C语言实现进制转换函数的实例详解
C语言实现进制转换函数的实例详解
前言:
写一个二进制,八进制,十六进制转换为十进制的函数
要求:
- 函数有两个参数,参数(1)是要转换为十进制的进制数,参数(2)是标示参数(1)是什么进制(2,8,16标示二进制,八进制,十六进制)。
- 要有报错信息,比如参数是1012,但参数(2)是2,显然是进制数表示有错误。
系统表 pg_proc 存储关于函数的信息
内部函数在编译之前需要先定义在 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 */ /* 函数名,sql 中 select 函数名(); */ Oid pronamespace; /* OID of namespace containing this proc */ /* 模式OID */ Oid proowner; /* procedure owner */ /* 用户OID */ Oid prolang; /* OID of pg_language entry */ float4 procost; /* estimated execution cost */ /* 估计执行成本 */ float4 prorows; /* estimated # of rows out (if proretset) */ /* 结果行估计数 */ Oid provariadic; /* element type of variadic array, or 0 */ regproc protransform; /* transforms calls to it during planning */ bool proisagg; /* is it an aggregate? */ /* 是否为聚集函数 */ bool proiswindow; /* is it a window function? */ /* 是否为窗口函数 */ bool prosecdef; /* security definer */ /* 函数是一个安全定义器,也就是一个“setuid"函数 */ bool proleakproof; /* is it a leak-proof function? */ /* 有无其他影响 */ bool proisstrict; /* strict with respect to NULLs? */ /* 遇到 NULL 值是否直接返回 NULL */ bool proretset; /* returns a set? */ /* 函数返回一个集合 */ char provolatile; /* see PROVOLATILE_ categories below */ int16 pronargs; /* number of arguments */ /* 参数个数 */ int16 pronargdefaults; /* number of arguments with defaults */ /* 默认参数的个数 */ Oid prorettype; /* OID of result type */ /* 返回参数类型OID */ /* * variable-length fields start here, but we allow direct access to * proargtypes */ oidvector proargtypes; /* parameter types (excludes OUT params) */ /* 存放函数参数类型的数组 */ #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 */ /* 函数处理器如何调用函数,实现函数的函数名 */ 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 添加函数定义:
/* 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,不能与其他定义 OID 重复 */ x_to_dec /* sql 中 select x_to_dec(); */ 2 0 23 "25 23" /* 传递两个参数; 默认 0; 返回值类型 OID = 23; 参数1类型 OID = 25, 参数2类型 OID = 23 */ x_to_dec /* 自定义函数名 */
这里的传递参数类型和返回值类型都用的了 OID
系统表 pg_type 存储数据类型的信息
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 实现自定义的函数
首先创建函数的整体部分:
Datum /* Datum 类型是PG系统函数大量引用的类型,其定义为:typedef uintptr_c Datum */ x_to_dec (PG_FUNCTION_ARGS) /* 函数名; 参数 */ { /* 获取参数 */ text *arg1 = PG_GETARG_TEXT_P(0); int32 arg2 = PG_GETARG_INT32(1); /** 实现功能 **/ /* 返回 */ PG_RETURN_INT32(sum); }
这里的 PG_GETARG_XXXX() 和 PG_RETURN_XXXXX() 在 src/include/fmgr.h
知道了如何获取参数以及返回返回值,接下来是具体的实现:
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) ,类型转换的相关函数定义在 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; }
结果:
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.
以上就是进制转换的实例,如有疑问请留言或者到本站社区交流讨论,感谢阅读,希望能帮助到大家,谢谢大家对本站的支持!
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