This section is for those who want to develop the ecpg interface. It describes how the things work. The ambition is to make this section contain things for those that want to have a look inside and the section on How to use it should be enough for all normal questions. So, read this before looking at the internals of the ecpg. If you are not interested in how it really works, skip this section.
This version the preprocessor has some flaws:
to_date et al. do not exists. But then Postgres has some good conversion routines itself. So you probably won't miss these.
Structures and unions have to be defined in the declare section.
The following statements are not implemented thus far:
The error message for "no data" in an exec sql insert select from statement has to be 100.
sqlwarn[6] should be 'W' if the PRECISION or SCALE value specified in a SET DESCRIPTOR statement will be ignored.
The first four lines written to the output are constant additions by ecpg. These are two comments and two include lines necessary for the interface to the library.
Then the preprocessor works in one pass only, reading the input file and writing to the output as it goes along. Normally it just echoes everything to the output without looking at it further.
When it comes to an EXEC SQL statements it intervenes and changes them depending on what it is. The EXEC SQL statement can be one of these:
Declare sections begins with
exec sql begin declare section;
and ends with
exec sql end declare section;
In the section only variable declarations are allowed. Every
variable declare within this section is also entered in a list of
variables indexed on their name together with the corresponding
type.
In particular the definition of a structure or union also has to be listed inside a declare section. Otherwise ecpg cannot handle these types since it simply does not know the definition.
The declaration is echoed to the file to make the variable a normal C-variable also.
The special types VARCHAR and VARCHAR2 are converted into a named struct for every variable. A declaration like:
VARCHAR var[180];
is converted into
struct varchar_var { int len; char arr[180]; } var;
An include statement looks like:
exec sql include filename;
Note that this is NOT the same as
#include <filename.h>
Instead the file specified is parsed by ecpg itself. So the contents of the specified file is included in the resulting C code. This way you are able to specify EXEC SQL commands in an include file.
A connect statement looks like:
exec sql connect to connection target;
It creates a connection to the specified database.
The connection target can be specified in the following ways:
There are also different ways to specify the user name:
Finally the userid and the password. Each may be a constant text, a character variable or a chararcter string.
A disconnect statement looks loke:
exec sql disconnect [connection target];
It closes the connection to the specified database.
The connection target can be specified in the following ways:
An open cursor statement looks like:
exec sql open cursor;
and is ignore and not copied from the output.
A commit statement looks like
exec sql commit;
and is translated on the output to
ECPGcommit(__LINE__);
A rollback statement looks like
exec sql rollback;
and is translated on the output to
ECPGrollback(__LINE__);
Other SQL statements are other statements that start with exec sql and ends with ;. Everything inbetween is treated as an SQL statement and parsed for variable substitution.
Variable substitution occur when a symbol starts with a colon (:). Then a variable with that name is looked for among the variables that were previously declared within a declare section and depending on the variable being for input or output the pointers to the variables are written to the output to allow for access by the function.
For every variable that is part of the SQL request the function gets another 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. |
| 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 value of the indicator variable or a pointer to the pointer of the indicator variable. |
| 0. |
| Number of elements in the indicator array (for array fetches). |
| The offset to the next element in the indicator array (for array fetches) |
Here is a complete example describing the output of the preprocessor of a file foo.pgc:
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 that the preprocessor can do.)
The most important function in the library is the ECPGdo function. It takes a variable amount of arguments. Hopefully we will not run into machines with limits on the amount of variables that can be accepted by a vararg function. This could easily add up to 50 or so arguments.
The arguments are:
This is a line number for the original line used in error messages only.
This is the SQL request that is to be issued. This request is modified by the input variables, i.e. the variables that where not known at compile time but are to be entered in the request. Where the variables should go the string contains ";".
As described in the section about the preprocessor every input variable gets ten arguments.
An enum telling that there are no more input variables.
As described in the section about the preprocessor every input variable gets ten arguments. These variables are filled by the function.
An enum telling that there are no more variables.
All the SQL statements are performed in one transaction unless you issue a commit transaction. To get this auto-transaction going the first statement or the first after statement after a commit or rollback always begins a transaction. To disable this feature per default use the -t option on the commandline.
To be completed: entries describing the other entries.