Using malloc(), realloc(), free() in C

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http://en.wikipedia.org/wiki/Malloc

Malloc is a subrouitine for performing dynamic memory allocation in C and C++.

• must be freed by free()
• memory is uninitialized
• not guaranteed to succeed.  Return NULL if fails.

C manages memory statically, automatically, or dynamically.

static-duration variables are allocated in main (fixed) memory and persist for the lifetime of the program.

automatic-duration variables are allocated on the stack and come and go as function are called and return.  Size of allocation is required a compile time (a compile-time constant).

dynamic-duration variables are allocated on the heap in C using malloc.

• use free(ptr) free pointer and returns memory to heap.

http://en.wikipedia.org/wiki/Dynamic_memory_allocation

dynamic memory allocation (also known as heap-based memory allocation) is the allocation of memory storage in a computer program during runtime.

http://en.wikipedia.org/wiki/Heap_overflow

heap overflow 

• a buffer overflow that occurs in the heap data area
• typically contains program data

http://en.wikipedia.org/wiki/Stack-based_memory_allocation

stack-based memory allocation

• a computing architecture where regions of memory have data added or removed in a list-in-first-out manner
• each threat usually has its own stack
• typically faster than heap-based memory allocation
• programmer doesn't need to release the memory used.
• memory size for a given threat is small (a few dozen kilobytes)

http://publications.gbdirect.co.uk/c_book/chapter5/sizeof_and_malloc.html

Has some nice examples.

sizeof(char) is always 1.

malloc(10) // allocates ten chars.

malloc(sizeof(int[10])) // allocates ten ints

If malloc cannot find enough space to satisfy the request, a null pointer is returned.

There is a lot more here on malloc, but that is beyond what I need at the moment.  However, I may need to use there GROW_BY approach to allocating more memory.

http://en.wikipedia.org/wiki/Calloc#calloc

calloc()

• initializes memory block to 0.
• parameter are element count and size of each element.
• should be slower than malloc()

http://en.wikipedia.org/wiki/Realloc#realloc

realloc()

• data is unchanged
• new memory is unallocated
• if new size is smaller than old size, memory is truncated.
• if new returned pointer is set to original pointer, could get a memory if a NULL is returned.