IBM AS/400 manual Managing Your Own Heap, Managing Dynamically-Allocated Storage, Bindable APIs

Managing Dynamically-Allocated Storage

*-----------------------------------------------------------------			*
* Heap Storage Misuse			*
*-----------------------------------------------------------------			*
D Fld1	S	25A	BASED(Ptr1)
D Fld2	S	5A	BASED(Ptr2)
D Ptr1	S	*
D Ptr2	S	*
....
C	ALLOC	25	Ptr1
C	DEALLOC		Ptr1
*-----------------------------------------------------------------			*
* After this point, Fld1 should not be accessed since the
* basing pointer ptr1 no longer points to allocated storag￿e.
*-----------------------------------------------------------------			*
C	CALL	'SOMEPGM'
*-----------------------------------------------------------------			*
* During the previous call to 'SOMEPGM', several storage
* allocations may have been done.
* In any case, it is extremely dangerous to make the
* following assignment, since 25 bytes of storage will be
* filled with 'a'. It is impossible to know what that storage
* is currently being used for.
*-----------------------------------------------------------------			*
C	EVAL	Fld1 = *ALL'a'
Figure 53.	Heap Storage Misuse

Similarly, errors can occur in the following cases:

¹A similar error can be made if a pointer is copied before bei deallocated. Great care must be taken when copying pointers to a storage, to ensure that they are not used after the storage i reallocated.

¹If a pointer to heap storage is copied, the copy can be use

reallocate

the

storage.

In

this

case,

the

original

pointer

should

it

is

set

to

a

new

value.

¹ If a pointer to

heap

storage is passed as a parameter, the c

cate

or reallocate

the

storage. After the call returns, attempts

pointer

could

cause

problems.

¹ If a pointer to

heap

storage

is set

in the *INZSR, a later RES

could

cause

the

pointer

to

get

set

to

storage

that

is

no

longer

¹ Another

type

of

problem

can

be

caused

if

a pointer

to

heap

sto

being

cleared,

or

set

to

a

new

pointer

by

an ALLOC

operation,

fo

Once

the

pointer

is

lost,

the

storage it

pointed

to

cannot

be

f

is

unavailable

to

be

allocated

since

the

system

does

not

know

tha

is

no

longer

addressable.

The

storage

will

not

be

freed

until

the

activation

group

ends.

Managing Your Own Heap	Using	ILE	Bindable APIs
You can isolate the dynamic storage					used	by	some	programs	and	proc
within	an	activation		group by creating	one	or	more user-created heap
tion on creating			a	user-created ILEheapConceptsrefer.			to
The following example shows you how to					manage		dynamic	storage	for	a
array	with a user-created heap from				an	ILE	RPG procedure.		In	this

Chapter 9. Running a Program119