This Project mainly came from Redis 2.6.
String in C is weak and immutable since cstr is is just a char array and the length of char array is fixed.
Modern Language Like Java/Python has powerful support for string with with we can append ... quickly Redis has its sds(Simple Dynamic String) to deal with string operation and I found the concept it has is really good so I extract it out.
the sds has three component,the length str already occupied the space left and the array that actually store the data.
typedef char *sds;
struct sdshdr {
uint64_t len;//buf 已占用长度
uint64_t free; // 剩余长度
char buf[];//C99特性 柔性数组成员
};The memory the struct sdshdruse is as followed:
| len | free | 不包含'\0'的字符串 | 终结符'\0' |
|---|---|---|---|
| 8 | 8 | s | 1 |
struct sdshdr的大小实际上为8个字节，buf[]是C99特性，不占据实际空间；
在实际的使用中，我们分配一块内存，内存大小为sizeof(struct sdshdr) + len + free + 1.
将前一部分16个字节保留给结构体，最后一个字节留给'\0',中间的空间属于str.
这种实现方式比较类似Pascal等的实现，len()等操作O(1)时间内就可以完成，同时包含很多字符串操作函数。
在实现的时候，我们返回结构体后面str开始的地址，而如果需要访问结构体，只需对指向字符串的指针往回移动sizeof(struct sdshdr)个字节就好。