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SQLite: proper method for calling sqlite3 directly?

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  • D davecotter

    i KNOW i have to link against the sqlite3 library..

    but HOW?

    and if i'm linking against the existing library then why would i need to recompile... the existing library?

    JKSHJ Offline
    JKSHJ Offline
    JKSH
    Moderators
    wrote on last edited by
    #9

    @davecotter said in SQLite: proper method for calling sqlite3 directly?:

    can't i link directly to the existing sqlite3 that QSqlDatabase already provides?

    Unfortunately not. The SQLite library itself is statically linked (embedded) into qsqlite.dll (or .so or .dylib) so the raw SQLite functions are no longer callable.

    This contrasts with the PostgreSQL plugin: qsqlpsql.dll is simply a small wrapper that dynamically links to the raw PSQL library, so your app can link to it directly and bypass the Qt wrapper if you wish. This also means you must remember to bundle the PSQL library when you distribute your application.

    I have tried instead just compiling sqlite3 into my app, but that seems AWEFULLY wasteful of space to include the lib again for JUST ONE CALL.

    It's kind of like bundling a whole dynamic library with your app just to call one function from the library.

    Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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    • D Offline
      D Offline
      davecotter
      wrote on last edited by
      #10

      It's kind of like bundling a whole dynamic library with your app just to call one function from the library.

      right, but... it sounds like that's exactly what i must do???

      i'm surprised the SQLite impl in Qt doesn't provide forwarding function wrappers for things like this?

      JKSHJ 1 Reply Last reply
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      • D davecotter

        It's kind of like bundling a whole dynamic library with your app just to call one function from the library.

        right, but... it sounds like that's exactly what i must do???

        i'm surprised the SQLite impl in Qt doesn't provide forwarding function wrappers for things like this?

        JKSHJ Offline
        JKSHJ Offline
        JKSH
        Moderators
        wrote on last edited by
        #11

        @davecotter said in SQLite: proper method for calling sqlite3 directly?:

        i'm surprised the SQLite impl in Qt doesn't provide forwarding function wrappers for things like this?

        There are some database-specific settings available: https://doc.qt.io/qt-5/qsqldatabase.html#setConnectOptions You can set parameters like QSQLITE_BUSY_TIMEOUT but that doesn't let you register callback functions.

        If you want, you can present a case for including this kind of functionality by posting at https://bugreports.qt.io/

        Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

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        • eyllanescE eyllanesc

          @davecotter

          First you have to download the same version of sqlite3 amalgation that was used to compile the Qt you use and that can be found by checking the source code: https://github.com/qt/qtbase/tree/5.15.0/src/3rdparty/sqlite.

          Then to handle it in a simple way you can create a .pri and you must copy the sqlite3.c and sqlite3.h in the folders shown in the following structure:

          ├── your_app.pro
          ├── 3rdParty
          │   ├── sqlite
          │   │   ├── sqlite3.c
          │   │   └── sqlite3.h
          │   └── sqlite3.pri
          ├── other files
          └── main.cpp
          

          sqlite3.pri

          SOURCES += $$PWD/sqlite/sqlite3.c
          HEADERS += $$PWD/sqlite/sqlite3.h
          

          Then in the .pro add the following to include the files:

          include(3rdParty/sqlite3.pri)
          

          Then you can include the header and do the cast:

          #include "sqlite3.h"
          
          // ...
          
          QVariant v = db.driver()->handle();
          if (v.isValid() && (qstrcmp(v.typeName(), "sqlite3*") == 0)) {
              // v.data() returns a pointer to the handle
              sqlite3 *handle = *static_cast<sqlite3 **>(v.data());
              if (handle) {
                  sqlite3_busy_handler(handle, callback, ...);
              }
          }
          

          Note: In linux I had to add unix: LIBS += -ldl in the .pro.

          Christian EhrlicherC Online
          Christian EhrlicherC Online
          Christian Ehrlicher
          Lifetime Qt Champion
          wrote on last edited by
          #12

          @eyllanesc This does not work. You then have two different static sqlite instances which will crash sooner or later.

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          eyllanescE 1 Reply Last reply
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          • Christian EhrlicherC Christian Ehrlicher

            @eyllanesc This does not work. You then have two different static sqlite instances which will crash sooner or later.

            eyllanescE Offline
            eyllanescE Offline
            eyllanesc
            wrote on last edited by eyllanesc
            #13

            @Christian-Ehrlicher Are you sure? I have used that method many times and have never had a problem. The only thing I forgot to point out in my post was that after obtaining the handle: sqlite3 *handle = *static_cast<sqlite3 **>(v.data()); then the sqlite3_initialize() function must be called.

            If you check the way sqlite is included in the plugin you will see that it is not linked statically or dynamically but as files (the .h and .c are added): https://github.com/qt/qtbase/blob/5.15.0/src/plugins/sqldrivers/sqlite/sqlite.pro . It is only dynamically linked if the "system-sqlite" setting is enabled.

            If you want me to help you develop some work then you can write to my email: e.yllanescucho@gmal.com.

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            • Christian EhrlicherC Online
              Christian EhrlicherC Online
              Christian Ehrlicher
              Lifetime Qt Champion
              wrote on last edited by
              #14

              @eyllanesc said in SQLite: proper method for calling sqlite3 directly?:

              then the sqlite3_initialize() function must be called.

              And that's exactly the problem - you then have two different instances running. sqlite3_initialize() creates a local storage for some stuff (see the sources)

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              • C Offline
                C Offline
                ChrisW67
                wrote on last edited by
                #15

                A quick look at the Qt Sqlite plugin shows that it can be built against a system Sqlite (--system-sqlite).

                Does this result in a static copy of Sqlite embedded in the plugin or does it dynamically link? If it is the latter, then you should be able to link to the same dynamic library.

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                • Christian EhrlicherC Online
                  Christian EhrlicherC Online
                  Christian Ehrlicher
                  Lifetime Qt Champion
                  wrote on last edited by
                  #16

                  Since the system sqlite is a shared lib - no.

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                  • D Offline
                    D Offline
                    davecotter
                    wrote on last edited by
                    #17

                    so i ask again: can i use the built in sqlite plugin, the one that comes with the install of Qt, and link against the sqlite.cpp code? or must i build it myself? if the latter: is there a "for dummies" step by step wiki about exactly how to build it?

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                    • SGaistS Offline
                      SGaistS Offline
                      SGaist
                      Lifetime Qt Champion
                      wrote on last edited by
                      #18

                      Hi,

                      It's in the documentation.

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                      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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