1 系统函数realloc测试

    mac OS 系统 函数解释

    The realloc() function tries to change the size of the allocation pointed
     to by ptr to size, and returns ptr.  If there is not enough room to
     enlarge the memory allocation pointed to by ptr, realloc() creates a new
     allocation, copies as much of the old data pointed to by ptr as will fit
     to the new allocation, frees the old allocation, and returns a pointer to
     the allocated memory.  If ptr is NULL, realloc() is identical to a call
     to malloc() for size bytes.  If size is zero and ptr is not NULL, a new,
     minimum sized object is allocated and the original object is freed.  When
     extending a region allocated with calloc(3), realloc(3) does not guaran-
     tee that the additional memory is also zero-filled.

#include <stdio.h>

#include <stdlib.h>
typedef char *sds;

int main() {
    printf("Hello, World!\n");
    char t[] = {'a','b','c','d'};
    sds s = t+1;
    char  bb = s[0];
    char  cc = s[1];
    char  dd = s[2];
    char  flag = *(s - 1);
    printf("%c %c %c %c", flag, bb, cc ,dd);


    int * p=NULL;

    p=(int *)malloc(sizeof(int));

    *p=3;

    printf("\np=%p\n",p);

    printf("*p=%d\n",*p);

    // 空间不变
    p=(int *)realloc(p,sizeof(int));

    printf("p=%p\n",p);

    printf("*p=%d\n",*p);

    
    // 空间扩大三倍，同时内容会被复制（内存足够的情况下，起始地址不变）
    p=(int *)realloc(p,3*sizeof(int));

    printf("p=%p\n",p);

    printf("*p=%d",*p);

    //释放p指向的空间

    realloc(p,0);

    p=NULL;

    return 0;

}

2 结果：

Hello, World!
a b c d

p=0x7fa656405b50
*p=3
p=0x7fa656405b50
*p=3
p=0x7fa656405b50  // 扩容后起始地址没有变化（内存足够）
*p=3
Process finished with exit code 0

3 小结

   函数：void * realloc(void * p,int n);
// 指针p必须为指向堆内存空间的指针，即由malloc函数、calloc函数或realloc函数分配空间的指针。
// realloc函数将指针p指向的内存块的大小改变为n字节。如果n小于或等于p之前指向的空间大小，
// 那么。保持原有状态不变。如果n大于原来p之前指向的空间大小，那么，系统将重新为p从堆上
// 分配一块大小为n的内存空间，同时，将原来指向空间的内容依次复制到新的内存空间上，
// p之前指向的空间被释放。relloc函数分配的空间也是未初始化的。