Home Database Mysql Tutorial redis数据库中的sds.h中字符串数组的一个说明

redis数据库中的sds.h中字符串数组的一个说明

Jun 07, 2016 pm 03:55 PM
redis string database array

在redis源码sds.h中对于字符串结构体的定义如下:typedef char *sds;struct sdshdr { int len; int free; char buf[];};这里刚开始没有看明白char buf[]是什么个形式,不是说C语言里数组必须要初始化大小吗?那么这里是什么东西?我们先用一个简单的方法来查

在redis源码sds.h中对于字符串结构体的定义如下: typedef char *sds; struct sdshdr { int len; int free; char buf[]; }; 这里刚开始没有看明白char buf[]是什么个形式,不是说C语言里数组必须要初始化大小吗?那么这里是什么东西? 我们先用一个简单的方法来查看一下结构体sdshdr的大小: #include struct sds { int val; int size; char name[]; }; int main() { printf("%d\n",sizeof(sds)); return 0; } 我们发现输出结果是8字节,也就是说char name[]是不占空间的,这里我们可以把它看成一个占位符。 struct Database
{
char Name[];
};

这种结构体声明倒是对的,最新的C标准支持这种写法,但是,初始化它就没这么简单了,前面还没有一个人说对正确的初始化办法呢
并且,这个东西的初始化,只能手工,先用malloc(sizeof(struct Database) + size);
其中size是你希望的Name的长度,然后你只能用strcpy这样的方式初始化了
上面是查找到的资料,实际上这里可以把char buf[]看成一个指针,在struct默认只是占用了指针的空间而已,而不占用实际的存储空间,相反,在生成该结构体实例的时候,才进行空间分配。在redis中空间分配的函数在sds.c中: sds sdsnewlen(const void *init, size_t initlen) { struct sdshdr *sh; if (init) { sh = zmalloc(sizeof(struct sdshdr)+initlen+1); } else { sh = zcalloc(sizeof(struct sdshdr)+initlen+1); } if (sh == NULL) return NULL; sh->len = initlen; sh->free = 0; if (initlen && init) memcpy(sh->buf, init, initlen); sh->buf[initlen] = '\0'; return (char*)sh->buf; } 我们从上述代码可以看出, sh = zmalloc(sizeof(struct sdshdr)+initlen+1);这句话就是根据传进来的参数来申请内存,当然,申请的内存是大于存放字符串的空间的,因为要申请额外的空间用来存放 int len;int free; 两个变量,也就是sizeof(struct sdshdr),其中+1是用来存放表示字符串结束的'\0'。
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