Function modules perform tasks of general interest to other programmers. Usually these tasks are well-defined functions that all users need, regardless of application. Some well-defined tasks include performing tax calculations, determining factory calendar dates, and calling frequently-used dialogs.

When you write ABAP routines that other programmers might use, you should define these routines as function modules. This means that you develop them in the Function Builder as follows:

1. Check whether a suitable function module already exists. If not, proceed to step 2.
2. Create a function group, if no appropriate group exists yet.
3. Create the function module.
4. Define the function module interface by entering its parameters and exceptions.
5. Write the actual ABAP code for the function module, adding any relevant global data to the TOP include.
6. Activate the module.
7. Test the module.
8. Document the module and its parameters for other users.
9. Release the module for general use.

There are some runtime considerations you should be familiar with when writing function modules:

• The CALL FUNCTION statement can pass import, export, and changing parameters either by value or by reference. Table parameters are always transferred by reference.

• If you declare the parameters with reference to ABAP Dictionary fields or structures, the system checks the type and length when the parameters are transferred. If the parameters from the calling program do not pass this check, the calling program terminates.

• At runtime, all function modules belonging to a function group are loaded with the calling program. As a result, you should plan carefully which functions really belong in a group and which do not. Otherwise, calling your function modules will unnecessarily increase the amount of memory required by the user.