Function packages are basically several external functions and subroutines that are grouped or packaged together. When the language processor processes a function call or a call to a subroutine, the language processor searches the function packages before searching load libraries or exec libraries, such as SYSEXEC and SYSPROC. Grouping frequently used external functions and subroutines in a function package allows for faster access to the function and subroutine, and therefore, better performance. Search order describes the complete search order the language processor uses to locate a function or subroutine.

To provide function packages, there are several steps you must perform:

1. You must first write the individual external functions and subroutines you want to include in a function package. If you want to include an external function or subroutine in a function package, the function or subroutine must be link-edited into a load module. If you write the function or subroutine in REXX and the REXX exec is interpreted (that is, the TSO/E REXX interpreter executes the exec), you cannot include the function or subroutine in a function package. However, if you write the external function or subroutine in REXX and the REXX exec is compiled, you can include the function or subroutine in a function package because the compiled exec can be link-edited into a load module. For information about compiled execs, see the appropriate compiler publications.

   If you write the external function or subroutine in a programming language other than REXX, the language you use must support the system-dependent interfaces that the language processor uses to invoke the function or subroutine. Interface for writing external function and subroutine code describes the interfaces.

2. After you write the individual functions and subroutines, you must write the directory for the function package. You need a directory for each individual function package.

   The function package directory is contained in a load module. The directory contains a header followed by individual entries that define the names and/or the addresses of the entry points of your function or subroutine code. Directory for function packages describes the directory for function packages.

3. The name of the entry point at the beginning of the directory (the function package name) must be specified in the function package table for a language processor environment. Function package table describes the format of the table. After you write the directory, you must define the directory name in the function package table. There are several ways you can do this depending on the type of function package you are defining (user, local, or system) and whether you are providing only one or several user and local function packages.

   If you are providing a local or user function package, you can name the function package directory IRXFLOC (local package) or IRXFUSER (user package). TSO/E provides these two "dummy" directory names in the three default parameters modules IRXPARMS, IRXTSPRM, and IRXISPRM. By naming your local function package directory IRXFLOC and your user function package directory IRXFUSER, the external functions and subroutines in the packages are automatically available to REXX execs that run in non-TSO/E and the TSO/E address space.

   If you write your own system function package or more than one local or user function package, you must provide a function package table containing the name of your directory. You must also provide your own parameters module that points to your function package table. Your parameters module then replaces the default parameters module that the system uses to initialize a default language processor environment. Specifying directory names in the function package table describes how to define directory names in the function package table.

TSO/E provides the IRXEFMVS and IRXEFPCK system function packages. The two function packages provide the TSO/E external functions, which are described in TSO/E external functions. The IRXEFMVS and IRXEFPCK system function packages are defined in the default parameters modules TSO/E provides (see Values provided in the three default parameters modules).

Other IBM® products may also provide system function packages that you can use for REXX processing in TSO/E and MVS™. If you install a product that provides a system function package for TSO/E REXX, you must change the function package table and provide your own parameters modules. The product itself supplies the individual functions in the function package and the directory for their function package. To use the functions, you must do the following:

1. Change the function package table. The function package table contains information about the user, local, and system function packages for a particular language processor environment. Table 1 shows the format of the table. Add the name of the function package directory to the entries in the table. You must also change the SYSTEM_TOTAL and SYSTEM_USED fields in the table header (offsets +28 and +32). Increment the value in each field by 1 to indicate the additional function package supplied by the IBM product.
2. Provide your own IRXTSPRM, IRXISPRM, or IRXPARMS parameters module. The function package table is part of the parameters module that the system uses to initialize language processor environments. You need to code one or more parameters modules depending on whether you want the function package available to REXX execs that run in ISPF only, TSO/E only, TSO/E and ISPF, non-TSO/E only, or any address space.

   Language Processor Environments describes environments, their characteristics, and the format of the parameters modules. In the same chapter, Changing the default values for initializing an environment describes how to provide your own parameters modules.