PostgreSQL
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# How To Use ecpg

This section describes how to use the ecpg 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 almost all SQL statements 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
    {
     char sqlcaid[8];
     long sqlabc;
     long sqlcode;
     struct
     {
      int sqlerrml;
      char sqlerrmc[70];
     } sqlerrm;
     char sqlerrp[8];
     long sqlerrd[6];
     /* 0: empty                                         */
     /* 1: OID of processed tuple if applicable          */
     /* 2: number of rows processed in an INSERT, UPDATE */
     /*    or DELETE statement                           */
     /* 3: empty                                         */
     /* 4: empty                                         */
     /* 5: empty                                         */
     char sqlwarn[8];
     /* 0: set to 'W' if at least one other is 'W'       */
     /* 1: if 'W' at least one character string	     */
     /*    value was truncated when it was               */
     /*    stored into a host variable.                  */
     /* 2: empty                                         */
     /* 3: empty                                         */
     /* 4: empty                                         */
     /* 5: empty                                         */
     /* 6: empty                                         */
     /* 7: empty                                         */
     char sqlext[8];
    } 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 the line number in the source file.

List of errors that can occur:

-12, Out of memory in line %d.

  Does not normally occur. This is a sign that your virtual memory is exhausted.

-200, 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.

-201, Too many arguments line %d.

  This means that Postgres has returned more arguments than we have matching variables. Perhaps you have forgotten a couple of the host variables in the INTO :var1,:var2-list.

-202, Too few arguments line %d.

  This means that Postgres has returned fewer arguments than we have host variables. Perhaps you have too many host variables in the INTO :var1,:var2-list.

-203, Too many matches line %d.

  This means that the query has returned several lines but the variables specified are no arrays. The SELECT you made probably was not unique.

-204, 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.

-205, 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.

-206, Not correctly formatted floating point type: %s line %d.

  This means that the host variable is of a 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.

-207, Unable to convert %s to bool on line %d.

  This means that the host variable is of a bool type and the field in the Postgres database is neither 't' nor 'f'.

-208, Empty query line %d.

  Postgres returned PGRES_EMPTY_QUERY, probably because the query indeed was empty.

-220, No such connection %s in line %d.

  The program tries to access a connection that does not exist.

-221, Not connected in line %d.

  The program tries to access a connection that does exist but is not open.

-230, Invalid statement name %s in line %d.

  The statement you are trying to use has not been prepared.

-400, Postgres error: %s line %d.

  Some Postgres error. The message contains the error message from the Postgres backend.

-401, Error in transaction processing line %d.

  Postgres signalled to us that we cannot start, commit or rollback the transaction.

-402, connect: could not open database %s.

  The connect to the database did not work.

100, 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.

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