WinAPI FileExists Function Implemetation-CodePudding

I am coding a simple replacement for std::filesystem::exists() function using Windows API. Surprisingly, it turned out to be pretty hard. I want to keep my code simple, so I am using minimum functions. My function of choice is GetFileAttributesW(). Code is tested with fs::recursive_directory_iterator() function. My function thinks that all files in “C:\Windows\servicing\LCU*” don’t exist (ERROR_PATH_NOT_FOUND). This directory is responsible for storing Windows Update Caches and is famous for having extremely long file names. I couldn’t find anything else about this directory. Example of filenames and my code are included below. Hope this helps!

Edited: The solution to this problem is to prepend absolute file path with “\\?\” char sequence. It makes Windows handle short files correctly!

C:\Windows\servicing\LCU\Package_for_RollupFix~31bf3856ad364e35~amd64~~19041.2006.1.7\amd64_microsoft-windows-a..g-whatsnew.appxmain_31bf3856ad364e35_10.0.19041.1741_none_ee5d4a8d060d7653\f\new360videossquare44x44logo.targetsize-16_altform-unplated_contrast-black.png
C:\Windows\servicing\LCU\Package_for_RollupFix~31bf3856ad364e35~amd64~~19041.2006.1.7\amd64_microsoft-windows-a..g-whatsnew.appxmain_31bf3856ad364e35_10.0.19041.1741_none_ee5d4a8d060d7653\f\new360videossquare44x44logo.targetsize-16_altform-unplated_contrast-white.png
C:\Windows\servicing\LCU\Package_for_RollupFix~31bf3856ad364e35~amd64~~19041.2006.1.7\amd64_microsoft-windows-a..g-whatsnew.appxmain_31bf3856ad364e35_10.0.19041.1741_none_ee5d4a8d060d7653\f\new360videossquare44x44logo.targetsize-20_altform-unplated_contrast-black.png
C:\Windows\servicing\LCU\Package_for_RollupFix~31bf3856ad364e35~amd64~~19041.2006.1.7\amd64_microsoft-windows-a..g-whatsnew.appxmain_31bf3856ad364e35_10.0.19041.1741_none_ee5d4a8d060d7653\f\new360videossquare44x44logo.targetsize-20_altform-unplated_contrast-white.png

#include <windows.h>

#include <filesystem>
#include <iostream>
#include <string>

using namespace std;
namespace fs = std::filesystem;

int FileExists(wstring file_path) {
    /* TODO:
         1. Doesn't work with "C:\\Windows\\servicing\\LCU\\*".
         2. Improve error system.
    */

    DWORD attributes = GetFileAttributesW(file_path.c_str());
        
    // Valid attributes => File exists
    if (attributes != INVALID_FILE_ATTRIBUTES) {
        return true;
    }

    DWORD error_code = GetLastError();
    wcout << error_code << ' ' << file_path << '\n';

    // Path related error => File doesn't exist
    if (error_code == ERROR_PATH_NOT_FOUND || error_code == ERROR_INVALID_NAME ||
        error_code == ERROR_FILE_NOT_FOUND || error_code == ERROR_BAD_NETPATH)
    {
        return false;
    }

    // Other errors are logged before if statement
    // File is busy with IO operations, etc.
    return error_code;
}

int main() {

    for (fs::path path : fs::recursive_directory_iterator("C:\\", fs::directory_options::skip_permission_denied)) {
        FileExists(path);
    }

    return 0;
}

CodePudding user response：

The solution that worked for me is to prepend absolute file path with “\\?\” char sequence. Somehow, it makes Windows handle shortened file paths correctly!

Check out MSDN Article "Maximum File Path Limitation" for more info.

CodePudding user response：

Common way to check file existanse using WinAPI - use file search API e.g.

bool file_exist(const std::wstring& path) noexcept
{
    if( path.empty() )
        return false;
    ::WIN32_FIND_DATAW fdata;
    ::HANDLE handle = ::FindFirstFileW(path.data(), &fdata);
    bool result = handle != INVALID_HANDLE_VALUE;
    if(result) {
        ::FindClose(handle);
    }
    return result;
}