使用malloc()、calloc()、free()和realloc()在C中進行動態內存分配

因爲C是一種結構化語言, 所以它具備一些固定的編程規則。其中之一包括更改數組的大小。數組是存儲在連續內存位置的項目的集合。

能夠看出, 上述數組的長度(大小)爲9。可是, 若是須要更改此長度(大小), 該怎麼辦。例如,

若是存在只須要在此數組中輸入5個元素的狀況。在這種狀況下, 剩餘的4個索引只會浪費該數組中的內存。所以須要將數組的長度(大小)從9減小到5。

採起另外一種狀況。在這裏, 有9個元素組成的數組, 全部9個索引均已填充。可是須要在此數組中再輸入3個元素。在這種狀況下, 還須要3個索引。所以, 陣列的長度(大小)須要從9更改成12。

此過程稱爲C中的動態內存分配.

所以, C動態內存分配能夠定義爲在運行時更改數據結構(如Array)的大小的過程。

C提供了一些功能來完成這些任務。 C下定義了4個提供的庫函數<stdlib.h>頭文件, 以方便C編程中的動態內存分配。他們是：

1. malloc()
2. calloc()
3. 自由()
4. realloc()

讓咱們更詳細地研究它們。

C malloc()方法

" malloc"or"內存分配"C語言中的方法用於動態分配具備指定大小的單個大內存塊。它返回void類型的指針, 該指針能夠轉換爲任何形式的指針。它使用默認垃圾值初始化每一個塊。

語法以下：

ptr = (cast-type*) malloc(byte-size)

例如：

ptr =(int )malloc(100 sizeof(int));因爲int的大小爲4個字節, 所以此語句將分配400個字節的內存。而且, 指針ptr保存分配的存儲器中的第一個字節的地址。

若是空間不足, 分配將失敗並返回NULL指針。

例子：

#include <stdio.h>
#include <stdlib.h>
  
int main()
{
  
     // This pointer will hold the
     // base address of the block created
     int * ptr;
     int n, i;
  
     // Get the number of elements for the array
     n = 5;
     printf ( "Enter number of elements: %dn" , n);
  
     // Dynamically allocate memory using malloc()
     ptr = ( int *) malloc (n * sizeof ( int ));
  
     // Check if the memory has been successfully
     // allocated by malloc or not
     if (ptr == NULL) {
         printf ( "Memory not allocated.n" );
         exit (0);
     }
     else {
  
         // Memory has been successfully allocated
         printf ( "Memory successfully allocated using malloc.n" );
  
         // Get the elements of the array
         for (i = 0; i < n; ++i) {
             ptr[i] = i + 1;
         }
  
         // Print the elements of the array
         printf ( "The elements of the array are: " );
         for (i = 0; i < n; ++i) {
             printf ( "%d, " , ptr[i]);
         }
     }
  
     return 0;
}

輸出以下：

Enter number of elements: 5
Memory successfully allocated using malloc.
The elements of the array are: 1, 2, 3, 4, 5,

C calloc()方法

" calloc"or"連續分配"C語言中的方法用於動態分配指定數量的指定類型的內存塊。它使用默認值" 0"初始化每一個塊。

語法以下：

ptr = (cast-type*)calloc(n, element-size);

例如：

ptr =(float *)calloc(25, sizeof(float));該語句在內存中爲25個元素分配連續的空間, 每一個元素的大小爲float。

若是空間不足, 分配將失敗並返回NULL指針。

例子：

#include <stdio.h>
#include <stdlib.h>
  
int main()
{
  
     // This pointer will hold the
     // base address of the block created
     int * ptr;
     int n, i;
  
     // Get the number of elements for the array
     n = 5;
     printf ( "Enter number of elements: %dn" , n);
  
     // Dynamically allocate memory using calloc()
     ptr = ( int *) calloc (n, sizeof ( int ));
  
     // Check if the memory has been successfully
     // allocated by calloc or not
     if (ptr == NULL) {
         printf ( "Memory not allocated.n" );
         exit (0);
     }
     else {
  
         // Memory has been successfully allocated
         printf ( "Memory successfully allocated using calloc.n" );
  
         // Get the elements of the array
         for (i = 0; i < n; ++i) {
             ptr[i] = i + 1;
         }
  
         // Print the elements of the array
         printf ( "The elements of the array are: " );
         for (i = 0; i < n; ++i) {
             printf ( "%d, " , ptr[i]);
         }
     }
  
     return 0;
}

輸出以下：

Enter number of elements: 5
Memory successfully allocated using calloc.
The elements of the array are: 1, 2, 3, 4, 5,

C free()方法

"free"C中的方法用於動態取消分配內存。使用函數malloc()和calloc()分配的內存不會自行取消分配。所以, 每當發生動態內存分配時, 都會使用free()方法。它經過釋放內存來幫助減小內存浪費。

語法以下：

free(ptr);

例子：

#include <stdio.h>
#include <stdlib.h>
  
int main()
{
  
     // This pointer will hold the
     // base address of the block created
     int *ptr, *ptr1;
     int n, i;
  
     // Get the number of elements for the array
     n = 5;
     printf ( "Enter number of elements: %dn" , n);
  
     // Dynamically allocate memory using malloc()
     ptr = ( int *) malloc (n * sizeof ( int ));
  
     // Dynamically allocate memory using calloc()
     ptr1 = ( int *) calloc (n, sizeof ( int ));
  
     // Check if the memory has been successfully
     // allocated by malloc or not
     if (ptr == NULL || ptr1 == NULL) {
         printf ( "Memory not allocated.n" );
         exit (0);
     }
     else {
  
         // Memory has been successfully allocated
         printf ( "Memory successfully allocated using malloc.n" );
  
         // Free the memory
         free (ptr);
         printf ( "Malloc Memory successfully freed.n" );
  
         // Memory has been successfully allocated
         printf ( "nMemory successfully allocated using calloc.n" );
  
         // Free the memory
         free (ptr1);
         printf ( "Calloc Memory successfully freed.n" );
     }
  
     return 0;
}

輸出以下：

Enter number of elements: 5
Memory successfully allocated using malloc.
Malloc Memory successfully freed.

Memory successfully allocated using calloc.
Calloc Memory successfully freed.

C realloc()方法

"從新分配"or"從新分配"C中的方法用於動態更改先前分配的內存的內存分配。換句話說, 若是先前藉助malloc或calloc分配的內存不足, 則可使用realloc來動態從新分配內存。內存的從新分配將保持已經存在的值, 而且新塊將使用默認垃圾值進行初始化。

語法以下：

ptr = realloc(ptr, newSize);

where ptr is reallocated with new size 'newSize'.

若是空間不足, 分配將失敗並返回NULL指針。

例子：

#include <stdio.h>
#include <stdlib.h>
  
int main()
{
  
     // This pointer will hold the
     // base address of the block created
     int * ptr;
     int n, i;
  
     // Get the number of elements for the array
     n = 5;
     printf ( "Enter number of elements: %dn" , n);
  
     // Dynamically allocate memory using calloc()
     ptr = ( int *) calloc (n, sizeof ( int ));
  
     // Check if the memory has been successfully
     // allocated by malloc or not
     if (ptr == NULL) {
         printf ( "Memory not allocated.n" );
         exit (0);
     }
     else {
  
         // Memory has been successfully allocated
         printf ( "Memory successfully allocated using calloc.n" );
  
         // Get the elements of the array
         for (i = 0; i < n; ++i) {
             ptr[i] = i + 1;
         }
  
         // Print the elements of the array
         printf ( "The elements of the array are: " );
         for (i = 0; i < n; ++i) {
             printf ( "%d, " , ptr[i]);
         }
  
         // Get the new size for the array
         n = 10;
         printf ( "nnEnter the new size of the array: %dn" , n);
  
         // Dynamically re-allocate memory using realloc()
         ptr = realloc (ptr, n * sizeof ( int ));
  
         // Memory has been successfully allocated
         printf ( "Memory successfully re-allocated using realloc.n" );
  
         // Get the new elements of the array
         for (i = 5; i < n; ++i) {
             ptr[i] = i + 1;
         }
  
         // Print the elements of the array
         printf ( "The elements of the array are: " );
         for (i = 0; i < n; ++i) {
             printf ( "%d, " , ptr[i]);
         }
  
         free (ptr);
     }
  
     return 0;
}

輸出以下：

Enter number of elements: 5
Memory successfully allocated using calloc.
The elements of the array are: 1, 2, 3, 4, 5, Enter the new size of the array: 10
Memory successfully re-allocated using realloc.
The elements of the array are: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,

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