How can I calculate the complete buffer size for G

Question

The GetModuleFileName() takes a buffer and size of buffer as input; however its return value can only tell us how many characters is has copied, and if the size is not enough (ERROR_INSUFFICIENT_BUFFER).

How do I determine the real required buffer size to hold entire file name for GetModuleFileName()?

Most people use MAX_PATH but I remember the path can exceed that (260 by default definition)...

(The trick of using zero as size of buffer does not work for this API - I've already tried before)

Answer 1

Windows cannot handle properly paths longer than 260 characters, so just use MAX_PATH. You cannot run a program having path longer than MAX_PATH.

Answer 2

The usual recipe is to call it setting the size to zero and it is guaranteed to fail and provide the size needed to allocate sufficient buffer. Allocate a buffer (don't forget room for nul-termination) and call it a second time.

In a lot of cases MAX_PATH is sufficient because many of the file systems restrict the total length of a path name. However, it is possible to construct legal and useful file names that exceed MAX_PATH, so it is probably good advice to query for the required buffer.

Don't forget to eventually return the buffer from the allocator that provided it.

Edit: Francis points out in a comment that the usual recipe doesn't work for GetModuleFileName(). Unfortunately, Francis is absolutely right on that point, and my only excuse is that I didn't go look it up to verify before providing a "usual" solution.

I don't know what the author of that API was thinking, except that it is possible that when it was introduced, MAX_PATH really was the largest possible path, making the correct recipe easy. Simply do all file name manipulation in a buffer of length no less than MAX_PATH characters.

Oh, yeah, don't forget that path names since 1995 or so allow Unicode characters. Because Unicode takes more room, any path name can be preceeded by \\?\ to explicitly request that the MAX_PATH restriction on its byte length be dropped for that name. This complicates the question.

MSDN has this to say about path length in the article titled File Names, Paths, and Namespaces:

Maximum Path Length

In the Windows API (with some exceptions discussed in the following paragraphs), the maximum length for a path is MAX_PATH, which is defined as 260 characters. A local path is structured in the following order: drive letter, colon, backslash, components separated by backslashes, and a terminating null character. For example, the maximum path on drive D is "D:\<some 256 character path string><NUL>" where "<NUL>" represents the invisible terminating null character for the current system codepage. (The characters < > are used here for visual clarity and cannot be part of a valid path string.)

Note File I/O functions in the Windows API convert "/" to "\" as part of converting the name to an NT-style name, except when using the "\\?\" prefix as detailed in the following sections.

The Windows API has many functions that also have Unicode versions to permit an extended-length path for a maximum total path length of 32,767 characters. This type of path is composed of components separated by backslashes, each up to the value returned in the lpMaximumComponentLength parameter of the GetVolumeInformation function. To specify an extended-length path, use the "\\?\" prefix. For example, "\\?\D:\<very long path>". (The characters < > are used here for visual clarity and cannot be part of a valid path string.)

Note The maximum path of 32,767 characters is approximate, because the "\\?\" prefix may be expanded to a longer string by the system at run time, and this expansion applies to the total length.

The "\\?\" prefix can also be used with paths constructed according to the universal naming convention (UNC). To specify such a path using UNC, use the "\\?\UNC\" prefix. For example, "\\?\UNC\server\share", where "server" is the name of the machine and "share" is the name of the shared folder. These prefixes are not used as part of the path itself. They indicate that the path should be passed to the system with minimal modification, which means that you cannot use forward slashes to represent path separators, or a period to represent the current directory. Also, you cannot use the "\\?\" prefix with a relative path, therefore relative paths are limited to MAX_PATH characters as previously stated for paths not using the "\\?\" prefix.

When using an API to create a directory, the specified path cannot be so long that you cannot append an 8.3 file name (that is, the directory name cannot exceed MAX_PATH minus 12).

The shell and the file system have different requirements. It is possible to create a path with the Windows API that the shell user interface might not be able to handle.

So an easy answer would be to allocate a buffer of size MAX_PATH, retrieve the name and check for errors. If it fit, you are done. Otherwise, if it begins with "\\?\", get a buffer of size 64KB or so (the phrase "maximum path of 32,767 characters is approximate" above is a tad troubling here so I'm leaving some details for further study) and try again.

Overflowing MAX_PATH but not beginning with "\\?\" appears to be a "can't happen" case. Again, what to do then is a detail you'll have to deal with.

There may also be some confusion over what the path length limit is for a network name which begins "\\Server\Share\", not to mention names from the kernel object name space which begin with "\\.\". The above article does not say, and I'm not certain about whether this API could return such a path.

Answer 3

Implement some reasonable strategy for growing the buffer like start with MAX_PATH, then make each successive size 1,5 times (or 2 times for less iterations) bigger then the previous one. Iterate until the function succeeds.

Answer 4

While the API is proof of bad design, the solution is actually very simple. Simple, yet sad it has to be this way, for it's somewhat of a performance hog as it might require multiple memory allocations. Here is some keypoints to the solution:

• You can't really rely on the return value between different Windows-versions as it can have different semantics on different Windows-versions (XP for example).

• If the supplied buffer is too small to hold the string, the return value is the amount of characters including the 0-terminator.

• If the supplied buffer is large enough to hold the string, the return value is the amount of characters excluding the 0-terminator.

This means that if the returned value exactly equals the buffer size, you still don't know whether it succeeded or not. There might be more data. Or not. In the end you can only be certain of success if the buffer length is actually greater than required. Sadly...

So, the solution is to start off with a small buffer. We then call GetModuleFileName passing the exact buffer length (in TCHARs) and comparing the return result with it. If the return result is less than our buffer length, it succeeded. If the return result is greater than or equal to our buffer length, we have to try again with a larger buffer. Rinse and repeat until done. When done we make a string copy (strdup/wcsdup/tcsdup) of the buffer, clean up, and return the string copy. This string will have the right allocation size rather than the likely overhead from our temporary buffer. Note that the caller is responsible for freeing the returned string (strdup/wcsdup/tcsdup mallocs memory).

See below for an implementation and usage code example. I have been using this code for over a decade now, including in enterprise document management software where there can be a lot of quite long paths. The code can ofcourse be optimized in various ways, for example by first loading the returned string into a local buffer (TCHAR buf[256]). If that buffer is too small you can then start the dynamic allocation loop. Other optimizations are possible but that's beyond the scope here.

Implementation and usage example:

/* Ensure Win32 API Unicode setting is in sync with CRT Unicode setting */
#if defined(_UNICODE) && !defined(UNICODE)
#   define UNICODE
#elif defined(UNICODE) && !defined(_UNICODE)
#   define _UNICODE
#endif

#include <stdio.h> /* not needed for our function, just for printf */
#include <tchar.h>
#include <windows.h>

LPCTSTR GetMainModulePath(void)
{
    TCHAR* buf    = NULL;
    DWORD  bufLen = 256;
    DWORD  retLen;

    while (32768 >= bufLen)
    {
        if (!(buf = (TCHAR*)malloc(sizeof(TCHAR) * (size_t)bufLen))
        {
            /* Insufficient memory */
            return NULL;
        }

        if (!(retLen = GetModuleFileName(NULL, buf, bufLen)))
        {
            /* GetModuleFileName failed */
            free(buf);
            return NULL;
        }
        else if (bufLen > retLen)
        {
            /* Success */
            LPCTSTR result = _tcsdup(buf); /* Caller should free returned pointer */
            free(buf);
            return result;
        }

        free(buf);
        bufLen <<= 1;
    }

    /* Path too long */
    return NULL;
}

int main(int argc, char* argv[])
{
    LPCTSTR path;

    if (!(path = GetMainModulePath()))
    {
        /* Insufficient memory or path too long */
        return 0;
    }

    _tprintf("%s\n", path);

    free(path); /* GetMainModulePath malloced memory using _tcsdup */ 

    return 0;
}

Having said all that, I like to point out you need to be very aware of various other caveats with GetModuleFileName(Ex). There are varying issues between 32/64-bit/WOW64. Also the output is not necessarily a full, long path, but could very well be a short-filename or be subject to path aliasing. I expect when you use such a function that the goal is to provide the caller with a useable, reliable full, long path, therefor I suggest to indeed ensure to return a useable, reliable, full, long absolute path, in such a way that it is portable between various Windows-versions and architectures (again 32/64-bit/WOW64). How to do that efficiently is beyond the scope here.

While this is one of the worst Win32 APIs in existance, I wish you alot of coding joy nonetheless.

Answer 5

My example is a concrete implementation of the "if at first you don't succeed, double the length of the buffer" approach. It retrieves the path of the executable that is running, using a string (actually a wstring, since I want to be able to handle Unicode) as the buffer. To determine when it has successfully retrieved the full path, it checks the value returned from GetModuleFileNameW against the value returned by wstring::length(), then uses that value to resize the final string in order to strip the extra null characters. If it fails, it returns an empty string.

inline std::wstring getPathToExecutableW() 
{
    static const size_t INITIAL_BUFFER_SIZE = MAX_PATH;
    static const size_t MAX_ITERATIONS = 7;
    std::wstring ret;
    DWORD bufferSize = INITIAL_BUFFER_SIZE;
    for (size_t iterations = 0; iterations < MAX_ITERATIONS; ++iterations)
    {
        ret.resize(bufferSize);
        DWORD charsReturned = GetModuleFileNameW(NULL, &ret[0], bufferSize);
        if (charsReturned < ret.length())
        {
            ret.resize(charsReturned);
            return ret;
        }
        else
        {
            bufferSize *= 2;
        }
    }
    return L"";
}

Answer 6

My approach to this is to use argv, assuming you only want to get the filename of the running program. When you try to get the filename from a different module, the only secure way to do this without any other tricks is described already, an implementation can be found here.

// assume argv is there and a char** array

int        nAllocCharCount = 1024;
int        nBufSize = argv[0][0] ? strlen((char *) argv[0]) : nAllocCharCount;
TCHAR *    pszCompleteFilePath = new TCHAR[nBufSize+1];

nBufSize = GetModuleFileName(NULL, (TCHAR*)pszCompleteFilePath, nBufSize);
if (!argv[0][0])
{
    // resize memory until enough is available
    while (GetLastError() == ERROR_INSUFFICIENT_BUFFER)
    {
        delete[] pszCompleteFilePath;
        nBufSize += nAllocCharCount;
        pszCompleteFilePath = new TCHAR[nBufSize+1];
        nBufSize = GetModuleFileName(NULL, (TCHAR*)pszCompleteFilePath, nBufSize);
    }

    TCHAR * pTmp = pszCompleteFilePath;
    pszCompleteFilePath = new TCHAR[nBufSize+1];
    memcpy_s((void*)pszCompleteFilePath, nBufSize*sizeof(TCHAR), pTmp, nBufSize*sizeof(TCHAR));

    delete[] pTmp;
    pTmp = NULL;
}
pszCompleteFilePath[nBufSize] = '\0';

// do work here
// variable 'pszCompleteFilePath' contains always the complete path now

// cleanup
delete[] pszCompleteFilePath;
pszCompleteFilePath = NULL;

I had no case where argv didn't contain the file path (Win32 and Win32-console application), yet. But just in case there is a fallback to a solution that has been described above. Seems a bit ugly to me, but still gets the job done.