4.11. For the Developer

This section explain how ecpg works internally. This information can occasionally be useful to help users understand how to use ecpg.

4.11.1. The Preprocessor

The first four lines written by ecpg to the output are fixed lines. Two are comments and two are include lines necessary to interface to the library. Then the preprocessor reads through the file and writes output. Normally it just echoes everything to the output.

When it sees an EXEC SQL statement, it intervenes and changes it. The command starts with exec sql and ends with ;. Everything in between is treated as an SQL statement and parsed for variable substitution.

Variable substitution occurs when a symbol starts with a colon (:). The variable with that name is looked up among the variables that were previously declared within a EXEC SQL DECLARE section. Depending on whether the variable is being use for input or output, a pointer to the variable is output to allow access by the function.

For every variable that is part of the SQL query, the function gets other arguments:

Note that not all SQL commands are treated in this way. For instance, an open cursor statement like

EXEC SQL OPEN cursor;

is not copied to the output. Instead, the cursor's DECLARE command is used because it opens the cursor as well.

Here is a complete example describing the output of the preprocessor of a file foo.pgc (details may change with each particular version of the preprocessor):

EXEC SQL BEGIN DECLARE SECTION;
int index;
int result;
EXEC SQL END DECLARE SECTION;
...
EXEC SQL SELECT res INTO :result FROM mytable WHERE index = :index;

is translated into:

/* Processed by ecpg (2.6.0) */
/* These two include files are added by the preprocessor */
#include <ecpgtype.h>;
#include <ecpglib.h>;

/* exec sql begin declare section */

#line 1 "foo.pgc"

 int index;
 int result;
/* exec sql end declare section */
...
ECPGdo(__LINE__, NULL, "SELECT res FROM mytable WHERE index = ?     ",
        ECPGt_int,&(index),1L,1L,sizeof(int),
        ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EOIT,
        ECPGt_int,&(result),1L,1L,sizeof(int),
        ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EORT);
#line 147 "foo.pgc"

(The indentation in this manual is added for readability and not something the preprocessor does.)

4.11.2. The Library

The most important function in the library is ECPGdo. It takes a variable number of arguments. Hopefully there are no computers that limit the number of variables that can be accepted by a varargs() function. This can easily add up to 50 or so arguments.

The arguments are:

A line number

This is a line number of the original line; used in error messages only.

A string

This is the SQL query that is to be issued. It is modified by the input variables, i.e. the variables that where not known at compile time but are to be entered in the query. Where the variables should go the string contains ?.

Input variables

As described in the section about the preprocessor, every input variable gets ten arguments.

ECPGt_EOIT

An enum telling that there are no more input variables.

Output variables

As described in the section about the preprocessor, every input variable gets ten arguments. These variables are filled by the function.

ECPGt_EORT

An enum telling that there are no more variables.