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I am writing a C++ app that needd to download a file and show a download progress dialog. I am using cURL to complete the doenlaod request, however, for some reason, when I initiate the download by clicking the install button, the application immediately closes with no errors or other indication. Here is what I have so far:

#include <wx/wx.h>

#include <iostream>
#include <fstream>
#include <curl/curl.h>

#include <thread>

class FileDownloader {
public:
    FileDownloader() = default;
    ~FileDownloader() = default;

    static size_t WriteCallback(void* contents, size_t size, size_t nmemb, void* userp)
    {
        size_t totalSize = size * nmemb;
        wxThreadEvent* event = static_cast<wxThreadEvent*>(userp);
        wxString data(static_cast<const char*>(contents), wxConvUTF8, totalSize);
        event->SetString(data);
        wxQueueEvent(wxTheApp->GetTopWindow(), event);
        return totalSize;
    }

    bool DownloadFile(const std::string& url, const std::string& destination)
    {
        CURL* curl = curl_easy_init();
        if (!curl)
            return false;

        FILE* file = fopen(destination.c_str(), "wb");
        if (!file)
        {
            curl_easy_cleanup(curl);
            return false;
        }

        curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteCallback);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, file);

        CURLcode res = curl_easy_perform(curl);

        fclose(file);
        curl_easy_cleanup(curl);

        return res == CURLE_OK;
    }
};

class WindowsInstaller : public wxApp
{
public:
    bool OnInit() override;
};

// This defines the equivalent of main() for the current platform.
wxIMPLEMENT_APP(WindowsInstaller);


class NewFrame : public wxFrame
{
public:
   NewFrame();

private:
    void OnHello(wxCommandEvent& event);
    void OnExit(wxCommandEvent& event);
    void OnAbout(wxCommandEvent& event);
    void OnInstallButtonClick(wxCommandEvent& event);
    void OnSetButtonClick(wxCommandEvent& event);
    void OnCheckBoxClicked(wxCommandEvent& event);
    


    wxButton* install_button;
    wxButton* set_location;

    wxTextCtrl* textCtrl;

    wxCheckBox* accept_checkbox;

    wxGauge* gauge;
};



bool WindowsInstaller::OnInit()
{
    NewFrame* frame = new NewFrame();
    frame->Show();
    return true;
}


enum
{
    ID_Hello = 1,
    ID_Install_Button = 2,
    ID_Set_Location = 3,
    ID_Accept_CheckBox = 4,
    ID_UPDATE_PROGRESS = 5,
    ID_DOWNLOAD_COMPLETE = 6,
    ID_DOWNLOAD_ERROR = 7,
    ID_ENABLE_INSTALL_BUTTON = 8

};


NewFrame::NewFrame()
    : wxFrame(nullptr, wxID_ANY, "Hello World", wxDefaultPosition, wxSize(700, 500), wxDEFAULT_FRAME_STYLE & ~(wxRESIZE_BORDER | wxMAXIMIZE_BOX))

{
    wxMenu* menuFile = new wxMenu;
    menuFile->Append(ID_Hello, "&Hello...\tCtrl+H",
        "Help string shown in status bar for this menu item");
    menuFile->AppendSeparator();
    menuFile->Append(wxID_EXIT);

    wxMenu* menuHelp = new wxMenu;
    menuHelp->Append(wxID_ABOUT);

    wxMenuBar* menuBar = new wxMenuBar;
    menuBar->Append(menuFile, "&File");
    menuBar->Append(menuHelp, "&Help");

    SetMenuBar(menuBar);

    CreateStatusBar();
    SetStatusText("Welcome to wxWidgets!");

    wxBoxSizer* mainSizer = new wxBoxSizer(wxVERTICAL);

    textCtrl = new wxTextCtrl(this, wxID_ANY, wxEmptyString, wxDefaultPosition, wxDefaultSize, wxTE_MULTILINE | wxTE_READONLY | wxHSCROLL);
    textCtrl->SetBackgroundColour(*wxWHITE);
    textCtrl->SetForegroundColour(*wxBLACK);
    textCtrl->SetFont(wxFont(wxFontInfo(10)));

    wxString licenseText = "Copyright 2023 Author \n\n"
        "Permission is hereby granted, free of charge, to any person obtaining a copy of this software "
        "and \nassociated documentation files(the “Software”), to deal in the Software without restriction, "
        "\nincluding without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, "
        "\nand/or sell copies of the Software, and to permit persons to whom the Software is furnished to do \n"
        "so, subject to the following conditions : \n"
        "The above copyright notice and this permission notice shall be included in all copies or substantial \n"
        "portions of the Software. \n\n"
        "THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR \n"
        "IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS \n"
        "FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE AUTHORS \n"
        "OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, \n"
        "WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN \n"
        "CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.";
    textCtrl->SetValue(licenseText);



    mainSizer->Add(textCtrl, 1, wxEXPAND | wxALL, 20);

    accept_checkbox = new wxCheckBox(this, ID_Accept_CheckBox, "I accept the license agreement");
    mainSizer->Add(accept_checkbox, 0, wxALIGN_LEFT | wxALL, 10);

    gauge = new wxGauge(this, wxID_ANY, 100, wxDefaultPosition, wxDefaultSize, wxGA_HORIZONTAL);
    mainSizer->Add(gauge, 0, wxEXPAND | wxALL, 20);

    install_button = new wxButton(this, ID_Install_Button, "Install", wxPoint(20, 20));
    set_location = new wxButton(this, ID_Set_Location, "Set Location", wxPoint(20, 20));

    install_button->Enable(false);

    mainSizer->AddStretchSpacer(); 
    mainSizer->Add(install_button, 0, wxALIGN_CENTER | wxALL, 10); 
    mainSizer->Add(set_location, 0, wxALIGN_CENTER | wxALL, 5);


    SetSizerAndFit(mainSizer);



    Bind(wxEVT_MENU, &NewFrame::OnHello, this, ID_Hello);
    Bind(wxEVT_MENU, &NewFrame::OnAbout, this, wxID_ABOUT);
    Bind(wxEVT_MENU, &NewFrame::OnExit, this, wxID_EXIT);
    Bind(wxEVT_BUTTON, &NewFrame::OnInstallButtonClick, this, ID_Install_Button);
    Bind(wxEVT_BUTTON, &NewFrame::OnSetButtonClick, this, ID_Set_Location);
    Bind(wxEVT_CHECKBOX, &NewFrame::OnCheckBoxClicked, this, ID_Accept_CheckBox);


    wxWindow::SetInitialSize(wxSize(700, 500));
}

void NewFrame::OnExit(wxCommandEvent& event)
{
    Close(true);
}

void NewFrame::OnAbout(wxCommandEvent& event)
{
    wxMessageBox("This is a wxWidgets Hello World example",
        "About Hello World", wxOK | wxICON_INFORMATION);
}

void NewFrame::OnHello(wxCommandEvent& event)
{
    wxLogMessage("Hello world from wxWidgets!");
}

void NewFrame::OnInstallButtonClick(wxCommandEvent& event)
{
    if (!accept_checkbox->IsChecked())
    {
        wxMessageBox("Please accept the license agreement.", "Error", wxOK | wxICON_ERROR);
        return;
    }

    // Test file download
    FileDownloader downloader;
    wxString url = "https://releases.ubuntu.com/22.04.2/ubuntu-22.04.2-desktop-amd64.iso";
    wxString destination = "F:/ubuntu-22.04.2-desktop-amd64.iso";

    install_button->Disable();

    std::thread downloadThread([this, &downloader, url, destination]() {
        wxThreadEvent* progressEvent = new wxThreadEvent(wxEVT_THREAD, ID_UPDATE_PROGRESS);
        progressEvent->SetInt(0);
        wxQueueEvent(wxTheApp->GetTopWindow(), progressEvent);

        if (downloader.DownloadFile(url.ToStdString(), destination.ToStdString()))
        {
            wxThreadEvent* completeEvent = new wxThreadEvent(wxEVT_THREAD, ID_DOWNLOAD_COMPLETE);
            wxQueueEvent(wxTheApp->GetTopWindow(), completeEvent);
        }
        else
        {
            wxThreadEvent* errorEvent = new wxThreadEvent(wxEVT_THREAD, ID_DOWNLOAD_ERROR);
            wxQueueEvent(wxTheApp->GetTopWindow(), errorEvent);
        }

        wxThreadEvent* enableEvent = new wxThreadEvent(wxEVT_THREAD, ID_ENABLE_INSTALL_BUTTON);
        wxQueueEvent(wxTheApp->GetTopWindow(), enableEvent);
        });

    downloadThread.detach();
}


void NewFrame::OnSetButtonClick(wxCommandEvent& event)
{
    wxMessageBox("Button clicked!", "Button", wxOK | wxICON_INFORMATION);
}

void NewFrame::OnCheckBoxClicked(wxCommandEvent& event)
{
    if (accept_checkbox->IsChecked())
        install_button->Enable(true);
    else
        install_button->Enable(false);
}

This is the exception that the debugger captures:

Exception thrown at 0x00007FFC285301E8 (wxbase32ud_vc_custom.dll) in WindowsInstaller.exe: 0xC0000005: Access violation writing location 0x00000080FDFDFDFD.
Ethan James
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  • Use a debugger like the one in Visual Studio to find the reason it crashes. By default it should give you the option to break into the debugger when a fatal exception is thrown. – drescherjm May 30 '23 at 00:29
  • @drescherjm I have updated the post with the exception. – Ethan James May 30 '23 at 00:38
  • As far as I know `curl_easy_setopt(curl, CURLOPT_WRITEDATA, file);` means the file pointer will be passed to the write callback function as `userp`. `wxThreadEvent* event = static_cast(userp);` tries to treat it as something completely different leading to your issue. – Retired Ninja May 30 '23 at 00:38
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    A debugger won't magically tell you what is wrong. It is just a tool you can use to figure things out (but a very effective tool). See [What is a debugger and how can it help me diagnose problems?](https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) and [How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/). – Jesper Juhl May 30 '23 at 00:49
  • "`bool WindowsInstaller::OnInit() { NewFrame* frame = new NewFrame(); ...`" - That looks like a memory leak. – Jesper Juhl May 30 '23 at 00:51
  • @RetiredNinja You appear to be correct, updating the WriteCallback function to account for this results in the application no longer crashing. – Ethan James May 30 '23 at 00:53
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    An additional point you should be aware of is that your `downloadThread` object captures a local variable `downloader` by reference, but that object drops out of scope after the `OnInstallButtonClick` method returns. The thread is detached and should be considered still active, possibly not even yet started executing. This is asking for trouble, as accessing `downloader` in any way has undefined behavior at that point. – paddy May 30 '23 at 00:55
  • `wxString data(static_cast(contents), wxConvUTF8, totalSize);` also seems like a really bad transformation for a binary file unless it is a utf-8 text file. – Retired Ninja May 30 '23 at 01:04

1 Answers1

2

In appreciate all the help I received in the comments, using a combination of them, I was able to fix the crashing problem, memory leak and poor binary transformation problems. Here is the updated code:

#include <wx/wx.h>

#include <iostream>
#include <fstream>
#include <curl/curl.h>
#include <thread>
#include <filesystem>
#include <future>


class BinaryFileDownloader
{
public:
    BinaryFileDownloader(wxGauge* progressGauge)
        : progressGauge_(progressGauge)
    {
    }

    bool DownloadFile(const std::string& url, const std::string& destination)
    {
        FILE* file = fopen(destination.c_str(), "wb");
        if (!file)
        {
            std::cerr << "Failed to open destination file: " << destination << std::endl;
            return false;
        }

        CURL* curl = curl_easy_init();
        if (!curl)
        {
            std::cerr << "Failed to initialize libcurl" << std::endl;
            fclose(file);
            return false;
        }

        curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteCallback);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, file);
        curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0L);
        curl_easy_setopt(curl, CURLOPT_XFERINFOFUNCTION, ProgressCallback);
        curl_easy_setopt(curl, CURLOPT_XFERINFODATA, this);

        CURLcode res = curl_easy_perform(curl);
        fclose(file);
        curl_easy_cleanup(curl);

        if (res != CURLE_OK)
        {
            std::cerr << "Failed to download file: " << curl_easy_strerror(res) << std::endl;
            return false;
        }

        std::cout << "File downloaded successfully!" << std::endl;
        return true;
    }

    static size_t WriteCallback(void* contents, size_t size, size_t nmemb, void* userdata)
    {
        FILE* file = static_cast<FILE*>(userdata);
        size_t written = fwrite(contents, size, nmemb, file);
        return written;
    }

    static int ProgressCallback(void* clientp, curl_off_t dltotal, curl_off_t dlnow, curl_off_t ultotal, curl_off_t ulnow)
    {
        BinaryFileDownloader* downloader = static_cast<BinaryFileDownloader*>(clientp);
        if (downloader->progressGauge_)
        {
            if (dltotal > 0)
            {
                double progress = static_cast<double>(dlnow) / static_cast<double>(dltotal) * 100.0;
                downloader->progressGauge_->SetValue(static_cast<int>(progress));
            }
        }
        return 0;
    }

private:
    wxGauge* progressGauge_;
};

enum
{
    ID_Hello = 1,
    ID_Install_Button = 2,
    ID_Set_Location = 3,
    ID_Accept_CheckBox = 4,
    ID_UPDATE_PROGRESS = 5,
    ID_DOWNLOAD_COMPLETE = 6,
    ID_DOWNLOAD_ERROR = 7,
    ID_ENABLE_INSTALL_BUTTON = 8

};

class NewFrame : public wxFrame
{
public:
    NewFrame();
    void OnHello(wxCommandEvent& event);
    void OnAbout(wxCommandEvent& event);
    void OnInstallButtonClick(wxCommandEvent& event);
    void OnSetButtonClick(wxCommandEvent& event);
    void OnCheckBoxClicked(wxCommandEvent& event);
    void OnExit(wxCommandEvent& event);
    void OnDownloadComplete(wxThreadEvent& event);


private:
    wxButton* install_button;
    wxButton* set_location;
    wxTextCtrl* textCtrl;
    wxTextCtrl* statusCtrl;
    wxCheckBox* accept_checkbox;
    wxGauge* gauge;
    std::thread downloadThread;
};

class WindowsInstallerApp : public wxApp
{
public:
    bool OnInit() override;
    int OnExit() override;

private:
    NewFrame* frame;
};

wxIMPLEMENT_APP(WindowsInstallerApp);

bool WindowsInstallerApp::OnInit()
{
    frame = new NewFrame();
    frame->Show(true);
    return true;
}


NewFrame::NewFrame()
    : wxFrame(nullptr, wxID_ANY, "Hello World", wxDefaultPosition, wxSize(700, 500), wxDEFAULT_FRAME_STYLE & ~(wxRESIZE_BORDER | wxMAXIMIZE_BOX))

{
    wxMenu* menuFile = new wxMenu;
    menuFile->Append(ID_Hello, "&Hello...\tCtrl+H",
        "Help string shown in status bar for this menu item");
    menuFile->AppendSeparator();
    menuFile->Append(wxID_EXIT);

    wxMenu* menuHelp = new wxMenu;
    menuHelp->Append(wxID_ABOUT);

    wxMenuBar* menuBar = new wxMenuBar;
    menuBar->Append(menuFile, "&File");
    menuBar->Append(menuHelp, "&Help");

    SetMenuBar(menuBar);

    CreateStatusBar();
    SetStatusText("Installer Ready!");

    wxBoxSizer* mainSizer = new wxBoxSizer(wxVERTICAL);

    textCtrl = new wxTextCtrl(this, wxID_ANY, wxEmptyString, wxDefaultPosition, wxDefaultSize, wxTE_MULTILINE | wxTE_READONLY | wxHSCROLL);
    textCtrl->SetBackgroundColour(*wxWHITE);
    textCtrl->SetForegroundColour(*wxBLACK);
    textCtrl->SetFont(wxFont(wxFontInfo(10)));

    wxString licenseText = "Copyright 2023 Ethan James \n\n"
        "Permission is hereby granted, free of charge, to any person obtaining a copy of this software "
        "and \nassociated documentation files(the “Software”), to deal in the Software without restriction, "
        "\nincluding without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, "
        "\nand/or sell copies of the Software, and to permit persons to whom the Software is furnished to do \n"
        "so, subject to the following conditions : \n"
        "The above copyright notice and this permission notice shall be included in all copies or substantial \n"
        "portions of the Software. \n\n"
        "THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR \n"
        "IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS \n"
        "FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE AUTHORS \n"
        "OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, \n"
        "WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN \n"
        "CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.";
    textCtrl->SetValue(licenseText);



    mainSizer->Add(textCtrl, 1, wxEXPAND | wxALL, 20);

    accept_checkbox = new wxCheckBox(this, ID_Accept_CheckBox, "I accept the license agreement");
    mainSizer->Add(accept_checkbox, 0, wxALIGN_LEFT | wxALL, 10);

    gauge = new wxGauge(this, wxID_ANY, 100, wxDefaultPosition, wxDefaultSize, wxGA_HORIZONTAL);
    mainSizer->Add(gauge, 0, wxEXPAND | wxALL, 20);
    gauge->SetValue(0);


    install_button = new wxButton(this, ID_Install_Button, "Install", wxPoint(20, 20));
    set_location = new wxButton(this, ID_Set_Location, "Set Location", wxPoint(20, 20));

    install_button->Enable(false);

    mainSizer->AddStretchSpacer();
    mainSizer->Add(install_button, 0, wxALIGN_CENTER | wxALL, 10);
    mainSizer->Add(set_location, 0, wxALIGN_CENTER | wxALL, 5);


    SetSizerAndFit(mainSizer);





    Bind(wxEVT_MENU, &NewFrame::OnHello, this, ID_Hello);
    Bind(wxEVT_MENU, &NewFrame::OnAbout, this, wxID_ABOUT);
    Bind(wxEVT_MENU, &NewFrame::OnExit, this, wxID_EXIT);
    Bind(wxEVT_BUTTON, &NewFrame::OnInstallButtonClick, this, ID_Install_Button);
    Bind(wxEVT_BUTTON, &NewFrame::OnSetButtonClick, this, ID_Set_Location);
    Bind(wxEVT_CHECKBOX, &NewFrame::OnCheckBoxClicked, this, ID_Accept_CheckBox);
    Bind(wxEVT_THREAD, &NewFrame::OnDownloadComplete, this, ID_DOWNLOAD_COMPLETE);


    wxWindow::SetInitialSize(wxSize(700, 500));
}

void NewFrame::OnAbout(wxCommandEvent& event)
{
    wxMessageBox("This is a simple windows application installer, written by Sadan#9264",
        "About This App", wxOK | wxICON_INFORMATION);
}

void NewFrame::OnHello(wxCommandEvent& event)
{

}

void NewFrame::OnInstallButtonClick(wxCommandEvent& event)
{
    if (!accept_checkbox->IsChecked())
    {
        wxMessageBox("Please accept the license agreement.", "Error", wxOK | wxICON_ERROR);
        return;
    }

    install_button->Enable(false);
    accept_checkbox->Enable(false);
    set_location->Enable(false);

    
    downloadThread = std::thread([this]() {
         BinaryFileDownloader downloader(gauge);
         std::string url = "https://releases.ubuntu.com/22.04.2/ubuntu-22.04.2-desktop-amd64.iso";
         std::string destination = "F:/ubuntu-22.04.2-desktop-amd64.iso";
         bool success = downloader.DownloadFile(url, destination);

         // Notify the main thread when the download is complete.
         wxThreadEvent* event = new wxThreadEvent(wxEVT_THREAD, ID_DOWNLOAD_COMPLETE);
         event->SetString(success ? "Download successful" : "Download failed");
         wxQueueEvent(this, event);
     });
   
    

    
}

void NewFrame::OnDownloadComplete(wxThreadEvent& event)
{
    /*
    std::string message = event.GetString();
    std::cout << message << std::endl;
    */

    // Cleanup the download thread
    if (downloadThread.joinable()) {
        downloadThread.join();
    }
}


void NewFrame::OnSetButtonClick(wxCommandEvent& event)
{
    wxMessageBox("Button clicked!", "Button", wxOK | wxICON_INFORMATION);
}

void NewFrame::OnCheckBoxClicked(wxCommandEvent& event)
{
    if (accept_checkbox->IsChecked())
        install_button->Enable(true);
    else
        install_button->Enable(false);
}

int WindowsInstallerApp::OnExit()
{

    delete frame;
    return wxApp::OnExit();
}


void NewFrame::OnExit(wxCommandEvent& event)
{

    Close(true);
}

The crashing was a problem with curl_easy_setopt(curl, CURLOPT_WRITEDATA, file); and wxThreadEvent* event = static_cast<wxThreadEvent*>(userp);, which are incompatible, thanks to drescherjm.

The memory leak with the frame, which was noticed by Jesper Juhl was fixed by closing it when the window is destroyed.

And finally the binary transformation was replaced in order to work with non utf-8 text files, thanks Retired Ninja.

Ethan James
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