Dynamic memory management (1)

1. Why is there dynamic memory allocation

At present, the way we know to open up space is ：

int val = 20;  // Open up four bytes in the stack space
char arr[10] = {0};  // Open up on the stack space 10 Bytes of contiguous space

  But there are two characteristics of the way to open up space ：
1. The size of the space opening is fixed .
2. Arrays are declared , You must specify the length of the array , The memory it needs is allocated at compile time .
But on the way to writing code , The situation is changeable , The fixed space size may not be enough for operation , Then we can't change the size , We introduced a new memory allocation method —— Dynamic memory .

2. The introduction of dynamic memory function

2.1 malloc and free

In the above question ,C Language provides a dynamic development function ——malloc function

1.  This function requests a piece of memory Continuously available Space , And return the pointer to this space .
2.  If the development is successful , Then return a pointer to open a good space .
3.  If the development fails , Returns a NULL The pointer , therefore malloc The return value of must be checked .
4.  The type of return value is void* , therefore malloc Function doesn't know the type of open space , Specifically, when using, the user can
It's up to me to decide .
5.  If parameters size by 0,malloc The standard is undefined , Depends on the compiler .

C Language provides another function free, It is specially used for dynamic memory release and recovery , The function prototype is as follows ：

void free (void* ptr);

 free Function is used to release dynamically opened memory .
1. If parameters ptr The pointed space is not opened dynamically , that free The behavior of a function is undefined .
2. If parameters ptr yes NULL The pointer , Then the function does nothing .
malloc and free It's all stated that stdlib.h Header file .

so to speak malloc and free It's used together .

2.2 calloc function

C The language also provides a function called calloc , calloc Function is also used for dynamic memory allocation . The prototype is as follows ：
 

 void* calloc (size_t num, size_t size);

1. The function is for num Size is size The elements of open up a space , And initialize each byte of the space to 0.
2. And functions malloc The only difference is calloc Initializes each byte of the requested space to full before returning the address 0.

So how do we request initialization for the contents of the requested memory space , So it's easy to use calloc function To get the job done .

2.3 realloc function

1. realloc Functions make dynamic memory management more flexible .
2. Sometimes we find that the application space is too small in the past , Sometimes we think the application space is too large , That's for reasonable time memory , We will make flexible adjustments to the size of memory . that realloc Function can achieve large memory for dynamic development
Small adjustments .

The function prototype is as follows :

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

1.  ptr Is the memory address to be adjusted
2. size New size after adjustment
3. The return value is the starting memory position after adjustment .
4. This function adjusts the size of the original memory space , It will also move the data in the original memory to a new space .