Wierd access violation error message when trying to use an openFile dialog window

Question

I am still kinda bad in c++ so pls dont mind my bad code or my missing knowledge. The project is about chosing a file and the paste it in the console for the user to read and i thought the best way would be using a dialog window (and i get more practice using the winapi).

Here my code for the window:

OPENFILENAMEA NameOfFile;
ZeroMemory(&NameOfFile, sizeof(NameOfFile));
NameOfFile.nFileOffset = 1;
char szFile[260];
NameOfFile.lpstrFile[0] = '\0';
NameOfFile.lpstrFile = szFile;
NameOfFile.nMaxFile = 4096;
NameOfFile.Flags = OFN_ALLOWMULTISELECT;
if (GetOpenFileName(&NameOfFile)) {
    
    cout << "opened";
}

Now the wierd thing. The Programm crashes with the error "-1073741819". Google said its an access violation of smth (no clue what exactly it means).

When i comment out the ZeroMemory function i got a linker and compiler error that NameOfFile is apparently not initialized??? (but if its not commented it compiles normally?!)

Answer 1

There are several issues in your code:

1. You need to set the lStructSize field of your OPENFILENAMEA variable.
2. You need to swap the 2 lines setting NameOfFile.lpstrFile, so that the pointer that you access (in order to write the zero termination) will point to a valid buffer.
3. NameOfFile.nMaxFile should be set to the size of the filename pointer by lpstrFile, i.e. the sizeof(szFile).
4. In order to set the file names filter, you can set lpstrFilter and nFilterIndex. The example below enabled to select either all or txt files.

Fixed version:

#include <Windows.h>
#include <iostream>

int main()
{
    OPENFILENAMEA NameOfFile;
    ZeroMemory(&NameOfFile, sizeof(NameOfFile));
    NameOfFile.lStructSize = sizeof(NameOfFile);   // Item 1 above
    char szFile[260];
    NameOfFile.lpstrFile = szFile;                 // Item 2 above
    NameOfFile.lpstrFile[0] = '\0';                // Item 2 above 
    NameOfFile.nMaxFile = sizeof(szFile);          // Item 3 above
    NameOfFile.Flags = OFN_ALLOWMULTISELECT;
    NameOfFile.lpstrFilter = "All\0*.*\0Text\0*.TXT\0"; // Item 4 above
    NameOfFile.nFilterIndex = 1;                        // Item 4 above
    if (GetOpenFileNameA(&NameOfFile))
    {
        std::cout << "opened " << szFile << std::endl;
    }
}

For additional info you can see the example in the MS documentation here under the "Opening a File" section.
You can also have a look at the OPENFILENAMEA structure for the list of fields you can potentially set for the API.

A side note: better to avoid using namespace std - see here Why is "using namespace std;" considered bad practice?.

Answer 2

So in the end its just the fault of one single line of code.

NameOfFile.lpstrFile[0] = '\0';

it was the fault for the whole thing and it was my mistake for pasting it from another stack overflow post without knowing what it does (it had an explanation that i didnt really understand)

just deleting it fixed the problem

Answer 3

here the code i use from now on. I fixed the problem with LPWSTR trough making szFile a TCHAR and converting it into LPWSTR in FILE. Not the best but enoght for me right now.

LPWSTR FILE{};
OPENFILENAME NameOfFile;
ZeroMemory(&NameOfFile, sizeof(NameOfFile));
NameOfFile.lStructSize = sizeof(NameOfFile);
TCHAR szFile[MAX_PATH];
NameOfFile.lpstrFile = szFile;
NameOfFile.nMaxFile = sizeof(szFile);
NameOfFile.Flags = OFN_ALLOWMULTISELECT;
if (GetOpenFileName(&NameOfFile))
{
    FILE = (LPWSTR)szFile;
}

For everyone that wants to paste this, don´t forget the file filters. Otherwise you wont be able to see any files if the windows opens for you. More of that in the documentation