总是记不住内存相关函数的API，特此记录。
1. malloc
void* malloc (size_t size);
Allocates a block of size bytes of memory, returning a pointer to the beginning of the block.
The content of the newly allocated block of memory is not initialized, remaining with indeterminate values.
2. free
void free (void* ptr);
A block of memory previously allocated by a call to malloc, calloc or realloc is deallocated, making it available again for further allocations.
If ptr does not point to a block of memory allocated with the above functions, it causes undefined behavior.
If ptr is a null pointer, the function does nothing.
3. memcpy
void * memcpy ( void * destination, const void * source, size_t num );
Copies the values of num bytes from the location pointed to by source directly to the memory block pointed to by destination.
4. memmove
void * memmove ( void * destination, const void * source, size_t num );
Copies the values of num bytes from the location pointed by source to the memory block pointed by destination. Copying takes place as if an intermediate buffer were used, allowing the destination and source to overlap.
5. realloc
void* realloc (void* ptr, size_t size);
Changes the size of the memory block pointed to by ptr.
The function may move the memory block to a new location (whose address is returned by the function).
The content of the memory block is preserved up to the lesser of the new and old sizes, even if the block is moved to a new location. If the new size is larger, the value of the newly allocated portion is indeterminate.
In case that ptr is a null pointer, the function behaves like malloc, assigning a new block of size bytes and returning a pointer to its beginning.
6. calloc
void* calloc (size_t num, size_t size);
Allocates a block of memory for an array of num elements, each of them size bytes long, and initializes all its bits to zero.
7. memset
void * memset ( void * ptr, int value, size_t num );
Sets the first num bytes of the block of memory pointed by ptr to the specified value (interpreted as an unsigned char).