WinAPI 文件存在 Function 实现

Question

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;
}

Answer 1

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

Answer 2

Common way to check file existanse using WinAPI - use file search API eg

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;
}