PostgreSQL 使用 Bytea 二进制数据和 C libpq 保存和选择文件

PostgreSQL Save and Pick files using Bytea binary data with C libpq

这是我使用 bytea 与 postgreSQL 和 libpq 进行实验的结果。我使用接收到的数据创建的结果文件的大小是原来上传的两倍加上 2 个字节 (picture.png)。我无法确切地了解我做错了多少操作,因为这个过程对我的小动物大脑来说非常混乱。任何帮助或建议对我来说都是一个很大的帮助,在此先感谢您。

<2021 年 5 月 27 日编辑>您可以在下方找到解决方案

#include <string>
#include <fstream>
#include <iostream>

#include "libpq/libpq-fs.h"
#include "libpq-fe.h"

#pragma comment(lib, "libpq.lib")   /*!< Only for windows compilation */

int main(int argc, char* argv[])
{
    //************************ SAVING FILE TO DB AS BYTEA **********
    manager.conn = manager.ConnectDB();  // my manager, working fine    

    FILE* file = fopen("powerup.png", "rb");
    if (file == NULL) cout << endl << "FILE WAS UNABLE TO BE READED" << endl;
    fseek(file, 0, SEEK_END);
    long int fileSize = ftell(file);

    rewind(file);

    unsigned char* buffRead = (unsigned char*)malloc(fileSize);
    size_t bytes_read = fread(buffRead, 1, fileSize, file);
    if (bytes_read != fileSize) cout << endl << "fread reading Error";
    fclose(file);

    const char* paramValues[3];
    paramValues[0] = "1";
    paramValues[1] = "powerup.png";
    paramValues[2] = reinterpret_cast<const char*>(buffRead);       //._.' type cast required to PQexeParams?

    const int paramFormats[3]{ 0,0,1 };
    const int paramLenghts[3]{ strlen(paramValues[0]), strlen(paramValues[1]), fileSize};

    PGresult *res = PQexecParams(manager.conn, "INSERT INTO filebyte (id, filename, file) VALUES(::text, ::text, ::bytea)",
        3,              /* params */
        NULL,           /* let the backend deduce param type */
        paramValues,
        paramLenghts,   /* don't need param lengths since text */
        paramFormats,   /* default to all text params */
        1);

    if (res && PQresultStatus(res) == PGRES_COMMAND_OK) cout << endl << "Inserted data to filebyte OK";

    PQfreemem(res);

    //********************** PICKING FILE FROM DB AS BYTEA **********
    
    const char* bytesFromDB = new const char[fileSize];
    int sizeR = -1;
    const char *sentenceEx = "SELECT file FROM filebyte WHERE id='1'";

    res = PQexec(manager.conn, sentenceEx);

    if (res && PQresultStatus(res) == PGRES_TUPLES_OK) 
    {
        sizeR = PQgetlength(res, 0, 0);
        bytesFromDB = PQgetvalue(res, 0, 0);
    }

    ofstream myFile("picture.png", ios::out | ios::binary);

    myFile.write(bytesFromDB, sizeR);
    myFile.close();

    PQfreemem(res);
    free(buffRead);
    delete sizeP;

    manager.CloseConn(manager.conn);

    return true;
}

我觉得我很接近,我错过了一些关于数据类型的东西 treatment/conversion。我认为这有助于理解正在发生的事情。仍在努力。我在下图中只展示了观察到的数据的一小部分。

查询的结果好像是一个表示十六进制的字符串。事实上,我将输出文件写成二进制文件,它再次将图形化的 asci 十六进制表示形式转换为真正的十六进制形式,破坏了文件的实际内容并增加了大小。我刚刚添加了关于结果文件内容发生了什么的直观解释。

这是代码运行良好的结果。我现在有时间 post 它以防它能帮助到别人。理解 bytea 的工作原理对我来说有点困难,但终于解决了。向社区致以诚挚的问候。

#include <string>
#include <fstream>
#include <iostream>
#include <sstream>

#include "libpq/libpq-fs.h"
#include "libpq-fe.h"

#pragma comment(lib, "libpq.lib")   /*!< Only for windows compilation */

int main(int argc, char* argv[])
{
    ManageDB manager;       

    manager.conn = manager.ConnectDB();  // My manager, connects ok

    const char* fileName = {"test.png"};
    const char* fileNameOut = { "testOut.png" };

    FILE* file = fopen(fileName, "rb");
    if (file == NULL) cout << endl << "FILE WAS UNABLE TO BE READED" << endl;
    fseek(file, 0, SEEK_END);
    long int fileSize = ftell(file);

    rewind(file);

    unsigned char* buffRead = (unsigned char*)malloc(fileSize);
    size_t bytes_read = fread(buffRead, 1, fileSize, file);
    if (bytes_read != fileSize) cout << endl << "fread reading Error";
    fclose(file);
    
    size_t* sizeP = new size_t(fileSize);

    const char* paramValues[3];
    paramValues[0] = "1";
    paramValues[1] = fileName;
    paramValues[2] = reinterpret_cast<const char*>(buffRead);   // type cast required to PQexeParams

    const int paramFormats[3]{ 0,0,1 };   //0 = text, 1 = binary
    const int paramLenghts[3]{ strlen(paramValues[0]), strlen(paramValues[1]), fileSize};

    PGresult *res = PQexecParams(manager.conn, "INSERT INTO filebyte (id, filename, file) VALUES(::text, ::text, ::bytea)",
        3,              /* params num */
        NULL,           /* let the backend deduce param type */
        paramValues,
        paramLenghts,
        paramFormats,
        1);

    if (res && PQresultStatus(res) == PGRES_COMMAND_OK) cout << endl << "Inserted data to filebyte OK";

    PQfreemem(res);

    //************************download from DB*************************
    
    const char* bytesFromDB = new const char[fileSize];
    int sizeR;
    // Table columns = id(text) | filename(text) | file(bytea)
    const char* sentenceEx = "SELECT file FROM filebyte WHERE id='1'";

    res = PQexec(manager.conn, sentenceEx);

    if (res && PQresultStatus(res) == PGRES_TUPLES_OK)
    {
        sizeR = PQgetlength(res, 0, 0);
        bytesFromDB = PQgetvalue(res, 0, 0);
    }
    else cout << endl << "Error at inserting file data in filebyte table";

    string hex(bytesFromDB, 2, sizeR-2);  //removing the first '\x' characters of the result.

    // vars for converting to real bytes process
    std::basic_string<uint8_t> bytes;
    bytes.clear();
    std::string nextbyte; 
    nextbyte.clear();
    uint16_t byte;

    // Iterate over every pair of hex values in the input string (e.g. "18", "0f", ...)
    for (size_t i = 0; i < hex.length(); i += 2)
    {
        // Get current pair and store in nextbyte
        nextbyte = hex.substr(i, 2);

        // Put the pair into an istringstream and stream it through std::hex for
        // conversion into an integer value.
        // This will calculate the byte value of your string-represented hex value.
        std::istringstream(nextbyte) >> std::hex >> byte;

        // As the stream above does not work with uint8 directly, we have to cast it now.
        // As every pair can have a maximum value of "ff",
        // which is "11111111" (8 bits), we will not lose any information during this cast.
        bytes.push_back(static_cast<uint8_t>(byte));
    }

    // string obj from bytes-"array"
    std::string result(begin(bytes), end(bytes));

    ofstream myFile(fileNameOut, ios::out | ios::ios_base::binary);
    
    if (myFile.is_open())
    {
        myFile << result;
        myFile.close();
    }
    else cout << endl << "Impossible to writte the file " << fileNameOut;
    
    manager.CloseConn(manager.conn);   //closes connection with database internally ok

    PQfreemem(res);
    free(buffRead);
    delete sizeP;
    free((char*)fileName[8]);
    free((char*)fileNameOut[11]);
    
    return true;
}