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TechTip: Lock and Lock

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Create your own utility to lock what the Allocate Object (ALCOBJ) command does not allow you to lock.

The Allocate Object (ALCOBJ) command is used to acquire one or more lock states on each of a list of objects. The lock states are allocated to the requesting thread, its containing job (i.e., MI process), or a lock space (i.e., an MI transaction control structure (TCS) object). Generally, the ALCOBJ command and its partner, the Deallocate Object (DLCOBJ) command, can achieve tasks similar to the Lock Object (LOCK) and Unlock Object (UNLOCK) MI instructions. But not exactly.

The ALCOBJ command does not allow the user to lock specific object types—for example, the User Profile (*USRPRF) objects (with MI object type code hex 08), and it can't lock internal objects (MI objects that are not visible at the operation system level). ALCOBJ also adds many limitations on what kinds of lock states can be allocated on specific object types. For example, ALCOBJ forbids allocating an *EXCL lock state on a Library (*LIB) object or a Device Description (*DEVD) object. Also, lock states except *EXCL can't be allocated on a Subsystem Description (*SBSD) object via ALCOBJ. These restrictions are also true for the DLCOBJ command.

However, the ability to allocate or deallocate any kind of lock state on any kind of MI object is sometimes necessary for an IBM i developer or administrator. For example, allocating a *SHRRD lock on a *SBSD object can prevent accidental deletion of the *SBSD object. Actually, that's the method used by the system to protect the subsystem descriptions of active subsystems. (Try issuing a Work with Object Lock (WRKOBJLCK) command upon the *SBSD of an active subsystem.)

This TechTip shows you how to create your own utility to lock/unlock what the ALCOBJ/DCLOBJ commands do not allow.

The LCKLCK Program

The example program called LCKLCK implements the tasks mentioned above. The working mechanism of the LCKLCK program is quite simple—issuing the LOCK or the UNLOCK MI instruction on an MI object on behalf of its caller. The LCKLCK program accepts the following parameters:

  • A system pointer to the target MI object to lock/unlock
  • CHAR(1) requested lock state
  • CHAR(1) flag that indicates whether a LOCK or UNLOCK MI instruction is to be issued on the target MI object

The source program (lcklck.rpgle) of LCKLCK is the following:

     /**

     * @file lcklck.rpgle

     *

     * Lock/Unlock an MI object idendified by a system pointer.

     * @param [in] SYSPTR to the MI object

     * @param [in] Requested lock state

     * @param [in] Flag. 'U' = Unlock, anything else = Lock

     */

     h dftactgrp(*no) actgrp('LCKLCK')

     d lock_request   ds                 qualified

     d       num_requests...

     d                               10i 0 inz(1)

     d       offset_lock_state...

     d                              5i 0 inz(32)

     d       time_out                 20u 0

     d       lock_opt                   2a     inz(x'4200')

     d       obj@                       *

     d       lock_state                 1a

     d                               15a

     * Lock request option related constants

     d LOCK_REQUEST_TYPE_IMMED...

     d                 c                   x'0000'

     d LOCK_REQUEST_TYPE_SYNC...

     d                 c                   x'4000'

     d LOCK_REQUEST_TYPE_ASYNC...

   d                 c                   x'8000'

     d LOCK_WAIT_4EVER...

     d                 c                   x'0200'

     d LOCK_SCOPED_TO_OBJ...

     d                 c                   x'0000'

     d LOCK_SCOPED_TO_THREAD...

     d                  c                   x'0080'

     d LOCK_SCOPE_OBJ_MI_PROCESS...

     d                 c                   x'0000'

     d LOCK_SCOPE_OBJ_TCS...

     d                 c                   x'0040'

     * The Lock Object (LOCK) MI instruction

     d lockobj         pr                 extproc('_LOCK')

     d       lock_request...

     d                                       likeds(lock_request)

     * The Unlock Object (UNLOCK) MI instruction

     d unlockobj       pr                 extproc('_UNLOCK')

     d       unlock_request...

     d                                       likeds(lock_request)

     * Prototype of the LCKLCK program

     d i_main         pr                 extpgm('LCKLCK')

     d       object                     *

     d    lock_state                 1a

     d       flag                       1a

     d i_main         pi

     d       object@                     *

     d       lock_state                 1a

     d       flag                       1a

     /free

           lock_request.lock_opt = %bitor(

             LOCK_REQUEST_TYPE_SYNC

             : LOCK_WAIT_4EVER

             : LOCK_SCOPED_TO_OBJ

             : LOCK_SCOPE_OBJ_MI_PROCESS );

           lock_request.obj@ = object@;

           lock_request.lock_state = lock_state;

           if flag = 'U';

               unlockobj(lock_request);

           else;

               lockobj(lock_request);

           endif;

           *inlr = *on;

     /end-free

An OMI version of the LCKLCK program (lcklck.emi) is also available as follows:

/**

* @file lcklck.emi

*

* Lock/Unlock an MI object idendified by a system pointer.

* @param [in] SYSPTR to the MI object

* @param [in] Requested lock state

* @param [in] Flag. 'U' = Unlock, anything else = Lock

*/

dcl spcptr obj@@ parm           ;

dcl spcptr lck-sts@ parm       ;

dcl spcptr flag@ parm           ;

dcl ol pl-main (

       obj@@,

       lck-sts@,

       flag@

) parm ext                     ;

entry i-main(pl-main) ext       ;

dcl sysptr p-obj@ bas(obj@@)     ;

dcl dd p-lck-sts char(1) bas(lck-sts@) ;

dcl dd flag char(1) bas(flag@)       ;

dcl spcptr tmpl@ auto init(tmpl) ;

dcl dd tmpl char(64) auto       ;

       dcl dd num-requests bin(4) def(tmpl) pos(1) init(1) ;

       dcl dd off-lck-sts bin(2) def(tmpl) pos(5) init(32) ;

       dcl dd lck-opt char(2) def(tmpl) pos(15) init(x'4200') ; /* 01000010,00000000b */

         /* Lock request type = '01'. Synchronous request- Wait until all locks can be granted. */

         /* Time-out option = '1'. Wait indefinitely */

         /* Template extension specified = '0'. No template extension */

         /* Lock scope = '0'. Lock is scoped to the lock scope object type. */

         /* Lock scope object type = '0'. Process containing the current thread. */

       dcl sysptr obj@ def(tmpl) pos(17) ;

       dcl dd lck-sts char(1) def(tmpl) pos(33) ;

       cpybwp obj@, p-obj@   ;

       cpybla lck-sts, p-lck-sts ;

       cmpbla(b) flag, 'U' / neq(=+3) ;

       unlock tmpl@                 ;

        b         =+2                   ;

:       lock     tmpl@                 ;

:       rtx       *                     ;

pend                                   ;

Finally, to avoid the Object Domain or Hardware Storage Protection Violation (hex 4401) exception at security level 40 or above when locking/unlocking system domain objects, change the compiled LCKLCK program to system state by changing the program state field in the program header of LCKLCK from user state (hex 0001) to system state (hex 0080). At V5R4, the program state field is at offset hex 5C from the start of the program header.

Experiment with the LCKLCK Program

Now let's do a couple of experiments with the LCKLCK program. As a lock-anything utility, you will surely find more uses of the LCKLCK program. The following tiny ILE C program (oo08t1.c) accepts the name of a library (i.e., a context object) and then calls the LCKLCK program to acquire an *EXCL (i.e., LENR) lock state on the library.

/**

* @file oo08t1.c

*

* Test of the LCKLCK program.

*/

# include <stdlib.h>

# include <string.h>

# pragma linkage(LCKLCK, OS)

void LCKLCK(void **obj, char *lock_state, char *flag);

# pragma linkage(_RSLVSP2, builtin)

void _RSLVSP2(void**, void*);

int main(int argc, char *argv[]) {

void *ctx = NULL;

char lck_sts = 0x9;

char flag = 'L';

char rt[34] = {0};

char *lib = NULL;

if(argc < 2) {

   return -1;

}

lib = argv[1];

memcpy(rt, "\x04\x01", 2);

memset(rt + 2, 0x40, 30);

memcpy(rt + 2, lib, strlen(lib));

_RSLVSP2(&ctx, rt);

LCKLCK(&ctx, &lck_sts, &flag);

return 0;

}

Compile the OO08T1 program and call it to lock the LIBA library in job JOBA. Then try to move an object (say, *DTAQ #0A01#1) into LIBA in another job (say, JOBB) via a MOVOBJ command.

JOBA> CALL OO08T1 LIBA

JOBB> MOVOBJ #0A01#1 *DTAQ TOLIB(LIBA)

Now the Work with Object Locks display of a WRKOBJLCK #0A01#1 *DTAQ command issued from JOBA might look like the following:

Opt   Job         User         Lock     Status         Scope     Thread  

     JOBB         LJL         *SHRUPD   WAIT           *THREAD   00000966

     JOBA         LJL         *EXCL     HELD         *JOB            

This tells us that the Move Object (MOVOBJ) command (or more exactly, the Command Processing Program (CPP) of MOVOBJ, QSYS/QLIMVOBJ) requests a *SHRUPD (i.e., LSUP) lock state synchronously before actually moving an object from its containing library to a new library.

Now let's do another experiment to protect a subsystem description object from being deleted by other jobs. The following ILE C program (oo08t2.c) allocates a *SHRRD (i.e., LSRD) lock state on a specified *SBSD.

/**

* @file oo08t2.c

*

* Test of LCKLCK program.

*/

# include <stdlib.h>

# include <string.h>

# pragma linkage(LCKLCK, OS)

void LCKLCK(void **obj, char *lock_state, char *flag);

# pragma linkage(_RSLVSP2, builtin)

void _RSLVSP2(void**, void*);

int main(int argc, char *argv[]) {

void *spc1909 = NULL;

char lck_sts = 0x81;

char flag = 'L';

char rt[34] = {0};

char *sbsd = NULL;

if(argc < 2) {

   return -1;

}

sbsd = argv[1];

memcpy(rt, "\x19\x09", 2);

memset(rt + 2, 0x40, 30);

memcpy(rt + 2, sbsd, strlen(sbsd));

_RSLVSP2(&spc1909, rt);

LCKLCK(&spc1909, &lck_sts, &flag);

return 0;

}

Compile the OO08T2 program and call it to allocate a *SHRRD (LSRD) lock state on a *SBSD (say, DEC) in job JOBA. Then try to delete DEC in job JOBB via a DLTSBSD command.

JOBA> CALL OO08T2 DEC

JOBB> DLTSBSD DEC

Now the Work with Object Locks display of a WRKOBJLCK DEC *SBSD command issued from JOBA might look like the following:

Opt   Job         User         Lock     Status         Scope     Thread  

     JOBB         LJL         *EXCL     WAIT           *THREAD   00000966

     JOBA         LJL         *SHRRD     HELD           *JOB            

This tells us that the DLTSBSD command (the CPP of which is QSYS/QLIDLOB) needs to acquire an *EXCL (i.e., LENR) lock state before destroying the *SBSD. And the *EXCL lock state requested by JOBB will not be granted due to the *SHRRD lock stated allocated on the *SBSD object by JOBA. The DLTSBD command issued from JOBB will fail after a wait time-out interval.

 

Junlei Li

Junlei Li is a programmer from Tianjin, China, with 10 years of experience in software design and programming. Junlei Li began programming under i5/OS (formerly known as AS/400, iSeries) in late 2005. He is familiar with most programming languages available on i5/OS—from special-purpose languages such as OPM/ILE RPG to CL to general-purpose languages such as C, C++, Java; from strong-typed languages to script languages such as QShell and REXX. One of his favorite programming languages on i5/OS is machine interface (MI) instructions, through which one can discover some of the internal behaviors of i5/OS and some of the highlights of i5/OS in terms of operating system design.

 

Junlei Li's Web site is http://i5toolkit.sourceforge.net/, where his open-source project i5/OS Programmer's Toolkit (https://sourceforge.net/projects/i5toolkit/) is documented.

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