C语言中动态内存管理函数

内存分配

malloc

function

<cstdlib>

void * malloc ( size_t size );

Allocate memory block

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.

Parameters

size

Size of the memory block, in bytes.

Return Value

On success, a pointer to the memory block allocated by the function.
The type of this pointer is always void*, which can be cast to the desired type of data pointer in order to be dereferenceable.
If the function failed to allocate the requested block of memory, a null pointer is returned.

realloc

function

<cstdlib>

void * realloc ( void * ptr, size_t size );

Reallocate memory block

The size of the memory block pointed to by the ptr parameter is changed to the size bytes, expanding or reducing the amount of memory available in the block.

The function may move the memory block to a new location, in which case the new location is returned. The content of the memory block is preserved up to the lesser of the new and old sizes, even if the block is moved. If the new size is larger, the value of the newly allocated portion is indeterminate.

In case that ptr is NULL, the function behaves exactly as malloc, assigning a new block of size bytes and returning a pointer to the beginning of it.

In case that the size is 0, the memory previously allocated in ptr is deallocated as if a call to free was made, and a NULL pointer is returned.

Parameters

ptr

Pointer to a memory block previously allocated with malloc, calloc or realloc to be reallocated.
If this is NULL, a new block is allocated and a pointer to it is returned by the function.

size

New size for the memory block, in bytes.
If it is 0 and ptr points to an existing block of memory, the memory block pointed by ptr is deallocated and a NULL pointer is returned.

Return Value

A pointer to the reallocated memory block, which may be either the same as the ptr argument or a new location.
The type of this pointer is void*, which can be cast to the desired type of data pointer in order to be dereferenceable.
If the function failed to allocate the requested block of memory, a NULL pointer is returned. 

calloc

function

<cstdlib>

void * calloc ( size_t num, size_t size );

Allocate space for array in memory

Allocates a block of memory for an array of num elements, each of them size bytes long, and initializes all its bits to zero.

The effective result is the allocation of an zero-initialized memory block of (num * size) bytes.

Parameters

num

Number of elements to be allocated.

size

Size of elements.

Return Value

A pointer to the memory block allocated by the function.
The type of this pointer is always void*, which can be cast to the desired type of data pointer in order to be dereferenceable.
If the function failed to allocate the requested block of memory, a NULL pointer is returned.

内存释放

free

function

<cstdlib>

void free ( void * ptr );

Deallocate space in memory

A block of memory previously allocated using a call to malloc, calloc or realloc is deallocated, making it available again for further allocations.

Notice that this function leaves the value of ptr unchanged, hence it still points to the same (now invalid) location, and not to the null pointer.

Parameters

ptr

Pointer to a memory block previously allocated with malloc, calloc or realloc to be deallocated.
If a null pointer is passed as argument, no action occurs.