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Chapter 12 - Dynamic Allocation
WHAT IS DYNAMIC ALLOCATION?
Dynamic allocation is very intimidating to a person the
first time he comes across it, but that need not be. Simply
relax and read this chapter carefully and you will have a
good grounding in a very valuable programming resource. All
of the variables in every program up to this point have been
static variables as far as we are concerned. (Actually,
some of them have been "automatic" and were dynamically
allocated for you by the system, but it was transparent to
you.) In this chapter, we will study some dynamically
allocated variables. They are variables that do not exist
when the program is loaded, but are created dynamically as
they are needed. It is possible, using these techniques, to
create as many variables as needed, use them, and deallocate
their space for use by other variables. As usual, the best
teacher is an example, so load and display the program named
DYNLIST.C.
We begin by defining a named structure "animal" with a
few fields pertaining to dogs. We do not define any
variables of this type, only three pointers. If you search
through the remainder of the program, you will find no
variables defined so we have nothing to store data in. All
we have to work with are three pointers, each of which point
to the defined structure. In order to do anything, we need
some variables, so we will create some dynamically.
DYNAMIC VARIABLE CREATION
The first program statement, which assigns something to
the pointer "pet1" will create a dynamic structure
containing three variables. The heart of the statement is
the "malloc" function buried in the middle of the statement.
This is a "memory allocate" function that needs the other
things to completely define it. The "malloc" function, by
default, will allocate a piece of memory on a "heap" that is
"n" characters in length and will be of type character. The
"n" must be specified as the only argument to the function.
We will discuss "n" shortly, but first we need to define a
"heap".
WHAT IS A HEAP?
Every compiler has a set of limitations on it as to how
big the executable file can be, how many variables can be
used, how long the source file can be, etc. One limitation
placed on users by most C compilers is a limit of 64K for
the executable code if you happen to be in the small memory
model. This is because the IBM-PC uses a microprocessor
with a 64K segment size, and it requires special calls to
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Chapter 12 - Dynamic Allocation
use data outside of a single segment. In order to keep the
program small and efficient, these calls are not used, and
the memory space is limited but still adequate for most
programs.
A heap is an area outside of this 64K boundary which
can be accessed by the program to store data and variables.
The data and variables are put on the "heap" by the system
as calls to "malloc" are made. The system keeps track of
where the data is stored. Data and variables can be
deallocated as desired leading to holes in the heap. The
system knows where the holes are and will use them for
additional data storage as more "malloc" calls are made.
The structure of the heap is therefore a very dynamic
entity, changing constantly.
MORE ABOUT SEGMENTS
Most C compilers give the user a choice of memory
models to use. The user has a choice of using a model with a
64K limitation for either program or data leading to a small
fast program or selecting a 640K limitation and requiring
longer address calls leading to less efficient addressing.
Using the larger address space requires inter segment
addressing, resulting in the slightly slower running time.
The time is probably insignificant in most programs, but
there are other considerations.
If a program uses no more than 64K bytes for the total
of its code and memory and if it doesn't use a stack, it can
be made into a .COM file. Since a .COM file is already in a
memory image format, it can be loaded very quickly whereas a
file in an .EXE format must have its addresses relocated as
it is loaded. Therefore a tiny memory model can generate a
program that loads faster than one generated with a larger
memory model. Don't let this worry you, it is a fine point
that few programmers worry about.
Using dynamic allocation, it is possible to store the
data on the "heap" and that may be enough to allow you to
use the small memory model. Of course, you wouldn't store
local variables such as counters and indexes on the heap,
only very large arrays or structures.
Even more important than the need to stay within the
small memory model is the need to stay within the computer.
If you had a program that used several large data storage
areas, but not at the same time, you could load one block
storing it dynamically, then get rid of it and reuse the
space for the next large block of data. Dynamically storing
each block of data in succession, and using the same storage
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Chapter 12 - Dynamic Allocation
for each block may allow you to run your entire program in
the computer without breaking it up into smaller programs.
BACK TO THE "MALLOC" FUNCTION
Hopefully the above description of the "heap" and the
overall plan for dynamic allocation helped you to understand
what we are doing with the "malloc" function. It simply
asks the system for a block of memory of the size specified,
and gets the block with the pointer pointing to the first
element of the block. The only argument in the parentheses
is the size of the block desired and in our present case, we
desire a block that will hold one of the structures we
defined at the beginning of the program. The "sizeof" is a
new function, new to us at least, that returns the size in
bytes of the argument within its parentheses. It therefore,
returns the size of the structure named "animal", in bytes,
and that number is sent to the system with the "malloc"
call. At the completion of that call, we have a block on
the heap allocated to us, with "pet1" pointing to the block
of data.
WHAT IS A CAST?
We still have a funny looking construct at the
beginning of the "malloc" function call. That is called a
"cast". The "malloc" function returns a block with the
pointer pointing to it being a pointer of type "char" by
default. Many times, if not most, you do not want a pointer
to a "char" type variable, but to some other type. You can
define the pointer type with the construct given on the
example line. In this case we want the pointer to point to
a structure of type "animal", so we tell the compiler with
this strange looking construct. Even if you omit the cast,
most compilers will return a pointer correctly, give you a
warning, and go on to produce a working program. It is
better programming practice to provide the compiler with the
cast to prevent getting the warning message.
USING THE DYNAMICALLY ALLOCATED MEMORY BLOCK
If you remember our studies of structures and pointers,
you will recall that if we have a structure with a pointer
pointing to it, we can access any of the variables within
the structure. In the next three lines of the program, we
assign some silly data to the structure for illustration.
It should come as no surprise to you that these assignment
statements look just like assignments to statically defined
variables.
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Chapter 12 - Dynamic Allocation
In the next statement, we assign the value of "pet1" to
"pet2" also. This creates no new data, we simply have two
pointers to the same object. Since "pet2" is pointing to
the structure we created above, "pet1" can be reused to get
another dynamically allocated structure which is just what
we do next. Keep in mind that "pet2" could have just as
easily been used for the new allocation. The new structure
is filled with silly data for illustration.
Finally, we allocate another block on the heap using
the pointer "pet3", and fill its block with illustrative
data.
Printing the data out should pose no problem to you
since there is nothing new in the three print statements.
It is left for you to study.
Even though it is not illustrated in this tutorial, you
can dynamically allocate and use simple variables such as a
single "char" type variable. This should be discouraged
however since it is very inefficient.
GETTING RID OF THE DYNAMICALLY ALLOCATED DATA
Another new function is used to get rid of the data and
free up the space on the heap for reuse, the function
"free". To use it, you simply call it with the pointer to
the block as the only argument, and the block is
deallocated.
In order to illustrate another aspect of the dynamic
allocation and deallocation of data, an additional step is
included in the program on your monitor. The pointer "pet1"
is assigned the value of "pet3" in line 38. In doing this,
the block that "pet1" was pointing to is effectively lost
since there is no pointer that is now pointing to that
block. It can therefore never again be referred to,
changed, or disposed of. That memory, which is a block on
the heap, is wasted from this point on. This is not
something that you would ever purposely do in a program. It
is only done here for illustration.
The first "free" function call removes the block of
data that "pet1" and "pet3" were pointing to, and the second
"free" call removes the block of data that "pet2" was
pointing to. We therefore have lost access to all of our
data generated earlier. There is still one block of data
that is on the heap but there is no pointer to it since we
lost the address to it. Trying to "free" the data pointed
to by "pet1" would result in an error because it has already
been "freed" by the use of "pet3". There is no need to
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Chapter 12 - Dynamic Allocation
worry, when we return to DOS, the entire heap will be
disposed of with no regard to what we have put on it. The
point does need to made that, if you lose a pointer to a
block of the heap, it forever removes that block of data
storage from our use and we may need that storage later.
Compile and run the program to see if it does what you
think it should do based on this discussion.
THAT WAS A LOT OF DISCUSSION
It took nearly four pages to get through the discussion
of the last program but it was time well spent. It should
be somewhat exciting to you to know that there is nothing
else to learn about dynamic allocation, the last four pages
covered it all. Of course, there is a lot to learn about
the technique of using dynamic allocation, and for that
reason, there are two more files to study. But the fact
remains, there is nothing more to learn about dynamic
allocation than what was given so far in this chapter.
AN ARRAY OF POINTERS
Load and display the file BIGDYNL.C for another example
of dynamic allocation. This program is very similar to the
last one since we use the same structure, but this time we
define an array of pointers to illustrate the means by which
you could build a large database using an array of pointers
rather than a single pointer to each element. To keep it
simple we define 12 elements in the array and another
working pointer named "point".
The "*pet[12]" is new to you so a few words would be in
order. What we have defined is an array of 12 pointers, the
first being "pet[0]", and the last "pet[11]". Actually,
since an array is itself a pointer, the name "pet" by itself
is a pointer to a pointer. This is valid in C, and in fact
you can go farther if needed but you will get quickly
confused. I know of no limit as to how many levels of
pointing are possible, so a definition such as "int ****pt"
is legal as a pointer to a pointer to a pointer to a pointer
to an integer type variable, if I counted right. Such usage
is discouraged until you gain considerable experience.
Now that we have 12 pointers which can be used like any
other pointer, it is a simple matter to write a loop to
allocate a data block dynamically for each and to fill the
respective fields with any data desirable. In this case,
the fields are filled with simple data for illustrative
purposes, but we could be reading in a database, readings
from some test equipment, or any other source of data.
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Chapter 12 - Dynamic Allocation
A few fields are randomly picked to receive other data
to illustrate that simple assignments can be used, and the
data is printed out to the monitor. The pointer "point" is
used in the printout loop only to serve as an illustration,
the data could have been easily printed using the "pet[n]"
means of definition. Finally, all 12 blocks of data are
freed before terminating the program.
Compile and run this program to aid in understanding
this technique. As stated earlier, there was nothing new
here about dynamic allocation, only about an array of
pointers.
A LINKED LIST
We finally come to the grandaddy of all programming
techniques as far as being intimidating. Load the program
DYNLINK.C for an example of a dynamically allocated linked
list. It sounds terrible, but after a little time spent
with it, you will see that it is simply another programming
technique made up of simple components that can be a
powerful tool.
In order to set your mind at ease, consider the linked
list you used when you were a child. Your sister gave you
your birthday present, and when you opened it, you found a
note that said, "Look in the hall closet." You went to the
hall closet, and found another note that said, "Look behind
the TV set." Behind the TV you found another note that
said, "Look under the coffee pot." You continued this
search, and finally you found your pair of socks under the
dogs feeding dish. What you actually did was to execute a
linked list, the starting point being the wrapped present
and the ending point being under the dogs feeding dish. The
list ended at the dogs feeding dish since there were no more
notes.
In the program DYNLINK.C, we will be doing the same
thing as your sister forced you to do. We will however, do
it much faster and we will leave a little pile of data at
each of the intermediate points along the way. We will also
have the capability to return to the beginning and
retraverse the entire list again and again if we so desire.
THE DATA DEFINITIONS
This program starts similarly to the last two with the
addition of the definition of a constant to be used later.
The structure is nearly the same as that used in the last
two programs except for the addition of another field within
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Chapter 12 - Dynamic Allocation
the structure in line 11, the pointer. This pointer is a
pointer to another structure of this same type and will be
used to point to the next structure in order. To continue
the above analogy, this pointer will point to the next note,
which in turn will contain a pointer to the next note after
that.
We define three pointers to this structure for use in
the program, and one integer to be used as a counter, and we
are ready to begin using the defined structure for whatever
purpose we desire. In this case, we will once again
generate nonsense data for illustrative purposes.
THE FIRST FIELD
Using the "malloc" function, we request a block of
storage on the "heap" and fill it with data. The additional
field in this example, the pointer, is assigned the value of
NULL, which is only used to indicate that this is the end of
the list. We will leave the pointer "start" at this
structure, so that it will always point to the first
structure of the list. We also assign "prior" the value of
"start" for reasons we will see soon. Keep in mind that the
end points of a linked list will always have to be handled
differently than those in the middle of a list. We have a
single element of our list now and it is filled with
representative data.
FILLING ADDITIONAL STRUCTURES
The next group of assignments and control statements
are included within a "for" loop so we can build our list
fast once it is defined. We will go through the loop a
number of times equal to the constant "RECORDS" defined at
the beginning of our program. Each time through, we
allocate memory, fill the first three fields with nonsense,
and fill the pointers. The pointer in the last record is
given the address of this new record because the "prior"
pointer is pointing to the prior record. Thus "prior->next"
is given the address of the new record we have just filled.
The pointer in the new record is assigned the value "NULL",
and the pointer "prior" is given the address of this new
record because the next time we create a record, this one
will be the prior one at that time. That may sound
confusing but it really does make sense if you spend some
time studying it.
When we have gone through the "for" loop 6 times, we
will have a list of 7 structures including the one we
generated prior to the loop. The list will have the
following characteristics.
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Chapter 12 - Dynamic Allocation
1. "start" points to the first structure in the list.
2. Each structure contains a pointer to the next structure.
3. The last structure has a pointer that points to NULL and
can be used to detect the end.
start->struct1 This diagram should aid in
name understanding the structure of
breed the data at this point.
age
point->struct2
name
breed
age
point->struct3
name
breed
age
point-> . . . . struct7
name
breed
age
point->NULL
It should be clear to you, if you understand the above
structure, that it is not possible to simply jump into the
middle of the structure and change a few values. The only
way to get to the third structure is by starting at the
beginning and working your way down through the structure
one record at a time. Although this may seem like a large
price to pay for the convenience of putting so much data
outside of the program area, it is actually a very good way
to store some kinds of data.
A word processor would be a good application for this
type of data structure because you would never need to have
random access to the data. In actual practice, this is the
basic type of storage used for the text in a word processor
with one line of text per record. Actually, a program with
any degree of sophistication would use a doubly linked list.
This would be a list with two pointers per record, one
pointing down to the next record, and the other pointing up
to the record just prior to the one in question. Using this
kind of a record structure would allow traversing the data
in either direction.
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Chapter 12 - Dynamic Allocation
PRINTING THE DATA OUT
To print the data out, a similar method is used as that
used to generate the data. The pointers are initialized and
are then used to go from record to record reading and
displaying each record one at a time. Printing is
terminated when the NULL on the last record is found, so the
program doesn't even need to know how many records are in
the list. Finally, the entire list is deleted to make room
in memory for any additional data that may be needed, in
this case, none. Care must be taken to assure that the last
record is not deleted before the NULL is checked. Once the
data is gone, it is impossible to know if you are finished
yet.
MORE ABOUT DYNAMIC ALLOCATION AND LINKED LISTS
It is not difficult, and it is not trivial, to add
elements into the middle of a linked lists. It is necessary
to create the new record, fill it with data, and point its
pointer to the record it is desired to precede. If the new
record is to be installed between the 3rd and 4th, for
example, it is necessary for the new record to point to the
4th record, and the pointer in the 3rd record must point to
the new one. Adding a new record to the beginning or end of
a list are each special cases. Consider what must be done
to add a new record in a doubly linked list.
Entire books are written describing different types of
linked lists and how to use them, so no further detail will
be given. The amount of detail given should be sufficient
for a beginning understanding of C and its capabilities.
ANOTHER NEW FUNCTION - CALLOC
One more function must be mentioned, the "calloc"
function. This function allocates a block of memory and
clears it to all zeros which may be useful in some
circumstances. It is similar to "malloc" and will be left
as an exercise for you to read about and use "calloc" if you
desire.
PROGRAMMING EXERCISES
1. Rewrite the example program STRUCT1.C from chapter 11 to
dynamically allocate the two structures.
2. Rewrite the example program STRUCT2.C from chapter 11 to
dynamically allocate the 12 structures.
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