C语言实现进制转换函数的实例详解

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;

/* 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 */

postgres=# select oid,typname from pg_type where typname = 'text' or typname = 'int4';
 oid | typname 
-----+---------
 23 | int4
 25 | text
(2 rows)

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);
}

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);
}

/*
 * 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;
}

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.