checking if it is windows or unix before creating a directory in C++

Question

I wrote this piece of code to check if a directory exists in both Windows and Unix, but I'm not sure whether it is correct:

int writeFiles(std::string location)
{

        // USED TO FILE SYSTEM OPERATION
        struct stat st;
        // DEFINE THE mode WHICH THE FILE WILL BE CREATED
        const char * mode = "w+b";
        /* local curl variable */

        // CHECK IF THE DIRECTORY TO WHERE THE FILE ARE GOING EXIST
        // IF NOT, CREATE IT
        if(stat(location.c_str(), &st) != 0){
                #ifndef (defined  _WIN32 || defined __WIN64)    /* WIN32 SYSTEM */
                if (!CreateDirectory(location.c_str(), NULL)){
                        std::string msg("The location directory did not exists, can't be created\n");
                        throw std::runtime_error(msg);
                }
                #elif defined __unix__          /* in the case of unix system */
                if(mkdir(location.c_str(), S_IRWXU) != 0){
                        std::string msg("The dest_loc directory did not exist, can't be created\n");
                        throw std::runtime_error(msg);
                }
                #endif

 ... more code down here.

location is the path to where the files are supposed to copied. However, before I start copying the files, I have to check whether the directory exists or not, for both Windows and Linux. Could someone please give me some input on that problem? Thanks

Answer 1

The preprocessor directives (see list of Microsoft Predefined Macros ) I would write as:

#ifdef _WIN32

#else

// Assume UNIX system,
// depending on what you are compiling your code on,
// by that I mean you only building on Windows or UNIX
// (Linux, Solaris, etc) and not on Mac or other.
#endif

The CreateDirectory() will fail (return FALSE ) if the directory already exists but will set the last error to ERROR_ALREADY_EXISTS . Change your use of CreateDirectory() to handle this correctly:

if (!CreateDirectory(location.c_str(), NULL) &&
    ERROR_ALREADY_EXISTS != GetLastError())
{
    // Error message more useful if you include last error code.
    std::ostringstream err;
    err << "CreateDirectory() failure on "
        << location
        << ", last-error="
        << GetLastError();

    throw std::runtime_exception(err.str());
}

Having said that, if you have access to boost consider using the boost::filesystem library.

Answer 2

You need to change:

            #ifndef (defined  _WIN32 || defined __WIN64)    /* WIN32 SYSTEM */

to:

            #if (defined _WIN32 || defined __WIN64)    /* WIN32 SYSTEM */

This tests whether either _WIN32 or __WIN64 is defined and then uses the WINAPI code if that is the case.

You could probably also change:

            #elif defined __unix__          /* in the case of unix system */

to just:

            #else          /* in the case of non-Windows system */

since most non-Windows OS are likely to have POSIX-ish APIs for mkdir etc, and you currently don't have any other OS-specific code.

Answer 3

如果必须编写与文件系统交互的跨平台代码，则可以使用跨平台文件系统API，例如Boost FileSystem 。

Answer 4

If you can assume that Windows has stat() , why can't you also just use mkdir() ?

But really, on Windows you can call CreateDirectory unconditionally (no preceding stat call) and check whether GetLastError() returns ERROR_ALREADY_EXISTS .

Also, std::string is a match for the ANSI function, CreateDirectoryA . Using the CreateDirectory macro leaves you open to Unicode mismatch.