Does opendir() / FindFirstFile() get a snapshot of a directory?

Question

If I do an opendir() under Posix or a FindFirstFile() under Windows. Do these operations get a snaphot of the directory contents ? I.e. a readdir() or FindNextFile() wouldn't get additional files added to the directory after opedir() or FindFirstFile() ?

Answer 1

No. For WIn32, see the question linked in the comment. For opendir, see the documentation of readdir.

If a file is removed from or added to the directory after the most recent call to opendir() or rewinddir(), whether a subsequent call to readdir() returns an entry for that file is unspecified.

(emphasis mine)

Answer 2

For my Windows system with WSL2 and my Ubuntu PC readdir does not take a snapshot:

#include <iostream>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <dirent.h>

using namespace std;

int main()
{
    constexpr char const *FILENAME = "file.bin";
    unlink( FILENAME );
    DIR *dir = opendir( "." );
    if( !dir )
        return EXIT_FAILURE;
    if( creat( FILENAME, 0 ) == -1 )
        return EXIT_FAILURE;
    for( dirent *de; de = readdir( dir ); )
        cout << de->d_name << endl;
    return !errno ? EXIT_SUCCESS : EXIT_FAILURE;
}

With my Windows 11 PC it's the same:

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

using namespace std;

int main()
{
    constexpr wchar_t const *FILENAME = L"file.bin";
    DeleteFileW( FILENAME );
    WIN32_FIND_DATAW fdw;
    HANDLE hFind = FindFirstFileW( L"*", &fdw );
    auto ret = [] { return GetLastError() == ERROR_FILE_NOT_FOUND ? EXIT_SUCCESS : EXIT_FAILURE; };
    if( hFind == INVALID_HANDLE_VALUE )
        return ret();
    HANDLE hFile = CreateFileW( FILENAME, GENERIC_READ | GENERIC_WRITE, 0, nullptr, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL );
    if( hFile == INVALID_HANDLE_VALUE )
        return EXIT_FAILURE;
    do
        wcout << fdw.cFileName << endl;
    while( FindNextFileW( hFind, &fdw ) );
    return ret();
}

Both programs find file.bin, although this isn't guaranteed.