C语言中的字符串

c语言的字符串，本质上是一个以\0字符结尾的字符数组，使用起来操作想去繁琐，不少库都会对其进行封装。下面看一些比较有代表性的

典型库里面的字符串的封装：

• Git里面的strbuf
• Dbus里面的字符串DBusRealString
• Glib中的GString
• nginx中的ngx_str_t
• Redis中的sds

Git里面的strbuf

struct strbuf {
    size_t alloc;
    size_t len;
    char*  buf;
};

Dbus里面的字符串DBusRealString

Dbus是DBusString包装了一层DBusRealString，对外DBusString，对内DBusRealString

typedef struct {
    unsigned char* str;              /**< String data, plus nul termination */
    int            len;              /**< Length without nul */
    int            allocated;        /**< Allocated size of data */
    unsigned int   constant : 1;     /**< String data is not owned by DBusString */
    unsigned int   locked : 1;       /**< DBusString has been locked and can't be changed */
    unsigned int   valid : 1;        /**< DBusString is valid (initialized and not freed) */
    unsigned int   align_offset : 3; /**< str - align_offset is the actual malloc block */
} DBusRealString;

Glib中的GString

struct _GString {
    gchar* str;
    gsize  len;
    gsize  allocated_len;
};

nginx中的ngx_str_t

typedef struct {
    size_t  len;
    u_char* data;
} ngx_str_t;

Redis中的sds

typedef char* sds;

/* Note: sdshdr5 is never used, we just access the flags byte directly.
 * However is here to document the layout of type 5 SDS strings. */
struct __attribute__((__packed__)) sdshdr5 {
    unsigned char flags; /* 3 lsb of type, and 5 msb of string length */
    char          buf[];
};
struct __attribute__((__packed__)) sdshdr8 {
    uint8_t       len;   /* used */
    uint8_t       alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char          buf[];
};
struct __attribute__((__packed__)) sdshdr16 {
    uint16_t      len;   /* used */
    uint16_t      alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char          buf[];
};
struct __attribute__((__packed__)) sdshdr32 {
    uint32_t      len;   /* used */
    uint32_t      alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char          buf[];
};
struct __attribute__((__packed__)) sdshdr64 {
    uint64_t      len;   /* used */
    uint64_t      alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char          buf[];
};

SDS：Simple Dynamic Strings

其它几没有太大差别：容量、长度、数据，到是这个SDS SDS是在redis中使用的字符串，后面独立成了单独的仓库：https://github.com/antirez/sds

• __attribute__((__packed__)) 属性: https://wshibin.github.io/2022/08/Compiler-Attributes/#packed-属性-同上
• 弹性数组成员: https://wshibin.github.io/2022/09/Flexible-array-member/

设计：

+--------+-------------------------------+-----------+
| Header | Binary safe C alike string... | Null term |
+--------+-------------------------------+-----------+
         |
         `-> Pointer returned to the user.

示例：HELLOWORLD

+------------+------------------------+-----------+---------------\
| Len | Free | H E L L O W O R L D \n | Null term |  Free space   \
+------------+------------------------+-----------+---------------\
             |
             `-> Pointer returned to the user.

使用：

todo

其它：

LSB： least significant bit