35.16. Internals

35.16. Internals
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+ This section explains how ECPG works + internally. This information can occasionally be useful to help + users understand how to use ECPG. +

+ 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. +

+ The most important function in the library is + ECPGdo, which takes care of executing most + commands. It takes a variable number of arguments. This can easily + add up to 50 or so arguments, and we hope this will not be a + problem on any platform. +

+ The arguments are: + +

A line number: + This is the line number of the original line; used in error + messages only. +
A string: + This is the SQL command 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 + command. Where the variables should go the string contains + ?. +
Input variables: + Every input variable causes ten arguments to be created. (See below.) +
ECPGt_EOIT: + An enum telling that there are no more input + variables. +
Output variables: + Every output variable causes ten arguments to be created. + (See below.) These variables are filled by the function. +
ECPGt_EORT: + An enum telling that there are no more variables. +

+ For every variable that is part of the SQL + command, the function gets ten arguments: + +

+ The type as a special symbol. +
+ A pointer to the value or a pointer to the pointer. +
+ The size of the variable if it is a char or varchar. +
+ The number of elements in the array (for array fetches). +
+ The offset to the next element in the array (for array fetches). +
+ The type of the indicator variable as a special symbol. +
+ A pointer to the indicator variable. +
+ 0 +
+ The number of elements in the indicator array (for array fetches). +
+ The offset to the next element in the indicator array (for + array fetches). +

+ 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 at the position of the OPEN command + because it indeed opens the cursor. +

+ Here is a complete example describing the output of the + preprocessor of a file foo.pgc (details might + 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 here is added for readability and not + something the preprocessor does.) +

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