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DYNAMIC MEMORY ALLOCATION USING THE HEAP.
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The normal variables used in C programs are 'static' variables, which are
declared in the program before they are used and allocated space in the
data segment of memory at compile time. Except for the Huge Memory Model,
the data segment is limited to 64K.
Sometimes it is desirable to allow the size of the data to grow as the
program is being executed, typically with databases where the number of
structures (or records) needs to increase progressively with each new
addition, frequently beyond the 64K limit of the data segment.
This situation is catered for by 'dynamic memory allocation' using a
function such as malloc(size) which allocates a block of 'size' bytes
from the C memory heap. In the large data models, all the space beyond
the program stack to the end of phsical memory is available for the heap.
An example of dynamic memory allocation is given on page 186 of the
User's Guide (version 2.0) as:
#include <alloc.h>
main()
{
int *iptr;
iptr = (int *) malloc(sizeof(int));
.....
.....
}
malloc() is declared in the header file alloc.h which has to be included
as shown.
iptr is a pointer to an integer and it is assigned the value returned by
the function malloc().
The expression sizeof(int) returns the number of bytes that a variable of
type int requires.
The function malloc(size) allocates 'size' number of consecutive bytes of
heap memory.
The expression (int *) means that iptr will be a pointer to type int.
The dynamically created integer variable can be referred to as *iptr.
Another example of the use of *malloc(size); can be seen in the program
LINKLIST.C (line 41) as shown below:
this = (struct person *)malloc(sizeof(struct person));
Here the structure is declared as:
struct person {
char name[20];
char job[20];
struct person *next;
};
and the pointer 'this' is a pointer to struct person as declared below:
struct person *first, *last, *this;
Here the memory allocated is the sizeof(struct person) which is 44 bytes
for the large memory model with a pointer to both the segment and the
offset addresses (4 bytes), but only 42 for the small memory model with
only an offset value required for the pointer (2 bytes).
Other examples of the use of malloc are found in the programs BINTREE.C
and TWOLINKS.C
calloc(numberofitems,sizeofitem) is a similar function, except that it
has two parameters or arguments, the first is the number of items and the
second is the size of each item.
free(ptr) is the function used to free the memory pointed to by ptr.
Refer to the Reference Guide for further details of these and other memory
allocation functions.