MySQL 之动态字符串处理
MySQL 之动态字符串处理
MySQL中,常常会看到一些关于动态字符串的处理,列如:DYNAMIC_STRING。
为了记录动态字符串的实际长度,缓冲区的最大长度,以及每次字符串需要调整时,及时分配新的内存,以及调整长度。MySQL使用了DYNAMIC_STRING来保存动态字符串相关的信息:- typedef struct st_dynamic_string
- {
- char *str;
- size_t length,max_length,alloc_increment;
- } DYNAMIC_STRING;
下面看看这个结构体的初始化过程:
- my_bool init_dynamic_string(DYNAMIC_STRING *str, const char *init_str,size_t init_alloc, size_t alloc_increment)
- {
- size_t length;
- DBUG_ENTER("init_dynamic_string");
- if (!alloc_increment)
- alloc_increment=128;
- length=1;
- if (init_str && (length= strlen(init_str)+1) < init_alloc)
- init_alloc=((length+alloc_increment-1)/alloc_increment)*alloc_increment;
- if (!init_alloc)
- init_alloc=alloc_increment;
- if (!(str->str=(char*) my_malloc(init_alloc,MYF(MY_WME))))
- DBUG_RETURN(TRUE);
- str->length=length-1;
- if (init_str)
- memcpy(str->str,init_str,length);
- str->max_length=init_alloc;
- str->alloc_increment=alloc_increment;
- DBUG_RETURN(FALSE);
- }
初始化这些内容之后,如果下次需要在该缓冲区添加更多字符,就可以根据这些值来判断是否需要对该缓冲区扩容:
- my_bool dynstr_append_mem(DYNAMIC_STRING *str, const char *append,
- size_t length)
- {
- char *new_ptr;
- if (str->length+length >= str->max_length) //如果新增字符串后,总长度超过缓冲区大小
- {
- //需要分配多少个alloc_increment 大小的内存,才能存下新增后的字符串
- size_t new_length=(str->length+length+str->alloc_increment)/
- str->alloc_increment;
- new_length*=str->alloc_increment;
- if (!(new_ptr=(char*) my_realloc(str->str,new_length,MYF(MY_WME))))
- return TRUE;
- str->str=new_ptr;
- str->max_length=new_length;
- }
- //将新分配的内容,append到str之后
- memcpy(str->str + str->length,append,length);
- str->length+=length; //扩容之后str新的长度
- str->str[str->length]=0; /* Safety for C programs */ //字符串最后一个字符为’\0'
- return FALSE;
- }
- my_bool dynstr_trunc(DYNAMIC_STRING *str, size_t n)
- {
- str->length-=n;
- str->str[str->length]= '\0';
- return FALSE;
- }
- char *strcend(register const char *s, register pchar c)
- {
- for (;;)
- {
- if (*s == (char) c) return (char*) s;
- if (!*s++) return (char*) s-1;
- }
- }
- my_bool dynstr_realloc(DYNAMIC_STRING *str, size_t additional_size)
- {
- DBUG_ENTER("dynstr_realloc");
- if (!additional_size) DBUG_RETURN(FALSE);
- if (str->length + additional_size > str->max_length) //如果新的字符串内容超过缓冲区的最大长度
- {
- str->max_length=((str->length + additional_size+str->alloc_increment-1)/
- str->alloc_increment)*str->alloc_increment;
- if (!(str->str=(char*) my_realloc(str->str,str->max_length,MYF(MY_WME))))
- DBUG_RETURN(TRUE);
- }
- DBUG_RETURN(FALSE);
- }
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- /*
- Concatenates any number of strings, escapes any OS quote in the result then
- surround the whole affair in another set of quotes which is finally appended
- to specified DYNAMIC_STRING. This function is especially useful when
- building strings to be executed with the system() function.
- @param str Dynamic String which will have addtional strings appended.
- @param append String to be appended.
- @param ... Optional. Additional string(s) to be appended.
- @note The final argument in the list must be NullS even if no additional
- options are passed.
- @return True = Success.
- */
- my_bool dynstr_append_os_quoted(DYNAMIC_STRING *str, const char *append, ...)
- {
- const char *quote_str= "\'";
- const uint quote_len= 1;
- my_bool ret= TRUE;
- va_list dirty_text;
- ret&= dynstr_append_mem(str, quote_str, quote_len); /* Leading quote */
- va_start(dirty_text, append);
- while (append != NullS)
- {
- const char *cur_pos= append;
- const char *next_pos= cur_pos;
- /* Search for quote in each string and replace with escaped quote */
- while(*(next_pos= strcend(cur_pos, quote_str[0])) != '\0')
- {
- ret&= dynstr_append_mem(str, cur_pos, (uint) (next_pos - cur_pos));
- ret&= dynstr_append_mem(str ,"\\", 1);
- ret&= dynstr_append_mem(str, quote_str, quote_len);
- cur_pos= next_pos + 1;
- }
- ret&= dynstr_append_mem(str, cur_pos, (uint) (next_pos - cur_pos));
- append= va_arg(dirty_text, char *);
- }
- va_end(dirty_text);
- ret&= dynstr_append_mem(str, quote_str, quote_len); /* Trailing quote */
- return ret;
- }
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