DYNAMIC MEMORY ALLOCATION
Malloc, Sizeof, and Free
The malloc Function is used to allocate a block of memory on run time.void *malloc(number_of_bytes)
That is to say it returns a pointer of type void * that is the start in memory of the reserved portion of size number_of_bytes. If memory cannot be allocated a NULL pointer is returned.
Since a void * is returned the C standard states that this pointer can be converted to any type. The size_t argument type is defined in stdlib.h and is an unsigned type.
So:
char *cp; cp = malloc(50);
attempts to get 50 bytes and assigns the start address to cp.
Also it is usual to use the sizeof() function to specify the number of bytes:
int *ip; ip = (int *) malloc(100*sizeof(int));
Some C compilers may require to cast the type of conversion. The (int *) means coercion to an integer pointer. Coercion to the correct pointer type is very important to ensure pointer arithmetic is performed correctly. I personally use it as a means of ensuring that I am totally correct in my coding and use cast all the time.
It is good practice to use sizeof() even if you know the actual size you want -- it makes for device independent (portable) code.
sizeof can be used to find the size of any data type, variable or structure. Simply supply one of these as an argument to the function.
SO:
int i; struct Node
{
float x,y,z
}; typedef struct Node sn; sizeof(int), sizeof(i), sizeof(struct Node) and sizeof(sn) are all ACCEPTABLE
In the above we can use the link between pointers and arrays to treat the reserved memory like an array. i.e we can do things like:
ip[0] = 100;
or
for(i=0;i<100;++i) scanf("%d",ip++);
When you have finished using a portion of memory you should always free() it. This allows the memory freed to be available again, possibly for further malloc() calls
The function free() takes a pointer as an argument and frees the memory to which the pointer refers.
Calloc and Realloc
There are two additional memory allocation functions, Calloc() and Realloc(). Their prototypes are given below:void *calloc(size_t num_elements, size_t element_size}; void *realloc( void *ptr, size_t new_size);Malloc does not initialise memory (to zero) in any way. If you wish to initialise memory then use calloc. Calloc there is slightly more computationally expensive but, occasionally, more convenient than malloc. Also note the different syntax between calloc and malloc in that calloc takes the number of desired elements, num_elements, and element_size, element_size, as two individual arguments.
Thus to assign 100 integer elements that are all initially zero you would do:
int *ip; ip = (int *) calloc(100, sizeof(int));Realloc is a function which attempts to change the size of a previous allocated block of memory. The new size can be larger or smaller.
If the block is made larger then the old contents remain unchanged and memory is added to the end of the block. If the size is made smaller then the remaining contents are unchanged.
If the original block size cannot be resized then realloc will attempt to assign a new block of memory and will copy the old block contents.
Note a new pointer (of different value) will consequently be returned. You must use this new value. If new memory cannot be reallocated then realloc returns NULL.
Thus to change the size of memory allocated to the *ip pointer above to an array block of 50 integers instead of 100, simply do:
ip = (int *) calloc( ip, 50);
