C語言實現進制轉換函數的實例詳解
前言：
寫一個二進制，八進制，十六進制轉換為十進制的函數
要求：
系統表 pg_proc 存儲關于函數的信息
內部函數在編譯之前需要先定義在 pg_proc.h 中，src/include/catalog/pg_proc.h
CATALOG(pg_proc,1255) BKI_BOOTSTRAP BKI_ROWTYPE_OID(81) BKI_SCHEMA_MACRO{ NameData proname; /* procedure name */ /* 函數名，sql 中 select 函數名(); */ Oid pronamespace; /* OID of namespace containing this proc */ /* 模式OID */ Oid proowner; /* procedure owner */ /* 用戶OID */ Oid prolang; /* OID of pg_language entry */ float4 procost; /* estimated execution cost */ /* 估計執行成本 */ float4 prorows; /* estimated # of rows out (if proretset) */ /* 結果行估計數 */ Oid provariadic; /* element type of variadic array, or 0 */ regproc protransform; /* transforms calls to it during planning */ bool proisagg; /* is it an aggregate? */ /* 是否為聚集函數 */ bool proiswindow; /* is it a window function? */ /* 是否為窗口函數 */ bool prosecdef; /* security definer */ /* 函數是一個安全定義器，也就是一個“setuid"函數 */ bool proleakproof; /* is it a leak-proof function? */ /* 有無其他影響 */ bool proisstrict; /* strict with respect to NULLs? */ /* 遇到 NULL 值是否直接返回 NULL */ bool proretset; /* returns a set? */ /* 函數返回一個集合 */ char provolatile; /* see PROVOLATILE_ categories below */ int16 pronargs; /* number of arguments */ /* 參數個數 */ int16 pronargdefaults; /* number of arguments with defaults */ /* 默認參數的個數 */ Oid prorettype; /* OID of result type */ /* 返回參數類型OID */ /* * variable-length fields start here, but we allow direct access to * proargtypes */ oidvector proargtypes; /* parameter types (excludes OUT params) */ /* 存放函數參數類型的數組 */#ifdef CATALOG_VARLEN Oid proallargtypes[1]; /* all param types (NULL if IN only) */ char proargmodes[1]; /* parameter modes (NULL if IN only) */ text proargnames[1]; /* parameter names (NULL if no names) */ pg_node_tree proargdefaults;/* list of expression trees for argument * defaults (NULL if none) */ Oid protrftypes[1]; /* types for which to apply transforms */ text prosrc BKI_FORCE_NOT_NULL; /* procedure source text */ /* 函數處理器如何調用函數，實現函數的函數名 */ text probin; /* secondary procedure info (can be NULL) */ text proconfig[1]; /* procedure-local GUC settings */ aclitem proacl[1]; /* access permissions */#endif} FormData_pg_proc;在 proc.h 添加函數定義：
/* myfunc */DATA(insert OID = 6663 ( x_to_dec PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 23 "25 23" _null_ _null_ _null_ _null_ _null_ x_to_dec _null_ _null_ _null_ ));DESCR("x_to_dec.");OID = 6663 /* OID 唯一，不能與其他定義 OID 重復 */x_to_dec /* sql 中 select x_to_dec(); */2 0 23 "25 23" /* 傳遞兩個參數; 默認 0; 返回值類型 OID = 23; 參數1類型 OID = 25, 參數2類型 OID = 23 */x_to_dec /* 自定義函數名 */這里的傳遞參數類型和返回值類型都用的了 OID
系統表 pg_type 存儲數據類型的信息
postgres=# select oid,typname from pg_type where typname = 'text' or typname = 'int4'; oid | typname -----+--------- 23 | int4 25 | text(2 rows)
在 src/backend/utils/adt/myfuncs.c 實現自定義的函數
首先創建函數的整體部分：
Datum /* Datum 類型是PG系統函數大量引用的類型，其定義為：typedef uintptr_c Datum */x_to_dec (PG_FUNCTION_ARGS) /* 函數名; 參數 */{ /* 獲取參數 */ text *arg1 = PG_GETARG_TEXT_P(0); int32 arg2 = PG_GETARG_INT32(1); /** 實現功能 **/ /* 返回 */ PG_RETURN_INT32(sum);}這里的 PG_GETARG_XXXX() 和 PG_RETURN_XXXXX() 在 src/include/fmgr.h
知道了如何獲取參數以及返回返回值，接下來是具體的實現：
Datum x_to_dec (PG_FUNCTION_ARGS){ int n = 0, i = 0, sum = 0, t = 0; text *arg1 = PG_GETARG_TEXT_P(0); int32 arg2 = PG_GETARG_INT32(1); char *str = text_to_cstring(arg1); n = strlen(str); switch(arg2) { case 2: for(i = n - 1; i >= 0; i--) { if((str[i] - '0') != 1 && (str[i] - '0') != 0) { ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE), errmsg("Please enter the correct binary number, such as '110011'."))); } sum += (str[i] - '0') * ((int)pow(2, n - 1 - i)); } break; case 8: for(i = n - 1; i >= 0; i--) { if(!(str[i] >= '0' && str[i] <= '7')) { ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE), errmsg("Please enter the correct octal number, for example '34567'."))); } sum += (str[i] - '0') * ((int)pow(8, n - 1 - i)); } break; case 16: for(i = n - 1; i >= 0; i--) { if( !(str[i] >= '0' && str[i] <= '9') ) { if(str[i] >= 'A' && str[i] <= 'F') { // Uppercase to lowercase str[i] = str[i] + 32; } else if ( !(str[i] >= 'a' && str[i] <= 'f') ) { ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE), errmsg("Please enter the correct hexadecimal number, for example '9f'."))); } } if(str[i] <= '9') { t = str[i] - '0'; } else { t = str[i] - 'a' + 10; } sum = sum * 16 + t; } break; default: ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE), errmsg("Out of range! The second parameter, please enter: 2, 4, 16."))); } PG_RETURN_INT32(sum);}其中用到了text_to_cstring(arg1) ,類型轉換的相關函數定義在 src/backend/utils/adt/varlena.c
/* * text_to_cstring * * Create a palloc'd, null-terminated C string from a text value. * * We support being passed a compressed or toasted text value. * This is a bit bogus since such values shouldn't really be referred to as * "text *", but it seems useful for robustness. If we didn't handle that * case here, we'd need another routine that did, anyway. */char *text_to_cstring(const text *t){ /* must cast away the const, unfortunately */ text *tunpacked = pg_detoast_datum_packed((struct varlena *) t); int len = VARSIZE_ANY_EXHDR(tunpacked); char *result; result = (char *) palloc(len + 1); memcpy(result, VARDATA_ANY(tunpacked), len); result[len] = '/0'; if (tunpacked != t) pfree(tunpacked); return result;}結果：
postgres=# select x_to_dec('111',2); x_to_dec ---------- 7(1 row)postgres=# select x_to_dec('aA',16); x_to_dec ---------- 170(1 row)postgres=# select x_to_dec('aA',1);ERROR: Out of range! The second parameter, please enter: 2, 4, 16. 以上就是進制轉換的實例，如有疑問請留言或者到本站社區交流討論，感謝閱讀，希望能幫助到大家，謝謝大家對本站的支持！
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