Linux
Tolke
Michael
Meskes
1996-1997
Linus Tolke
1998
Michael Meskes
Transcribed 1998-02-12
ecpg - Embedded SQL
in C
This describes an embedded SQL in C
package for Postgres.
It is written by Linus Tolke
and Michael Meskes.
Permission is granted to copy and use in the same way as you are allowed
to copy and use the rest of the PostgreSQL.
Why Embedded SQL?
Embedded SQL has some small advantages over other ways
to handle SQL
queries. It takes care of all the tedious moving of information to and
from variables in your C program.
Many RDBMS packages
support this embedded language.
There is an ANSI-standard describing how the embedded language should
work. ecpg was designed to meet this standard
as much as possible. So it is
possible to port programs with embedded SQL written for
other RDBMS packages to
Postgres and thus promoting the spirit of free
software.
The Concept
You write your program in C with some
special SQL things.
For declaring variables that can be used in
SQL statements you need to
put them in a special declare section.
You use a special syntax for the SQL queries.
Before compiling you run the file through
the embedded SQL C
preprocessor and it converts the SQL statements you used
to function
calls with the variables used as arguments. Both variables that are used
as input to the SQL statements and variables that will
contain the
result are passed.
Then you compile and at link time you link with a special library that
contains the functions used. These functions (actually it is mostly one
single function) fetches the information from the arguments, performs
the SQL query using the ordinary interface
(libpq) and puts back
the result in the arguments dedicated for output.
Then you run your program and when the control arrives to
the SQL
statement the SQL statement is performed against
the database and you
can continue with the result.
How To Use egpc
This section describes how to use the egpc tool.
Preprocessor
The preprocessor is called ecpg.
After installation it resides in
the Postgres bin/ directory.
Library
The ecpg library is called
libecpg.a or
libecpg.so. Additionally, the library
uses the libpq library for communication to the
Postgres server so you will
have to link your program with -lecpg -lpq.
The library has some methods that are "hidden" but that could prove very
useful sometime.
ECPGdebug(int on, FILE *stream)
turns on debug logging if called with the first argument non-zero.
Debug logging is done on stream.
Most SQL statement logs its arguments and result.
The most important one (ECPGdo)
that is called on all SQL
statements except EXEC SQL COMMIT,
EXEC SQL ROLLBACK,
EXEC SQL CONNECT logs both its expanded string,
i.e. the string
with all the input variables inserted, and the result from the
Postgres server.
This can be very useful when searching for errors
in your SQL statements.
ECPGstatus()
This method returns TRUE if we are connected to a database and FALSE if not.
Error handling
To be able to detect errors from the Postgres
server you include a line like
exec sql include sqlca;
in the include section of your file. This will define a struct and a
variable with the name sqlca as following:
struct sqlca {
int sqlcode;
struct {
int sqlerrml;
char sqlerrmc[1000];
} sqlerrm;
} sqlca;
If an error occured in the last SQL statement
then sqlca.sqlcode
will be non-zero. If sqlca.sqlcode is less that 0
then this is
some kind of serious error, like the database definition does not match
the query given. If it is bigger than 0 then this is a normal error like
the table did not contain the requested row.
sqlca.sqlerrm.sqlerrmc will contain a string that describes the error.
The string ends with line 23.
where the line is the line number
in the source file (actually the file generated by the preprocessor but
I hope I can fix this to be the line number in the input file.)
List of errors that can occur:
-1, Unsupported type %s on line %d.
Does not normally occur. This is a sign that the preprocessor has
generated something that the library does not know about. Perhaps you
are running incompatible versions of the preprocessor and the library.
-1, Too many arguments line %d.
The preprocessor has goofed up and generated some incorrect code.
-1, Too few arguments line %d.
The preprocessor has goofed up and generated some incorrect code.
-1, Error starting transaction line %d.
Postgres signalled to us that we cannot open
the connection.
-1, Postgres error: %s line %d.
Some Postgres error.
The message contains the error message from the
Postgres backend.
1, Data not found line %d.
This is a "normal" error that tells you that what you are quering cannot
be found or we have gone through the cursor.
-1, To many matches line %d.
This means that the query has returned several lines.
The SELECT you made probably was not unique.
-1, Not correctly formatted int type: %s line %d.
This means that the host variable is of an int type and the field
in the Postgres database is of another type and
contains a value that cannot be interpreted as an int.
The library uses strtol
for this conversion.
-1, Not correctly formatted unsigned type: %s line %d.
This means that the host variable is of an unsigned int type and
the field in the Postgres database is of another
type and contains a
value that cannot be interpreted as an unsigned int. The library
uses strtoul for this conversion.
-1, Not correctly formatted floating point type: %s line %d.
This means that the host variable is of an float type and
the field in the Postgres database is of another
type and contains a
value that cannot be interpreted as an float. The library
uses strtod for this conversion.
-1, Too few arguments line %d.
This means that Postgres has returned more records
than we have
matching variables. Perhaps you have forgotten a couple of the host
variables in the INTO :var1,:var2-list.
-1, Too many arguments line %d.
This means that Postgres has returned fewer records
than we have
host variables. Perhaps you have to many host variables in the
INTO :var1,:var2-list.
-1, Empty query line %d.
Postgres returned PGRES_EMPTY_QUERY.
-1, Error: %s line %d.
This means that Postgres returned on of the errors
PGRES_NONFATAL_ERROR, PGRES_FATAL_ERROR or PGRES_BAD_RESPONSE. Which one
and why is explained in the message.
-1, Postgres error line %d.
Postgres returns something that the library does
not know how to
handle. This is probably because the version of
Postgres does not
match the version of the ecpg library.
-1, Error committing line %d.
Error during COMMIT. EXEC SQL COMMIT
is translated to an
end operation in Postgres
and that is the operation that could
not be performed.
-1, Error rolling back line %d.
Error during ROLLBACK.
EXEC SQL ROLLBACK is translated to
an abort operation in Postgres
and that is the operation that
could not be performed.
-1, ECPGconnect: could not open database %s.
The connect to the database did not work.
Limitations
What will never be included and why or what cannot be done with this
concept.
oracles single tasking possibility
Oracle version 7.0 on AIX 3 uses the OS-supported locks on the shared
memory segments and allows the application designer to link an
application in a so called single tasking way. Instead of starting one
client process per application process both the database part and the
application part is run in the same process. In later versions of oracle
this is no longer supported.
This would require a total redesign of the Postgres access model and
that effort can not justify the performance gained.
Porting From Other RDBMS Packages
To be written by someone who knows the different
RDBMS packages and who
actually does port something...
Installation
Since version 0.5 ecpg is distributed
together with Postgres. So you
should get your precompiler, libraries and header files compiled and
installed by default as a part of your installation.
For the Developer
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.
ToDo List
This version the preprocessor has some flaws:
no restriction to strings only
The PQ interface, and most of all the PQexec function, that is used by
the ecpg relies on that the request is built
up as a string. In some
cases, like when the data contains the null character, this will be a
serious problem.
error codes
There should be different error numbers for the different errors instead
of just -1 for them all.
library functions
to_date et al.
records
Records or structures have to be defined in the declare section.
missing statements
The following statements are not implemented thus far:
exec sql type
exec sql prepare
exec sql allocate
exec sql free
exec sql whenever sqlwarning
SQLSTATE
message īno data foundī
The error message for "no data" in an exec sql insert select from statement
has to be 100.
sqlwanr[6]
sqlwarn[6] should be 'W' if the PRECISION or SCALE value specified in a SET
DESCRIPTOR statement will be ignored.
conversion of scripts
To set up a database you need a few scripts with table definitions and
other configuration parameters. If you have these scripts for an old
database you would like to just apply them to get a
Postgres database that works in the same way.
To set up a database you need a few scripts with table definitions and
The functionality could be accomplished with some conversion scripts.
Speed will never be accomplished in this way. To do this you need a
bigger insight in the database construction and the use of the database
than could be realised in a script.
The Preprocessor
First four lines are written to the output. 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 interviens and
changes them depending on what iit is.
The EXEC SQL statement can be one of these:
Declare sections
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.
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;
Include statements
An include statement looks like:
exec sql include filename;
It is converted into
#include <filename.h>
Connect statement
A connect statement looks like:
exec sql connect 'database';
That statement is converted into
ECPGconnect("database");
Open cursor statement
An open cursor statement looks like:
exec sql open cursor;
and is ignore and not copied from the output.
Commit statement
A commit statement looks like
exec sql commit;
and is translated on the output to
ECPGcommit(__LINE__);
Rollback statement
A rollback statement looks like
exec sql rollback;
and is translated on the output to
ECPGrollback(__LINE__);
Other statements
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 found among the variables
that were previously declared within a declare section and depending on
whether or not the SQL statements knows it to be
a variable 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 five arguments:
The type as a special symbol
A pointer to the value
The size of the variable if it is a varchar
Number of elements in the array (for array fetches)
The offset to the next element in the array (for array fetches)
Since the array fetches are not implemented yet the two last arguments
are not really important. They could perhaps have been left out.
A Complete Example
Here is a complete example describing the output 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:
/* These two include files are added by the preprocessor */
#include <ecpgtype.h>
#include <ecpglib.h>
/* exec sql begin declare section */
int index;
int result;
/* exec sql end declare section */
...
ECPGdo(__LINE__, "select res from mytable where index = ;;",
ECPGt_int,&index,0,0,sizeof(int),
ECPGt_EOIT,
ECPGt_int,&result,0,0,sizeof(int),
ECPGt_EORT );
(the indentation in this manual is added for readability and not
something that the preprocessor can do.)
The Library
The most important function in the library is the ECPGdo
function. It takes a variable amount of arguments. Hopefully we wont run
into machines with limits on the amount of variables that can be
accepted by a varchar function. This could easily add up to 50 or so
arguments.
The arguments are:
A line number
This is a line number for the original line used in error messages only.
A string
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 ;
.
Input variables
As described in the section about the preprocessor every input variable
gets five 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 five arguments. These variables are filled by the function.
ECPGt_EORT
An enum telling that there are no more variables.
All the SQL statements are performed in one transaction
unless you issue
a commit transaction. This works so that the first transaction or the
first after a commit or rollback always begins a transaction.
To be completed: entries describing the other entries.