类型转换。我该怎么做在C中的libpq PostgreSQL的OID值？ [英] Type conversion. What do I do with a PostgreSQL OID value in libpq in C?

问题描述

我与PostgreSQL的C API工作，libpq的。我需要能够将值转换在的PGresult * 成的Ruby等效的数据类型。我目前只是选择所有的数据，并使用 PQgetvalue（），这给了我一个的char * ，我可以转换成字符串红宝石。这很容易。但是否有任何的例子有人可以共享做从的char * 来，类型转换发言权， INT ，浮动或双击，根据 PQftype（）返回的OID ？

I'm working with the PostgreSQL C API, libpq. I need to be able to convert the values in a PGresult* into their equivalent data types in Ruby. I'm currently just selecting all the data and using PQgetvalue(), which gives me a char* that I can convert into a ruby String. That's easy. But are there any examples somebody can share that do a type conversion from the char* to, say, int, float or double, according to the OID returned by PQftype()?

其实，总之我不知道如何跨preT OID和的文档似乎并没有给予任何指针。我发现此页面，但这并不能帮助了解如何使用此OID来做C API的类型转换。我猜有常量列表地方我可以做一个大的switch语句？

Actually, in short I have no idea how to interpret the OID and the documentation doesn't seem to give any pointers. I found this page, but that doesn't help understand how to use this OID to do a type conversion in the C API. I'm guessing there's a list of constants somewhere I can make a big switch statement from?

推荐答案

我找到了答案，询问此之后。基本上有一个名为目录/文件pg_type.h文件，旁边的libpq-fe.h和了postgres.h。您需要包含libpq-fe.h和了postgres.h，然后就可以访问诸如 TEXTOID ， BOOLOID ， INT4OID 等。

I found the answer after asking this. Basically there's a file called catalog/pg_type.h, alongside libpq-fe.h and postgres.h. You need to include after including libpq-fe.h and postgres.h, then you can access the definitions like TEXTOID, BOOLOID, INT4OID etc.

#include <stdio.h>
#include <postgres.h>
#include <libpq-fe.h>
#include <catalog/pg_type.h>

// ... snip ...

if (PQgetisnull(result, row, col)) {
  // value is NULL, nothing more to do
} else {
  char * value  = PQgetvalue(result, row, col);
  int    length = PQgetlength(result, row, col);

  switch (PQftype(result, col)) {
    case INT2OID:
    case INT4OID:
    case INT8OID:
      // process value as an integer
      break;

    default:
      // just default to a text representation
  }
}

您需要看看在pg_type.h文件里所有的OID实际上有一个广泛的名单，或者只是考什么，你回来干什么基本 SELECT'T'::布尔类型的查询等，并建立只因为你需要支持新型开关。

You need to look at all the OIDs in pg_type.h to actually have an extensive list, or just test what you get back doing basic SELECT 't'::boolean type queries etc and build up the switch only as you need a new type supporting.

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