Programmer's Guide to the Pro*C/C++ Precompiler

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Writing User Exits

What Is a User Exit?
Why Write a User Exit?
Developing a User Exit
Writing a User Exit
Calling a User Exit
Passing Parameters to a User Exit
Returning Values to a Form
An Example
Precompiling and Compiling a User Exit
Using the GENXTB Utility
Linking a User Exit into SQL*Forms
Guidelines
EXEC TOOLS Statements

This chapter focuses on writing user exits for your Oracle Tools applications. You learn how C subroutines can do certain jobs more quickly and easily than SQL*Forms and Oracle Forms. The following topics are covered:

common uses for user exits

writing a user exit

passing values between the tool and a user exit

implementing a user exit

calling a user exit

guidelines for user exits

using the EXEC TOOLS routines for applications that use the Oracle Toolset

This chapter is supplemental. For more information about user exits, refer to the SQL*Forms Designer's Reference, the Oracle Forms Reference Manual, Vol. 2, and your system-specific Oracle documentation.

What Is a User Exit?

A user exit is a C subroutine written by you and called by Oracle Forms to do special-purpose processing. You can embed SQL statements and PL/SQL blocks in your user exit, then precompile it as you would a host program.

When called by an Oracle Forms V3 trigger, the user exit runs, then returns a status code to Oracle Forms. Your exit can display messages on the Oracle Forms status line, get and set field values, do high-speed computations and table lookups, and manipulate Oracle data.

Figure 13 - 1 shows how an Oracle Forms application interacts with a user exit.

Figure 13 - 1. Oracle Forms and a User Exit

Why Write a User Exit?

SQL*Forms Version 3 allows you to use PL/SQL blocks in triggers. So, in most cases, instead of calling a user exit, you can use the procedural power of PL/SQL. If the need arises, you can call user exits from a PL/SQL block with the USER_EXIT function. User exits are harder to write and implement than SQL, PL/SQL, or SQL*Forms commands. So, you will probably use them only to do processing that is beyond the scope of SQL, PL/SQL, and SQL*Forms. Some common uses follow:

operations more quickly or easily done in a third generation languages like C (numerical integration, for instance)

controlling real time devices or processes (issuing a sequence of instructions to a printer or graphics device, for example)

data manipulations that need extended procedural capabilities (recursive sorting, for example)

special file I/O operations

Developing a User Exit

This section outlines the way to develop a SQL*Forms 3.0 user exit; later sections go into more detail. For information about the EXEC TOOLS options available with SQL*Forms 4, see the section ``EXEC TOOLS Statements''

. To incorporate a user exit into a form, you take the following steps:

1. Write the user exit in Pro*C.

4. Use the GENXTB utility to create a database table, IAPXTB.

5. Use the GENXTB form in SQL*Forms to insert your user exit information into the table.

8. In the form, define a trigger to call the user exit.

9. Instruct operators to use the new IAP when running the form. This is unnecessary if the new IAP replaces the standard one. For details, see the Oracle installation or user's guide for your system.

Writing a User Exit

You can use the following kinds of statements to write your SQL*Forms user exit:

C code

EXEC SQL

EXEC ORACLE

EXEC IAF GET

EXEC IAF PUT

This section focuses on the EXEC IAF GET and PUT statements, which let you pass values between SQL*Forms and a user exit.

Requirements for Variables

The variables used in EXEC IAF statements must correspond to field names used in the form definition. If a field reference is ambiguous because you did not specify a block name, EXEC IAF defaults to the context block--the block that calls the user exit. An invalid or ambiguous reference to a form field generates an error. Host variables must be prefixed with a colon (:) in EXEC IAF statements.

Note: Indicator variables are not allowed in EXEC IAF GET and PUT statements.

The IAF GET Statement

This statement allows your user exit to ``get" values from fields on a form and assign them to host variables. The user exit can then use the values in calculations, data manipulations, updates, and so on. The syntax of the GET statement follows:

EXEC IAF GET field_name1, field_name2, ...

    INTO :host_variable1, :host_variable2, ...;

where field_name can be any of the following SQL*Forms variables:

field

block.field

system variable

global variable

host variable (prefixed with a colon) containing the value of a field, block.field, system variable, or global variable

If a field_name is not qualified, the field must be in the context block.

Using IAF GET

The following example shows how a user exit GETs a field value and assigns it to a host variable:

EXEC IAF GET employee.job INTO :new_job;

All field values are character strings. If it can, GET converts a field value to the datatype of the corresponding host variable. If an illegal or unsupported datatype conversion is attempted, an error is generated.

In the last example, a constant is used to specify block.field. You can also use a host string to specify block and field names, as follows:

char blkfld[20] = "employee.job";

EXEC IAF GET :blkfld INTO :new_job;

Unless the field is in the context block, the host string must contain the full block.field reference with intervening period. For example, the following usage is invalid:

char blk[20] = "employee";

strcpy(fld, "job");

EXEC IAF GET :blk.:fld INTO :new_job;

You can mix explicit and stored field names in a GET statement field list, but not in a single field reference. For example, the following usage is invalid:

strcpy(fld, "job");

EXEC IAF GET employee.:fld INTO :new_job;

The IAF PUT Statement

This statement allows your user exit to ``put" the values of constants and host variables into fields on a form. Thus, the user exit can display on the SQL*Forms screen any value or message you like. The syntax of the PUT statement follows:

EXEC IAF PUT field_name1, field_name2, ...

    VALUES (:host_variable1, :host_variable2, ...);

where field_name can be any of the following SQL*Forms variables:

field

block.field

system variable

global variable

host variable (prefixed with a colon) containing the value of a field, block.field, system variable, or global variable

Using IAF PUT

The following example shows how a user exit PUTs the values of a numeric constant, string constant, and host variable into fields on a form:

EXEC IAF PUT employee.number, employee.name, employee.job

    VALUES (7934, 'MILLER', :new_job);

Like GET, PUT lets you use a host string to specify block and field names, as follows:

char blkfld[20] = "employee.job";

EXEC IAF PUT :blkfld VALUES (:new_job);

On character-mode terminals, a value PUT into a field is displayed when the user exit returns, rather than when the assignment is made, provided the field is on the current display page. On block-mode terminals, the value is displayed the next time a field is read from the device.

If a user exit changes the value of a field several times, only the last change takes effect.

Calling a User Exit

You call a user exit from a SQL*Forms trigger using a packaged procedure named USER_EXIT (supplied with SQL*Forms). The syntax you use is

USER_EXIT(user_exit_string [, error_string]);

where user_exit_string contains the name of the user exit plus optional parameters and error_string contains an error message issued by SQL*Forms if the user exit fails. For example, the following trigger command calls a user exit named LOOKUP:

USER_EXIT('LOOKUP');

Notice that the user exit string is enclosed by single (not double) quotes.

Passing Parameters to a User Exit

When you call a user exit, SQL*Forms passes it the following parameters automatically:

Command Line Is the user exit string.

Command Line Length Is the length (in characters) of the user exit string.

Error Message Is the error string (failure message) if one is defined.

Error Message Length Is the length of the error string.

In-Query Is a Boolean value indicating whether the exit was called in normal or query mode.

However, the user exit string allows you to pass additional parameters to the user exit. For example, the following trigger command passes two parameters and an error message to the user exit LOOKUP:

Notice that the user exit string is enclosed by single (not double) quotes.

USER_EXIT('LOOKUP 2025 A', 'Lookup failed');

You can use this feature to pass field names to the user exit, as the following example shows:

USER_EXIT('CONCAT firstname, lastname, address');

However, it is up to the user exit, not SQL*Forms, to parse the user exit string.

Returning Values to a Form

When a user exit returns control to SQL*Forms, it must also return a code indicating whether it succeeded, failed, or suffered a fatal error. The return code is an integer constant defined by SQL*Forms (see the next section). The three results have the following meanings:

success

The user exit encountered no errors. SQL*Forms proceeds to the success label or the next step, unless the Reverse Return Code switch is set by the calling trigger step.

failure

The user exit detected an error, such as an invalid value in a field. An optional message passed by the exit appears on the message line at the bottom of the SQL*Forms screen and on the Display Error screen. SQL*Forms responds as it does to a SQL statement that affects no rows.

fatal error

The user exit detected a condition that makes further processing impossible, such as an execution error in a SQL statement. An optional error message passed by the exit appears on the SQL*Forms Display Error screen. SQL*Forms responds as it does to a fatal SQL error. If a user exit changes the value of a field, then returns a failure or fatal error code, SQL*Forms does not discard the change. Nor does SQL*Forms discard changes when the Reverse Return Code switch is set and a success code is returned.

The IAP Constants

SQL*Forms defines three symbolic constants for use as return codes. Depending on the host language, they are prefixed with IAP or SQL. For example, they might be IAPSUCC, IAPFAIL, and IAPFTL.

Using the SQLIEM Function

By calling the function SQLIEM, your user exit can specify an error message that SQL*Forms will display on the message line if the trigger step fails or on the Display Error screen if the step causes a fatal error. The specified message replaces any message defined for the step. The syntax of the SQLIEM function call is

SQLIEM (char *error_message, int message_length);

where error_message and message_length are character and integer variables, respectively. The Pro*C Precompiler generates the appropriate external function declaration for you. You pass both parameters by reference; that is, you pass their addresses, not their values. SQLIEM is a SQL*Forms function; it cannot be called from other Oracle tools such as SQL*ReportWriter.

Using WHENEVER

You can use the WHENEVER statement in an exit to detect invalid datatype conversions (SQLERROR), truncated values PUT into form fields (SQLWARNING), and queries that return no rows (NOT FOUND).

An Example

The following example shows how a user exit that uses the EXEC IAF GET and PUT routines, as well as the sqliem function, is coded. Notice that, like a host program, the user exit has a Declare Section and a SQLCA.

int
myexit()
{
    char field1[20], field2[20], value1[20], value2[20];
    char result_value[20];
    char errmsg[80];
    int errlen;

    #include sqlca.h
    EXEC SQL WHENEVER SQLERROR GOTO sql_error;
    /* get field values into form */
    EXEC IAF GET :field1, :field2 INTO :value1, :value2;
    /* manipulate the values to obtain result_val */
    ...
    /* put result_val into form field result */
    EXEC IAF PUT result VALUES (:result_val);
    return IAPSUCC;   /* trigger step succeeded */

sql_error:
    strcpy(errmsg, CONCAT("MYEXIT", sqlca.sqlerrm.sqlerrmc);
    errlen = strlen(errmsg);
    sqliem(errmsg, &errlen); /* send error msg to Forms */
    return IAPFAIL;

Precompiling and Compiling a User Exit

User exits are precompiled like stand-alone host programs. Refer to Chapter 7, "Running the Pro*C Precompiler." For instructions on compiling a user exit, see the Oracle installation or user's guide for your system.

Sample Program: A User Exit

The example below shows a user exit.


/**************************************************************
Sample Program 5:  SQL*Forms User Exit

This user exit concatenates form fields.  To call the user 
exit from a SQL*Forms trigger, use the syntax

   user_exit('CONCAT field1, field2, ..., result_field');

where user_exit is a packaged procedure supplied with SQL*Forms
and CONCAT is the name of the user exit.  A sample form named
CONCAT invokes the user exit.
**************************************************************/

#define min(a, b) ((a < b) ? a : b)
#include <stdio.h>
#include <string.h>

/* Include the SQL Communications Area, a structure through which
   Oracle makes runtime status information such as error
   codes, warning flags, and diagnostic text available to the
   program. */

EXEC SQL INCLUDE sqlca;

/* All host variables used in embedded SQL must appear in the
   Declare Section. */

VARCHAR   field[81];
VARCHAR   value[81];
VARCHAR   result[241];

int concat(cmd, cmdlen, msg, msglen, query)

char *cmd;     /* command line in trigger step ("CONCAT...") */
int  *cmdlen;  /* length of command line */
char *msg;     /* trigger step failure message from form */
int  *msglen;  /* length of failure message */
int  *query;   /* TRUE if invoked by post-query trigger,
                  FALSE otherwise */
{
    char *cp = cmd + 7;    /* pointer to field list in
                              cmd string; 7 characters
                              are needed for "CONCAT " */
    char *fp = (char*)&field.arr[0]; /* pointer to a field name in
                                         cmd string */
    char  errmsg[81];      /* message returned to SQL*Forms
                              on error */
    int   errlen;          /* length of message returned
                              to SQL*Forms */

    /* Branch to label sqlerror if an Oracle error occurs. */
    EXEC SQL WHENEVER SQLERROR GOTO sqlerror;

    result.arr[0] = '\0';
    /* Parse field names from cmd string. */
    for (; *cp != '\0'; cp++)
    {
       if (*cp != ',' && *cp != ' ')
           /* Copy a field name into field.arr from cmd. */
       {
           *fp = *cp;
           fp++;
       }
       else
           if (*cp == ' ')
           {   /* Have whole field name now. */
               *fp = '\0';
               field.len = strlen((char *) field.arr);
               /* Get field value from form. */
               EXEC IAF GET :field INTO :value;
               value.arr[value.len] = '\0';
               strcat((char *) result.arr, (char *) value.arr);
              /* Reset field pointer. *
               fp = (char *)&field.arr[0];/
           }
    }

    /* Have last field name now. */
    *fp = '\0';
    field.len = strlen((char *) field.arr);
    result.len = strlen((char *) result.arr);
    /* Put result into form. */
    EXEC IAF PUT :field VALUES (:result);
    return(IAPSUCC);  /* Trigger step succeeded. */

sqlerror:
    strcpy(errmsg, "CONCAT: ");
    strncat(errmsg, sqlca.sqlerrm.sqlerrmc, min(72,
        sqlca.sqlerrm.sqlerrml));
    errlen = strlen(errmsg);
    /* Pass error message to SQL*Forms status line. */
    sqliem(errmsg, &errlen);
    return(IAPFAIL);  /* Trigger step failed. */
}

Using the GENXTB Utility

The IAP program table IAPXTB in module IAPXIT contains an entry for each user exit linked into IAP. IAPXTB tells IAP the name, location, and host language of each user exit. When you add a new user exit to IAP, you must add a corresponding entry to IAPXTB. IAPXTB is derived from a database table, also named IAPXTB. You can modify the database table by running the GENXTB form on the operating system command line, as follows:

RUNFORM GENXTB username/password

A form is displayed that allows you to enter the following information for each user exit you define:

exit name (see the section ``Guidelines'' )

C-language code

date created

date last modified

comments

After modifying the IAPXTB database table, use the GENXTB utility to read the table and create an Assembler or C source program that defines the module IAPXIT and the IAPXTB program table it contains. The source language used depends on your operating system. The syntax you use to run the GENXTB utility is

GENXTB username/password outfile

where outfile is the name you give the Assembler or C source program that GENXTB creates.

**Linking a User Exit into SQL*Forms**

Before running a form that calls a user exit, you must link the user exit into IAP, the SQL*Forms component that runs a form. The user exit can be linked into your standard version of IAP or into a special version for those forms that call the exit.

To produce a new executable copy of IAP, link your user exit object module, the standard IAP modules, the IAPXIT module, and any modules needed from the Oracle and C link libraries.

The details of linking are system-dependent. Check the Oracle installation or user's guide for your system.

Guidelines

The guidelines in this section will help you avoid some common pitfalls.

Naming the Exit

The name of your user exit cannot be an Oracle reserved word. Also avoid using names that conflict with the names of SQL*Forms commands, function codes, and externally defined names used by SQL*Forms. The name of the user exit entry point in the source code becomes the name of the user exit itself. The exit name must be a valid C function name, and a valid filename for your operating system.

SQL*Forms converts the name of a user exit to upper case before searching for the exit. Therefore, the exit name must be in upper case in your source code.

Connecting to Oracle

User exits communicate with Oracle via the connection made by SQL*Forms. However, a user exit can establish additional connections to any database via SQL*Net. For more information, see the section ``Concurrent Connections"

Issuing I/O Calls

File I/O is supported but screen I/O is not.

Using Host Variables

Restrictions on the use of host variables in a stand-alone program also apply to user exits. Host variables must be prefixed with a colon in EXEC SQL and EXEC IAF statements. The use of host arrays is not allowed in EXEC IAF statements.

Updating Tables

Generally, a user exit should not UPDATE database tables associated with a form. For example, suppose an operator updates a record in the SQL*Forms work space, then a user exit UPDATEs the corresponding row in the associated database table. When the transaction is COMMITted, the record in the SQL*Forms work space is applied to the table, overwriting the user exit UPDATE.

Issuing Commands

Avoid issuing a COMMIT or ROLLBACK command from your user exit because Oracle will commit or roll back work begun by the SQL*Forms operator, not just work done by the user exit. Instead, issue the COMMIT or ROLLBACK from the SQL*Forms trigger. This also applies to data definition commands (such as ALTER, CREATE, and GRANT) because they issue an implicit COMMIT before and after executing.

EXEC TOOLS Statements

EXEC TOOLS statements support the basic Oracle Toolset (Oracle Forms V4, Oracle Report V2, and Oracle Graphics V2) by providing a generic way to handle get, set, and exception callbacks from user exits. The following discussion focuses on Oracle Forms but the same concepts apply to Oracle Report and Oracle Graphics.

Writing a Toolset User Exit

Besides EXEC SQL, EXEC ORACLE, and host language statements, you can use the following EXEC TOOLS statements to write an Oracle Forms user exit:

SET CONTEXT

GET CONTEXT

MESSAGE

The EXEC TOOLS GET and SET statements replace the EXEC IAF GET and PUT statements used with earlier versions of Oracle Forms. Unlike IAF GET and PUT, however, TOOLS GET and SET accept indicator variables. The EXEC TOOLS MESSAGE statement replaces the message-handling function sqliem. Now, let us take a brief look at all the EXEC TOOLS statements. For more information, see the Oracle Forms Reference Manual, Vol 2.

EXEC TOOLS SET

The EXEC TOOLS SET statement passes values from a user exit to OracleForms. Specifically, it assigns the values of host variables and constants to Oracle Forms variables and items. Values passed to form items display after the user exit returns control to the form. To code the EXEC TOOLS SET statement, you use the syntax

EXEC TOOLS SET form_variable[, ...]

    VALUES ({:host_variable :indicator | constant}[, ...]);

where form_variable is an Oracle Forms field, block.field, system variable, or global variable, or a host variable (prefixed with a colon) containing the value of one of the foregoing items. In the following example, a user exit passes an employee name to Oracle Forms:

char ename[20];

short ename_ind;

...

    strcpy(ename, "MILLER");

    ename_ind = 0;

    EXEC TOOLS SET emp.ename VALUES (:ename :ename_ind);

In this example, emp.ename is an Oracle Forms block.field.

EXEC TOOLS GET

The EXEC TOOLS GET statement passes values from Oracle Forms to a user exit. Specifically, it assigns the values of Oracle Forms variables and items to host variables. As soon as the values are passed, the user exit can use them for any purpose. To code the EXEC TOOLS GET statement, you use the syntax

EXEC TOOLS GET form_variable[, ...]

    INTO :host_variable:indicator[, ...];

where form_variable is an Oracle Forms field, block.field, system variable, or global variable, or a host variable (prefixed with a colon) containing the value of one of the foregoing items. In the following example, Oracle Forms passes an item name from a block to your user exit:

...

char     name_buff[20];

VARCHAR  name_fld[20];

strcpy(name_fld.arr, "EMP.NAME");

name_fld.len = strlen(name_fld.arr);

EXEC TOOLS GET :name_fld INTO :name_buff;

EXEC TOOLS SET CONTEXT

The EXEC TOOLS SET CONTEXT statement saves context information from a user exit for later use in another user exit. A pointer variable points to a block of memory in which the context information is stored. With SET CONTEXT, you need not declare a global variable to hold the information. To code the EXEC TOOLS SET CONTEXT statement, you use the syntax

EXEC TOOLS SET CONTEXT :host_pointer_variable

    IDENTIFIED BY context_name;

where context_name is an undeclared identifier or a character host variable (prefixed with a colon) that names the context area.

...

char  *context_ptr;

char  context[20];

strcpy(context, "context1")

EXEC TOOLS SET CONTEXT :context IDENTIFIED BY application1;

EXEC TOOLS GET CONTEXT

The EXEC TOOLS GET CONTEXT statement retrieves context information (saved earlier by SET CONTEXT) into a user exit. A host-language pointer variable points to a block of memory in which the context information is stored. To code the EXEC TOOLS GET CONTEXT statement, you use the syntax

EXEC TOOLS GET CONTEXT context_name

    INTO :host_pointer_variable;

where context_name is an undeclared identifier or a character host variable (prefixed with a colon) that names the context area. In the following example, your user exit retrieves context information saved earlier:

...

char  *context_ptr;

EXEC TOOLS GET CONTEXT application1 INTO :context_ptr;

EXEC TOOLS MESSAGE

The EXEC TOOLS MESSAGE statement passes a message from a user exit to Oracle Forms. The message is displayed on the Oracle Forms message line after the user exit returns control to the form. To code the EXEC TOOLS MESSAGE statement, you use the syntax

EXEC TOOLS MESSAGE message_text [severity_code];

where message-text is a quoted string or a character host variable (prefixed with a colon), and the optional severity_code is an integer constant or an integer host variable (prefixed with a colon). The MESSAGE statement does not accept indicator variables. In the following example, your user exit passes an error message to Oracle Forms:

EXEC TOOLS MESSAGE 'Bad field name! Please reenter.';

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