C ++和NTFS文件：路径名VS开头

Question

This is an extension of this question: fstream not opening files with accent marks in pathname

The problem is the following: a program opening a simple NTFS text file with accent marks in pathname (eg à , ò , ...). In my tests I'm using a file with pathname I:\\università\\foo.txt ( università is the Italian translation of university )

The following is the test program:

#include <iostream>
#include <fstream>
#include <string>
#include <cstdio>
#include <errno.h>
#include <Windows.h>

using namespace std;

LPSTR cPath = "I:/università/foo.txt";
LPWSTR widecPath = L"I:/università/foo.txt";
string path("I:/università/foo.txt");

void tryWithStandardC();
void tryWithStandardCpp();
void tryWithWin32();

int main(int argc, char **argv) {
    tryWithStandardC();
    tryWithStandardCpp();
    tryWithWin32();

    return 0;
} 

void tryWithStandardC() {
    FILE *stream = fopen(cPath, "r");

    if (stream) {
        cout << "File opened with fopen!" << endl;
        fclose(stream);
    }

    else {
        cout << "fopen() failed: " << strerror(errno) << endl;
    }
}

void tryWithStandardCpp() {
    ifstream s;
    s.exceptions(ifstream::failbit | ifstream::badbit | ifstream::eofbit);      

    try {
        s.open(path.c_str(), ifstream::in);
        cout << "File opened with c++ open()" << endl;
        s.close();
    }

    catch (ifstream::failure f) {
        cout << "Exception " << f.what() << endl;
    }   
}

void tryWithWin32() {

    DWORD error;
    HANDLE h = CreateFile(cPath, GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

    if (h == INVALID_HANDLE_VALUE) {
        error = GetLastError();
        cout << "CreateFile failed: error number " << error << endl;
    }

    else {
        cout << "File opened with CreateFile!" << endl;
        CloseHandle(h);
        return;
    }

    HANDLE wideHandle = CreateFileW(widecPath, GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

    if (wideHandle == INVALID_HANDLE_VALUE) {
        error = GetLastError();
        cout << "CreateFileW failed: error number " << error << endl;
    }

    else {
        cout << "File opened with CreateFileW!" << endl;
        CloseHandle(wideHandle);
    }
}

The source file is saved with UTF-8 encoding. I'm using Windows 8.

This is the output of the program compiled with VC++ (Visual Studio 2012)

fopen() failed: No such file or directory
Exception ios_base::failbit set
CreateFile failed: error number 3
CreateFileW failed: error number 3

This is the output using MinGW g++

fopen() failed: No such file or directory
Exception basic_ios::clear
CreateFile failed: error number 3
File opened with CreateFileW!

So let's go to the questions:

1. Why fopen() and std::ifstream works in a similar test in Linux but they don't in Windows?
2. Why CreateFileW() works only compiling with g++?
3. Does exist a cross-platform alternative to CreateFile?

I hope that opening a generic file with a generic pathname could be done without the necessity of platform-specific code, but I have not idea how to do it.

Thanks in advance.

Answer 1

You write:

“The source file is saved with UTF-8 encoding.”

Well that's all well and good (so far) if you're using the g++ compiler, which has UTF-8 as its default basic source character set. However, Visual C++ will by default assume that the source file is encoded in Windows ANSI, unless it's clearly otherwise. So make very sure that it has a BOM (Byte Order Mark) at the start, which – undocumented, as far as I know – causes Visual C++ to treat it as encoded with UTF-8.

You then ask,

“1. Why fopen() and std::ifstream works in a similar test in Linux but they don't in Windows?”

For Linux it's likely to work because (1) modern Linux is UTF-8 oriented, so if the filename looks the same it is likely the same as the identical looking UTF-8 encoded filename in the source code, and (2) in *nix a filename is just a sequence of bytes, not a sequence of characters. Which means that regardless of how it looks, if you pass the identical sequence of bytes, the same values, then you have a match, otherwise not.

In contrast, in Windows a filename is a sequence of characters that can be encoded in various ways.

In your case the UTF-8 encoded filename in the source code is stored as Windows ANSI in the executable (and yes, the result of building with Visual C++ depends on the selected ANSI codepage in Windows, which also as far as I know is undocumented). Then this gobbledegook string is passed down a routine hierarchy and converted to UTF-16, which is the standard character encoding in Windows. The result doesn't match the filename at all.

---

You further ask,

“2. Why CreateFileW() works only compiling with g++?”

Presumably because you did not include a BOM at the start of the sourc code file (see above).

With a BOM everything works nicely with Visual C++, at least in Windows 7:

File opened with fopen!
File opened with c++ open()
File opened with CreateFile!

---

Finally, you ask,

“3. Does exist a cross-platform alternative to CreateFile?”

Not really. There is Boost filesystem. But while its version 2 did have a workaround for the standard library's lossy narrow character based encoding, that workaround was removed in version 3, which just uses a Visual C++ extension of the standard library where Visual C++ implementation provides wide character argument versions of the stream constructors and open . Ie, at least as far as I know (I haven't checked lately if things have been fixed), Boost filesystem only works in general with Visual C++, not with eg g++ – although it works for no-troublesome-characters filenames.

The workaround that v2 had, was to try with conversion to Windows ANSI (the codepage specified by the GetACP function), and if that didn't work, try GetShortPathName , which is practically guaranteed to be representable with Windows ANSI.

Part of the reason that the workaround in Boost filesystem was removed was, as I understand it, that it's in principle possible for the user to turn off the Windows short name functionality at least in Windows Vista and earlier. However that's not a practical concern. It just means that there is an easy fix available (namely turn it back on) if the user experiences problems due to having wilfully lobotomized the system.

Answer 2

The problem you're stumbling over is that the encoding you pass to fstreams as path is implementation-specific. Further, the behaviour of your program is implementation-defined because it uses characters outside of C++'s characterset in the code, ie the accented characters. The problem there is that there are many different encodings that can be used to represent these characters.

Now, there are solutions:

• Firstly, there is an MSC extension to tell the compiler which encoding it should assume.
• In order to get a path working with CreateFileW(), you can code the path like wchar_t const path[] = {'f', 0x20ac, '.', 't', 'x', 't'}; . This is not really comfortable, but in practice the paths are stored in files with some Unicode encoding or input by the user.
• Then, there is an extension in the implementation of the standard library that allows you to use wchar_t paths, there are both _wfopen() and fstream constructors.
• Then, there is Boost, which has a filesystem and iostream library that is specifically made to provide portable. I would definitely look at this.

Note that while the wchar_t paths are not portable, porting them to a new platform is usually not very complicated. A few #ifdefs and you're set.