Editor's Note: This article is an excerpt from RPG TnT: 101 Dynamite Tips 'n Techniques with RPG IV.

The User Space is an interesting object. User Spaces have been around on this architecture for nearly 20 years, and yet many people still don't know how to create them or why they would use them. In fact, the underlying object on which User Spaces are based was the first object created for this architecture back in the mid-1970s.

The reason most people don't know how to create user space objects is that OS/400 never shipped with a CL command to create this type of object. There is a DLTUSRSPC command, but no CRTUSRSPC command. Some people have created their own userwritten CRTUSRSPC command (one is included in RPG xTools).

The User Space is becoming more popular lately because RPG IV allows you to use it for dynamic memory allocation, which in turn allows you to create dynamically sized arrays, data structure arrays, and dynamic memory allocations without actually using the precious (and expensive) system RAM or main storage, as we like to call it.

User Spaces are arguably the most optimized objects on the system. They have the ability to automatically grow from a few bytes in length up to 16MB, which is their limit. This allows you to store information in a user space and not worry about the size of the user space—it automatically (and seemingly magically) grows as necessary.

To create a user space, the QUSCRTUS API is used. It creates a user space and sets the User Space's default properties or attributes. The following is the prototype for QUSCRTUS:

Prototype for QUSCRTUS (Create User Space) API

D QusCrtUsrSpace  PR                  extPgm('QUSCRTUS')

D  UsrSpace                     20A   Const

D  ExtAttr                      10A   Const

D  nSize                        10I 0 Const

D  InitValue                    1A    Const

D  PubAuth                      10A   Const

D  szTextDesc                   50A   Const

D  Replace                      10A   Const

D  api_error                          LikeDS(QUSEC_T)

D                                     OPTIONS(*VARSIZE)

D  bSysDomain                   10A   Const OPTIONS(*NOPASS)

Using this API to create a user space requires several parameters, but once a prototype is created, specifying values for those parameters is relatively easy:

D mySpaceName       DS                Qualified

D  UsrSpace                     10A   Inz('MYUSRSPC')

D  LibName                      10A   Inz('QGPL')

 /free

     CallP(e)  QusCrtUsrSpace(mySpaceName:

                    'MYSTUFF' : 6000 : X'00' :

                    '*ALL' : 'Bob''s Stuff' :

                    '*YES' : apiErrorDS);

 /end-free

In this example, a user space named MYUSRSPC is created in the QGPL library. The extended object attribute is 'MYSTUFF'. This attribute is visible using the DSPOBJD CL command and is stored just like the RPGLE attribute of an RPG IV program or the PF attribute of a database file.

The user space's initial length is set to 6,000 bytes and is initialized to X'00'. Any character can be used to initialize the user space. Most programmers tend to use blanks (X'40') but hex zeros (X'00') are much more efficient than any other character, including blanks. The object's text description is set to 'Bob's Stuff', and the Replace object parameter is '*YES'. Specifying '*YES' for the Replace parameter is just like specifying REPLACE(*YES) when (re)compiling a program.

If the user space needs to be auto-extending, then the QUSCUSAT (Change User Space Attributes) API is to be called. For some reason, this attribute can't be set when initially creating a user space.

The prototype for the QUSCUSAT API is as follows:

Change User Space Attributes (QUSCUSAT) API Prototype

D QusChgUsrSpaceAttr...

D                 PR                  extPgm('QUSCUSAT')

D  RtnLibName                   10A

D  UsrSpace                     20A   Const

D  USAttr                       64A   OPTIONS(*VARSIZE)

D  api_error                          LikeDS(QUSEC_T)

D                                     OPTIONS(*VARSIZE)

This API can change several attributes of a user space simultaneously or individually. Each attribute is specified in a data structure. For example, to set the Auto Extend attribute, the following data structure is used:

D usAutoExt         DS                Qualified

D  nRecdCount                   10I 0 Inz(1)

D  nAttrKey                     10I 0 Inz(3)

D  nAttrLen                     10I 0 Inz(1)

D  bExtend                      1A Inz('1')

The first subfield, nRecdCount, is the number of User Space Attribute "records" you have included in this data structure. To change the Auto-Extend attribute, we only need one record.

The second subfield, nAttrKey, contains the attribute key for the attribute being changed. To change the Auto Extend attribute, a key value of 3 is required.

The third subfield, nAttrLen, indicates the length of the value specified for the attribute being changed. The user space Auto Extend attribute is 1 byte long, so this field is initialized to 1.

The fourth subfield is the user space attribute value being applied. For Auto Extend, this field is a 1-byte character field. Setting this field to '1' enables the user space's Auto Extend attribute. A value of '0' disables the Auto Extend attribute.

To change the size of the user space, the following data structure is used:

D usSize            DS                Qualified

D  nRecdCount                   10I 0 Inz(1)

D  nAttrKey                     10I 0 Inz(1)

D  nAttrLen                     10I 0 Inz(4)

D  nNewSize                     10I 0 Inz(12000)

The second subfield, nAttrKey is set to 1 (the key used to indicate User Space size), and the fourth field is changed to an integer (10i0). The new size of the user space must be specified in the nNewSize subfield.

All user space objects are created with a fixed length. To set the auto-extend attribute, the QUSCUSAT APIs must be called after they are created, as follows:

Create User Space and Change Attribute to Auto-Extend

D userSpaceAttr     DS                Qualified

D  nRecdCount                   10I 0 Inz(1)

D  nAttrKey                     10I 0 Inz(3)

D  nAttrLen                     10I 0 Inz(1)

D  bExtend                       1A   Inz('1')

D  apiErrorDS       DS                LikeDS(QUSEC) Inz

D  rtnLib           S          10A

 /free

     QusCrtUsrSpace('MYUSRSPC QGPL':

                    'MYSTUFF' : 6000 : X'00' :

                    '*ALL' : 'Bob''s Stuff' :

                    '*YES' : apiErrorDS);

     QusChgUsrSpaceAttr(rtnLib : 'MYUSRSPC QGPL' :

                         userSpaceAttr : QUSEC);

 /end-free

In this example, the name of the user space is passed on the second parameter and followed by the USERSPACEATTR data structure. This data structure contains the elements necessary to set on the user space's auto-extend attribute, as follows:

• nRecdCount — Number of User Space Attributes being modified.
• nAttrKey — Identifier that indicates which key is being modified. To change the autoextend attribute, the key of 3 is needed.
• nAttrLen — Length of the data being used for the attribute's new value. Since autoextend is a yes/no value (i.e., a logical '1' or '0'), the length is set to 1.
• bExtend — Indicates if the auto-extend attribute is being turned on or turned off. A value of '1' means turn on auto-extend; '0' means turn off auto-extend.

Changing Multiple User Space Attributes

It is relatively easy to set up a data structure for each of the user space attributes and then a master data structure with nested data structures. This allows you to change any of the user space attributes as follows:

D newAttr           DS                Qualified

D  attrCount                    10I 0 Inz(3)

D  size                               LikeDS(QUSKeySize_T) inz(*LIKEDS)

D  initValue                          LikeDS(QUSKeyInit_T) inz(*LIKEDS)

D  autoExt                            LikeDS(QUSKeyAuto_T) inz(*LIKEDS)

The data structure NEWATTR contains a subfield named ATTRCOUNT that identifies the number of user space attributes being changed. This value varies based on the nested data structures being specified. In the example above, three (3) attributes are being changed: the size, initial value, and the auto-extend attribute.

The SIZE, INITVALUE, and AUTOEXT subfields are nested data structures—that is, they are qualified data structures inside the NEWATTR data structure. If you want to modify only the auto-extend attribute of the user space, only the AUTOEXT subfield should be specified:

D newAttr           DS                Qualified

D  attrCount                    10I 0 Inz(1)

D  autoExt                            LikeDS(QUSKeyAuto_T) inz(*LIKEDS)

Note that the ATTRCOUNT subfield is now set to 1 instead of 3, and only AUTOEXT is specified.

The nested subfields are based on data structure templates that I created. They are defined as follows:

User Space Attribute Data Structure Templates

D QUSKeySize_T      DS                Qualified

D  keyID                        10I 0 Inz(1)

D  len                          10I 0 Inz(%size(QUSKeySize_T.value))

D  value                        10I 0 Inz

D QUSKeyInit_T      DS                Qualified

D  keyID                        10I 0 Inz(2)

D  len                          10I 0 Inz(%size(QUSKeyInit_T.value))

D  value                         1A   Inz(X'00')

D QUSKeyAuto_T      DS                Qualified

D  keyID                        10I 0 Inz(3)

D  len                          10I 0 Inz(%size(QUSKeyAuto_T.value))

D  value                         1A   Inz('1')

To set the auto-extend attribute, declare the NEWATTR data structure as previously illustrated and then set the VALUE subfield of the AUTOEXT nested structure:

 /free

       newAttr.autoExt.value = '1';

       qusChgUserSpaceAttr(rtnLib : 'MYUSRSPC QGPL':

                          newAttr : apiError);

 /end-free

Changing the initial value and the auto-extend attribute simultaneously is just as easy:

D newAttr           DS                Qualified

D  attrCount                    10I 0 Inz(2)

D  autoExt                            LikeDS(QUSKeyAuto_T) inz(*LIKEDS)

D  initValue                          LikeDS(QUSKeyInit_T) inz(*LIKEDS)

 /free

       newAttr.autoExt.value = '1';

       newAttr.initValue.value = X'00';

       qusChgUserSpaceAttr(rtnLib : 'MYUSRSPC QGPL':

                          newAttr : apiError);

 /end-free

Editor's Note: This article is an excerpt from RPG TnT: 101 Dynamite Tips 'n Techniques with RPG IV.

as/400, os/400, iseries, system i, i5/os, ibm i, power systems, 6.1, 7.1, V7, V6R1

Bob Cozzi is a programmer/consultant, writer/author, and software developer. His popular RPG xTools add-on subprocedure library for RPG IV is fast becoming a standard with RPG developers. His book The Modern RPG Language has been the most widely used RPG programming book for more than a decade. He, along with others, speaks at and produces the highly popular RPG World conference for RPG programmers.

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