C和指针之动态内存分配malloc calloc realloc简单使用和区别

1、介绍

3个函数都在<stdlib.h>头文件 1）、void* malloc(unsigned size); 在内存的动态存储区中分配一块长度为size字节的连续区域，参数size为需要内存空间的长度，返回该区域的首地址，这里我们需要主要如果申请失败返回空指针 2）、void* realloc(void* ptr, unsigned newsize); 给一个已经分配了地址的指针重新分配空间,参数ptr为原有的空间地址,newsize是重新申请的地址长度，地址可能会变 3）、void* calloc(size_t numElements, size_t sizeOfElement); 参数sizeOfElement为申请地址的单位元素长度,numElements为元素个数，即在内存中申请numElements*sizeOfElement字节大小的连续地址空间 会将所分配的内存空间中的每一位都初始化为零,也就是说,如果你是为字符类型或整数类型的元素分配内存,那么这些元素将保证会被初始化为0 用完要记得释放内存 free(p); p = NULL; 都在堆区分配内存，用完都需要释放，都是连续的分配内存

2、测试Demo

3个函数简单使用

#include <stdio.h>#include <stdlib.h>//malloc、realloc、calloc简单使用int main(){int *p1 = NULL;p1 = malloc(10 * sizeof(int));if (p1 != NULL){//给p1赋值for (int i = 0; i < 10; ++i){*(p1 + i) = 5;}//打印值for (int i = 0; i < 10; ++i){printf("p1[%d] is %d\n", i, p1[i]);}free(p1);p1 = NULL;}int *p2 = NULL;p2 = calloc(5, sizeof(int));if (p2 != NULL){for (int i = 0; i < 5; i++){printf("p2[%d] is %d\n", i, p2[i]);}free(p2);p2 = NULL;}char *p3, *q4;p3 = (char *)malloc(5);q4 = p3;p3 = (char *)realloc(p3, 10)if (p3 != NULL){printf("p3 is %p\n", p3);printf("q3 is %p\n", q4);p3 = (char *)realloc(p3, 1000);printf("p3 is %p\n", p3);free(p3);p3 = NULL;}return 0;}

3、 运行结果

1111deMacBook-Pro:malloc a1111$ vim malloc.c1111deMacBook-Pro:malloc a1111$ gcc -g malloc.c -o malloc1111deMacBook-Pro:malloc a1111$ ./mallocp1[0] is 5p1[1] is 5p1[2] is 5p1[3] is 5p1[4] is 5p1[5] is 5p1[6] is 5p1[7] is 5p1[8] is 5p1[9] is 5p2[0] is 0p2[1] is 0p2[2] is 0p2[3] is 0p2[4] is 0p3 is 0x7ff323c01020q3 is 0x7ff323c01020p3 is 0x7ff323c03260