如何使用libpq获取double值？

问题描述

libpq文档中的显示如何获取通过将其转换为主机端表示法，该整数值。

我很好奇使用libpq（不使用libpqtyppes）获取双精度值必须做什么？我试过 reinterpret_cast ，没有运气。

另外为什么文本和字节数据不需要endian转换？ / p>

数据库在Windows 7上本地运行，我使用Visual C ++ 2013。

pptr 是我正在检索的双重密码。

  #include< iostream> 
 #include< memory> 
 #include< vector> 
 #include< libpq-fe.h> 
 #include< Winsock2.h> 
 
 
 static void 
 show_binary_results（PGresult * res）
 {
 int i，j; 
 int i_fnum，n_fnum，p_fnum; 
 
 / *使用PQfnumber来避免对结果* / 
中的字段顺序的假设b i_fnum = PQfnumber（res，id）; 
 n_fnum = PQfnumber（res，name）; 
 p_fnum = PQfnumber（res，price）; （i = 0; i< PQntuples（res）; i ++）
 {
 char * iptr; 
 char * nptr; 
 char * pptr; 
 int blen; 
 intival; 
 
 / *获取字段值（我们忽略可能性为null！）* / 
 iptr = PQgetvalue（res，i，i_fnum）; 
 nptr = PQgetvalue（res，i，n_fnum）; 
 pptr = PQgetvalue（res，i，p_fnum）; / *这是一个值我想要得到* / 
 
 / * 
 * INT4的二进制表示是网络字节顺序，其中
 *我们最好强制到本地字节顺序。 
 * / 
 ival = ntohl（*（（uint32_t *）iptr））; 
 
 / * 
 * TEXT的二进制表示形式是文本，因为libpq 
 *很好，可以附加一个零字节，它只能工作罚款
 *作为C字符串。 
 * 
 * BYTEA的二进制表示是一堆字节，可以
 *包含嵌入的空值，所以我们必须注意字段长度。 
 * / 
 // blen = PQgetlength（res，i，b_fnum）; 
 
 printf（tuple％d：got\\\
，i）; 
 printf（i =（％d bytes）％d\\\
，
 PQgetlength（res，i，i_fnum），ival）; 
 printf（t =（％d bytes）'％s'\\\
，
 PQgetlength（res，i，n_fnum），nptr）; 
 printf（p =（％d bytes）％f\\\
，
 PQgetlength（res，i，p_fnum），* reinterpret_cast< double *>（pptr））; 
 
 printf（\\\
\\\
）; 
} 
} 
 
 
 int main（int argc，char * argv []）
 {
 auto conn_string =postgresql：/ / postgres的：PWD @本地/分贝; 
 auto conn_deleter = []（PGconn * c）{PQfinish（c）; }; 
 auto res_deleter = []（PGresult * r）{PQclear（r）; std :: cout<< 删除<<的std :: ENDL; }; 
 std :: unique_ptr< PGconn，decltype（conn_deleter）> conn（PQconnectdb（conn_string），conn_deleter）; 
 
 if（PQstatus（conn.get（））！= ConnStatusType :: CONNECTION_OK）
 std :: cerr<< 问题<<的std :: ENDL; 
 
 std :: vector< const char *> params {1}; 
 std :: unique_ptr< PGresult，decltype（res_deleter）> res（PQexecParams（conn.get（），
SELECT * FROM table_with_double WHERE id = $ 1，
 params.size（），/ *一个参数* / 
 NULL，/ * let后端推导参数类型* / 
（const char * const *）& params.front（），
 NULL，/ *不需要参数长度，因为文本* / 
 NULL， / *默认为所有文本参数* / 
 1），/ *请求二进制结果* / 
 res_deleter）; 
 
 if（PQresultStatus（res.get（））！= ExecStatusType :: PGRES_TUPLES_OK）
 std :: cout<< SELECT failed：<< PQerrorMessage（conn.get（））<<的std :: ENDL; 
 show_binary_results（res.get（））; 
} 
  

解决方案

显然，双列的数据以大字符串形式出现，必须转换成一个小字母。与ints一样。基于这个优秀的中的文章我使用了一个简单的函数来交换双重值。

  double double_swap（double d）
 {
 union 
 {
 double d; 
 unsigned char bytes [8]; 
} src，dest; 
 
 src.d = d; 
 dest.bytes [0] = src.bytes [7]; 
 dest.bytes [1] = src.bytes [6]; 
 dest.bytes [2] = src.bytes [5]; 
 dest.bytes [3] = src.bytes [4]; 
 dest.bytes [4] = src.bytes [3]; 
 dest.bytes [5] = src.bytes [2]; 
 dest.bytes [6] = src.bytes [1]; 
 dest.bytes [7] = src.bytes [0]; 
 return dest.d; 
} 
  

应用此函数从DB中检索正确的值。

  printf（p =（％d bytes）％lf\\\
，
 PQgetlength（res，i，p_fnum） double_swap（*（（double *）pptr）））; 
  

The examples in the libpq documentation show how to get the the integer value by converting it to the host-endian representation.

I am curious what must be done to get the double precision value using libpq (without libpqtyppes)? I have tried reinterpret_cast with no luck.

Also why text and byte data doesn't need endian conversions?

The DB runs locally on Windows 7, I am using Visual C++ 2013.

pptr is a double vaule I am trying to retrieve.

#include <iostream>
#include <memory>
#include <vector>
#include <libpq-fe.h>
#include <Winsock2.h>


static void
show_binary_results(PGresult *res)
{
    int i, j;
    int i_fnum, n_fnum, p_fnum;

    /* Use PQfnumber to avoid assumptions about field order in result */
    i_fnum = PQfnumber(res, "id");
    n_fnum = PQfnumber(res, "name");
    p_fnum = PQfnumber(res, "price");

    for (i = 0; i < PQntuples(res); i++)
    {
        char* iptr;
        char* nptr;
        char* pptr;
        int         blen;
        int         ival;

        /* Get the field values (we ignore possibility they are null!) */
        iptr = PQgetvalue(res, i, i_fnum);
        nptr = PQgetvalue(res, i, n_fnum);
        pptr = PQgetvalue(res, i, p_fnum); /*THIS IS A VALUE I AM TRYING TO GET*/

        /*
        * The binary representation of INT4 is in network byte order, which
        * we'd better coerce to the local byte order.
        */
        ival = ntohl(*((uint32_t *) iptr));

        /*
        * The binary representation of TEXT is, well, text, and since libpq
        * was nice enough to append a zero byte to it, it'll work just fine
        * as a C string.
        *
        * The binary representation of BYTEA is a bunch of bytes, which could
        * include embedded nulls so we have to pay attention to field length.
        */
        //blen = PQgetlength(res, i, b_fnum);

        printf("tuple %d: got\n", i);
        printf(" i = (%d bytes) %d\n",
            PQgetlength(res, i, i_fnum), ival);
        printf(" t = (%d bytes) '%s'\n",
            PQgetlength(res, i, n_fnum), nptr);
        printf(" p = (%d bytes) %f\n",
            PQgetlength(res, i, p_fnum), *reinterpret_cast<double*>(pptr));

        printf("\n\n");
    }
}


int main(int argc, char* argv [])
{
    auto conn_string = "postgresql://postgres:pwd@localhost/db";
    auto conn_deleter = [](PGconn* c) { PQfinish(c); };
    auto res_deleter = [](PGresult* r) { PQclear(r); std::cout << "deleted" << std::endl; };
    std::unique_ptr<PGconn, decltype(conn_deleter)> conn(PQconnectdb(conn_string), conn_deleter);

    if (PQstatus(conn.get()) != ConnStatusType::CONNECTION_OK)
        std::cerr << "Problem" << std::endl;

    std::vector<const char *> params{ "1" };
    std::unique_ptr < PGresult, decltype(res_deleter)> res(PQexecParams(conn.get(),
        "SELECT * FROM table_with_double WHERE id = $1",
        params.size(),       /* one param */
        NULL,    /* let the backend deduce param type */
        (const char * const *)&params.front(),
        NULL,    /* don't need param lengths since text */
        NULL,    /* default to all text params */
        1), /* ask for binary results */
        res_deleter);      

    if (PQresultStatus(res.get()) != ExecStatusType::PGRES_TUPLES_OK)
        std::cout << "SELECT failed: " << PQerrorMessage(conn.get()) << std::endl;
    show_binary_results(res.get());
}

解决方案

Apparently data for a double column comes in as big endian and has to be converted to a little endian. Same as ints. Based on this excellent article from this answer I have used a simple function to swap the double value.

double double_swap(double d)
{
    union
    {
        double d;
        unsigned char bytes[8];
    } src, dest;

    src.d = d;
    dest.bytes[0] = src.bytes[7];
    dest.bytes[1] = src.bytes[6];
    dest.bytes[2] = src.bytes[5];
    dest.bytes[3] = src.bytes[4];
    dest.bytes[4] = src.bytes[3];
    dest.bytes[5] = src.bytes[2];
    dest.bytes[6] = src.bytes[1];
    dest.bytes[7] = src.bytes[0];
    return dest.d;
}

Applying this function a correct value is retrieved from the DB.

printf(" p = (%d bytes) %lf\n",
            PQgetlength(res, i, p_fnum), double_swap(*((double*) pptr)));

这篇关于如何使用libpq获取double值？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！