be parsed by ecpg. ecpg observes case sensitivity (SQLCA.h will not be found.) EXEC SQL INCLUDE can be used to include other header files as long as case sensitivity is observed.

The sqlprint command is used with the EXEC SQL WHENEVER statement to turn on error handling throughout the program:

EXEC SQL WHENEVER sqlerror sqlprint;
    

and

EXEC SQL WHENEVER not found sqlprint;
    

Note: This is not an exhaustive example of usage for the EXEC SQL WHENEVER statement. Further examples of usage may be found in SQL manuals (e.g., `The LAN TIMES Guide to SQL' by Groff and Weinberg).

Connecting to the Database Server

One connects to a database using the following:

EXEC SQL CONNECT dbname;
    

where the database name is not quoted. Prior to version 2.1.0, the database name was required to be inside single quotes.

Specifying a server and port name in the connect statement is also possible. The syntax is:

dbname[@server][:port]
    

or

<tcp|unix>:postgresql://server[:port][/dbname][?options]
    

Queries

In general, SQL queries acceptable to other applications such as psql can be embedded into your C code. Here are some examples of how to do that.

Create Table:

EXEC SQL CREATE TABLE foo (number int4, ascii char(16));
EXEC SQL CREATE UNIQUE index num1 on foo(number);
EXEC SQL COMMIT;
    

Insert:

EXEC SQL INSERT INTO foo (number, ascii) VALUES (9999, 'doodad');
EXEC SQL COMMIT;
    

Delete:

EXEC SQL DELETE FROM foo WHERE number = 9999;
EXEC SQL COMMIT;
    

Singleton Select:

EXEC SQL SELECT foo INTO :FooBar FROM table1 WHERE ascii = 'doodad';
    

Select using Cursors:

EXEC SQL DECLARE foo_bar CURSOR FOR
    SELECT number, ascii FROM foo
    ORDER BY ascii;
EXEC SQL FETCH foo_bar INTO :FooBar, DooDad;
...
EXEC SQL CLOSE foo_bar;
EXEC SQL COMMIT;
    

Updates:

EXEC SQL UPDATE foo
    SET ascii = 'foobar'
    WHERE number = 9999;
EXEC SQL COMMIT;