Page 130 - Programming Microcontrollers in C

Memory Management 115

not taken, none of the usual pointer arithmetic will work. If the
memory is not available to be allocated, the allocation program will
return a NULL.
A second dynamic memory allocation function is

void* calloc(int number, size_t n);
This function returns a pointer to a type void. It takes two arguments.
The first argument is the number of items and the second argument is
the size of the specific item. Therefore, if you wished to allocate space
for ten TNODEs from the above problem, you should use

(TNODE *) calloc(10, sizeof(TNODE));
calloc() differs from malloc() in that it returns a pointer to
initialized space. The calloc() function initializes all memory
assigned by the function to zero. When memory is allocated by ei­
ther malloc() or calloc(), this memory must be returned to
the system. If allocated memory is not returned to the system, even­
tually all of the available memory will be allocated. Further attempts
to allocate memory will fail. If the return pointer from the allocation
function is not tested and properly handled when a NULL is returned,
the program will exhibit undefined behavior. This problem can be
avoided by deallocating memory when it is no longer needed by the
program. The function
void free(void* );

will release allocated memory. The argument in this case must be a
pointer that was returned from an allocate function, either malloc()
or calloc().
An additional memory allocation function is

void *realloc(void *,size_t);

The first parameter here is a pointer to a previously allocated memory
block, and the second is the new size that you want for the allocated
memory. This function will change the size of the allocated memory
block. If you are reducing the size of the block, the pointer returned
will probably be the same as that passed to realloc. If you are
increasing the size of the block, and there is not enough contiguous
memory available, the function will search for a proper sized block.