Pointer learning diary (III)

Dynamic memory allocation
When an array is declared , The memory he needs is allocated at compile time .
But we always write similar code ：

int a[1000000] = { 0 };

In a sense , you 're right , And this kind of method can well avoid the problem of array overflow .
But at the same time , This is definitely not a good program , The reason is simple , Memory helps you allocate sizeof（int）*1000000;
But you may not use a lot of memory at all , This creates waste . Fortunately , stay C and C++ Programmers can manually allocate memory and free unused memory .

One 、malloc And free

Both are in the header file stdlib.h In a statement .
malloc Extract a suitable piece of memory from the memory pool , And return a Pointer to this memory .
When a previously allocated memory is no longer used , Program call free Function returns it to the memory pool for future needs .

ps：malloc Allocated is a continuous block of memory , If the available memory of the memory pool can meet this demand ,malloc Just Returns a pointer to the starting position of the allocated memory block . If the memory pool is empty , It will ask the system to get more memory , If the system cannot provide more memory , that Will return a NULL The pointer

therefore ！！ For each slave malloc All returned pointers should be checked , Make sure it's not NULL, It's very important .

ps:C++ Medium new It may be more convenient , But this study diary mainly introduces C, So I won't retell too much here

void *malloc（size_t size);
void free(void *pointer);

malloc Return to one void The pointer , That's why . Standard represents a void Pointers of type can be converted to pointers of any other type .

Two 、calloc And realloc

calloc Also used to allocate memory , And malloc The biggest difference is ,calloc Initialize it to... Before returning a pointer to memory 0.relloc Function is used to modify the size of a previously allocated memory block , You can expand or reduce a piece of memory

void *calloc（size_t num_elements,size_t element_size）;
void relloc(void *ptr,size_t new_size)

Pass to free The pointer to must be a malloc、calloc or realloc The pointer returned by the function .

！！ Be careful , Don't visit already free Memory freed by function . The most common situation is , A pointer to dynamically allocated memory has been copied , Then access the copy in other parts of the program .