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C gives programmers the standard sort of facilities to allocate and deallocate dynamic
heap memory. A word of warning: writing programs which manage their heap memory is
notoriously difficult. This partly explains the great popularity of languages such as Java
and Perl which handle heap management automatically. These languages take over a task
which has proven to be extremely difficult for the programmer. As a result Perl and Java
programs run a little more slowly, but they contain far fewer bugs. (For a detailed
discussion of heap memory see http://cslibrary.stanford.edu/102/, Pointers and Memory.)

C provides access to the heap features through library functions which any C code can call. The prototypes for these functions are in the file <stdlib.h>, so any code which wants to call these must #include that header file. The three functions of interest are...

void* malloc(size_t size) Request a contiguous block of memory
of the given size in the heap. malloc() returns a pointer to the heap block or NULL if
the request could not be satisfied. The type size_t is essentially an unsigned long which indicates how large a block the caller would like measured in bytes. Because the block pointer returned by malloc() is a void* (i.e. it makes no claim
about the type of its pointee), a cast will probably be required when storing the void*
pointer into a regular typed pointer.

void free(void* block) The mirror image of malloc() -- free takes a
pointer to a heap block earlier allocated by malloc() and returns that block to the heap
for re-use. After the free(), the client should not access any part of the block or
assume that the block is valid memory. The block should not be freed a second time.

void* realloc(void* block, size_t size); Take an existing heap block and try to relocate it to a heap block of the given size which may be larger or smaller than the original size of the block. Returns a pointer to the new block, or
NULL if the relocation was unsuccessful. Remember to catch and examine the return value of realloc() -- it is a common error to continue to use the old block pointer. Realloc() takes care of moving the bytes from the old block to the new block.
Realloc() exists because it can be implemented using low-level features which make
it more efficient than C code the client could write.
 

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