因为C是一种结构化语言, 因而它具备一些固定的编程规定。其中之一包含更改数组的大小。数组是存储在间断内存地位的我的项目的汇合。
能够看出, 上述数组的长度(大小)为9。然而, 如果须要更改此长度(大小), 该怎么办。例如,
如果存在只须要在此数组中输出5个元素的状况。在这种状况下, 残余的4个索引只会节约该数组中的内存。因而须要将数组的长度(大小)从9缩小到5。
采取另一种状况。在这里, 有9个元素组成的数组, 所有9个索引均已填充。然而须要在此数组中再输出3个元素。在这种状况下, 还须要3个索引。因而, 阵列的长度(大小)须要从9更改为12。
此过程称为C中的动态内存调配.
因而, C动态内存调配能够定义为在运行时更改数据结构(如Array)的大小的过程。
C提供了一些性能来实现这些工作。 C下定义了4个提供的库函数<stdlib.h>头文件, 以不便C编程中的动态内存调配。他们是：
- malloc()
- calloc()
- 自在()
- realloc()
让咱们更具体地钻研它们。
C malloc()办法
" malloc"or"内存调配"C语言中的办法用于动态分配具备指定大小的单个大内存块。它返回void类型的指针, 该指针能够转换为任何模式的指针。它应用默认垃圾值初始化每个块。
语法如下：
ptr = (cast-type*) malloc(byte-size)例如：
ptr =(int )malloc(100 sizeof(int));因为int的大小为4个字节, 因而此语句将调配400个字节的内存。并且, 指针ptr保留调配的存储器中的第一个字节的地址。
如果空间有余, 调配将失败并返回NULL指针。
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;}输入如下：
Enter number of elements: 5Memory successfully allocated using malloc.The elements of the array are: 1, 2, 3, 4, 5, C calloc()办法
" calloc"or"间断调配"C语言中的办法用于动态分配指定数量的指定类型的内存块。它应用默认值" 0"初始化每个块。
语法如下：
ptr = (cast-type*)calloc(n, element-size);例如：
ptr =(float *)calloc(25, sizeof(float));该语句在内存中为25个元素调配间断的空间, 每个元素的大小为float。
如果空间有余, 调配将失败并返回NULL指针。
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;}输入如下：
Enter number of elements: 5Memory successfully allocated using calloc.The elements of the array are: 1, 2, 3, 4, 5, C free()办法
"free"C中的办法用于动静勾销分配内存。应用函数malloc()和calloc()调配的内存不会自行勾销调配。因而, 每当产生动态内存调配时, 都会应用free()办法。它通过开释内存来帮忙缩小内存节约。
语法如下：
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;}输入如下：
Enter number of elements: 5Memory successfully allocated using malloc.Malloc Memory successfully freed.Memory successfully allocated using calloc.Calloc Memory successfully freed.C realloc()办法
"重新分配"or"重新分配"C中的办法用于动静更改先前调配的内存的内存调配。换句话说, 如果先前借助malloc或calloc调配的内存不足, 则能够应用realloc来动静从新分配内存。内存的重新分配将放弃曾经存在的值, 并且新块将应用默认垃圾值进行初始化。
语法如下：
ptr = realloc(ptr, newSize);where ptr is reallocated with new size 'newSize'.如果空间有余, 调配将失败并返回NULL指针。
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;}输入如下：
Enter number of elements: 5Memory successfully allocated using calloc.The elements of the array are: 1, 2, 3, 4, 5, Enter the new size of the array: 10Memory successfully re-allocated using realloc.The elements of the array are: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 更多C/C++开发相干内容请参考：lsbin - IT开发技术：https://www.lsbin.com/
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