本文介绍了如何使用libpq获取double值?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!

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!

09-21 00:10