Workaround for IShellLink::GetPath()

• Download ReadLnkFile - 2.35 KB

Introduction

IShellLink::GetPath() may fail and crash your application. This is a workaround to help you out of this.

Using the code

//*****************************************************************************
// File ..................: ReadLnkFile.cpp
// Description ...........: Decoder for .lnk files on WINDOWS
// Author ................: Peter Thoemmes (http://www.notes-about-cpp.com/)
//-----------------------------------------------------------------------------
// #                  !!! Disclaimer !!!                                      #
// # This source code is provided "AS-IS" basis, without any warranties or    #
// # representations express, implied or statutory; including, without        #
// # limitation, warranties of quality, performance, non-infringement,        #
// # merchantability or fitness for a particular purpose. Peter Thoemmes      #
// # does not warrant that this source code will meet your needs or be free   #
// # from errors.                                                             #
//-----------------------------------------------------------------------------
// Many thanks to Jesse Hager (jessehager@iname.com) for reverse engineering
// the shortcut file format and publishing the document 'The Windows Shortcut
// File Format'.
//-----------------------------------------------------------------------------
// Copyright (c) 2008 Peter Thoemmes, Weinbergstr. 3a, D-54441 Ockfen/Germany
//*****************************************************************************

#include <string>
#include <vector>
using namespace std;

//-----------------------------------------------------------------------------
// Get target file (full local path) from a link file (.lnk):
//-----------------------------------------------------------------------------

string ReadLnkFile(const string& strFullLinkFileName)
{
    //-------------------------------------------------------------------------
    // How to read the target's path from a .lnk-file:
    //-------------------------------------------------------------------------
    // Problem:
    //
    //    The COM interface to shell32.dll IShellLink::GetPath() fails!
    //
    // Solution:
    //
    //   We need to parse the file manually. The path can be found like shown
    //   here, if the shell item id list is present. In case it is not present
    //   we have to assume A = -6, but not to parse/decode byte 76 and 77.
    //
    //  +---------------------+-----------------------------------------------+
    //  | Index               | Description                                   |
    //  +---------------------+-----------------------------------------------+
    //  |                   0 | 'L' (magic value)                             |
    //  +---------------------+-----------------------------------------------+
    //  |                 ... | ...                                           |
    //  +---------------------+-----------------------------------------------+
    //  |                  76 | A_0                                           |
    //  +---------------------+-----------------------------------------------+
    //  |                  77 | A_1 (16 bit) [w/o shell item id list: A = -6] |
    //  +---------------------+-----------------------------------------------+
    //  |                 ... | ...                                           |
    //  +---------------------+-----------------------------------------------+
    //  |         78 + 16 + A | B_0                                           |
    //  +---------------------+-----------------------------------------------+
    //  |     78 + 16 + A + 1 | B_1                                           |
    //  +---------------------+-----------------------------------------------+
    //  |     78 + 16 + A + 2 | B_2                                           |
    //  +---------------------+-----------------------------------------------+
    //  |     78 + 16 + A + 3 | B_3 (32 bit)                                  |
    //  +---------------------+-----------------------------------------------+
    //  |                 ... | ...                                           |
    //  +---------------------+-----------------------------------------------+
    //  |          78 + A + B | PATH_STR_0                                    |
    //  +---------------------+-----------------------------------------------+
    //  |      78 + A + B + 1 | PATH_STR_1                                    |
    //  +---------------------+-----------------------------------------------+
    //  |      78 + A + B + 2 | PATH_STR_2                                    |
    //  +---------------------+-----------------------------------------------+
    //  |                 ... | ...                                           |
    //  +---------------------+-----------------------------------------------+
    //  |                 ... | 0x00                                          |
    //  +---------------------+-----------------------------------------------+
    //-------------------------------------------------------------------------

    //-------------------------------------------------------------------------
    // Get the .lnk-file content:
    //-------------------------------------------------------------------------

    FILE* pFile = fopen(strFullLinkFileName.c_str(),"rb");
    if(!pFile)
        return string("");
    vector<unsigned char> vectBuffer;
    unsigned char byte = '?';
    while((byte = (unsigned char) fgetc(pFile)) != (unsigned char) EOF)
        vectBuffer.push_back(byte);
    fclose(pFile);

    //-------------------------------------------------------------------------
    // Check the magic value (first byte) and the GUID (16 byte from 5th byte):
    //-------------------------------------------------------------------------
    // The GUID is telling the version of the .lnk-file format. We expect the
    // following GUID (HEX): 01 14 02 00 00 00 00 00 C0 00 00 00 00 00 00 46.
    //-------------------------------------------------------------------------

    if(vectBuffer.size() < 20)
        return string("");
    if(vectBuffer[0] != (unsigned char) 'L') //test the magic value
        return string("");
    if((vectBuffer[4] != 0x01) || //test the GUID
       (vectBuffer[5] != 0x14) ||
       (vectBuffer[6] != 0x02) ||
       (vectBuffer[7] != 0x00) ||
       (vectBuffer[8] != 0x00) ||
       (vectBuffer[9] != 0x00) ||
       (vectBuffer[10] != 0x00) ||
       (vectBuffer[11] != 0x00) ||
       (vectBuffer[12] != 0xC0) ||
       (vectBuffer[13] != 0x00) ||
       (vectBuffer[14] != 0x00) ||
       (vectBuffer[15] != 0x00) ||
       (vectBuffer[16] != 0x00) ||
       (vectBuffer[17] != 0x00) ||
       (vectBuffer[18] != 0x00) ||
       (vectBuffer[19] != 0x46))
    {
        return string("");
    }

    //-------------------------------------------------------------------------
    // Get the flags (4 byte from 21st byte):
    //-------------------------------------------------------------------------
    // Check if it points to a file or directory!
    //-------------------------------------------------------------------------
    // Flags (4 byte little endian):
    //        Bit 0 -> has shell item id list
    //        Bit 1 -> points to file or directory
    //        Bit 2 -> has description
    //        Bit 3 -> has relative path
    //        Bit 4 -> has working directory
    //        Bit 5 -> has commandline arguments
    //        Bit 6 -> has custom icon
    //-------------------------------------------------------------------------

    unsigned int i = 20;
    if(vectBuffer.size() < (i + 4))
        return string("");
    unsigned int dwFlags = (unsigned int) vectBuffer[i]; //little endian format
    dwFlags |= (((unsigned int) vectBuffer[++i]) << 8);
    dwFlags |= (((unsigned int) vectBuffer[++i]) << 16);
    dwFlags |= (((unsigned int) vectBuffer[++i]) << 24);

    bool bHasShellItemIdList = (dwFlags & 0x00000001) ? true : false;
    bool bPointsToFileOrDir = (dwFlags & 0x00000002) ? true : false;
    
    if(!bPointsToFileOrDir)
        return string("");

    //-------------------------------------------------------------------------
    // Shell item id list (starts at 76 with 2 byte length -> so we can skip):
    //-------------------------------------------------------------------------

    int32 A = -6;
    if(bHasShellItemIdList)
    {
        i = 76;
        if(vectBuffer.size() < (i + 2))
            return string("");
        A = (unsigned char) vectBuffer[i]; //little endian format
        A |= (((unsigned char) vectBuffer[++i]) << 8);
    }

    //-------------------------------------------------------------------------
    // File location info:
    //-------------------------------------------------------------------------
    // Follows the shell item id list and starts with 4 byte structure length,
    // followed by 4 byte offset for skipping.
    //-------------------------------------------------------------------------

    i = 78 + 4 + A;
    if(vectBuffer.size() < (i + 4))
        return string("");
    unsigned int B = (unsigned int) vectBuffer[i]; //little endian format
    B |= (((unsigned int) vectBuffer[++i]) << 8);
    B |= (((unsigned int) vectBuffer[++i]) << 16);
    B |= (((unsigned int) vectBuffer[++i]) << 24);

    //-------------------------------------------------------------------------
    // Local volume table:
    //-------------------------------------------------------------------------
    // Follows the file location info and starts with 4 byte table length for
    // skipping the actual table and moving to the local path string.
    //-------------------------------------------------------------------------

    i = 78 + A + B;
    if(vectBuffer.size() < (i + 4))
        return string("");
    unsigned int C = (unsigned int) vectBuffer[i]; //little endian format
    C |= (((unsigned int) vectBuffer[++i]) << 8);
    C |= (((unsigned int) vectBuffer[++i]) << 16);
    C |= (((unsigned int) vectBuffer[++i]) << 24);

    //-------------------------------------------------------------------------
    // Local path string (ending with 0x00):
    //-------------------------------------------------------------------------

    i = 78 + A + B + C;
    if(vectBuffer.size() < (i + 1))
        return string("");

    string strLinkedTarget = "";
    for(;i < vectBuffer.size();++i)
    {
        strLinkedTarget.append(1,(char) vectBuffer[i]);
        if(!vectBuffer[i])
            break;
    }

    //Return if empty:
    if(strLinkedTarget.empty())
        return string("");

    //-------------------------------------------------------------------------
    // Convert the target path into the long format (format without ~):
    //-------------------------------------------------------------------------
    // GetLongPathNameA() fails it the target file doesn't exist!
    //-------------------------------------------------------------------------

    char* szLinkedTargetLongFormat = new char[MAX_PATH + 1];
    if(!szLinkedTargetLongFormat)
        return string("");
    if(!::GetLongPathNameA(
                strLinkedTarget.c_str(),
                szLinkedTargetLongFormat,
                MAX_PATH))
    {
        return strLinkedTarget; //file doesn't exist
    }
    string strLinkedTargetLongFormat(szLinkedTargetLongFormat);
    delete[] szLinkedTargetLongFormat;

    return strLinkedTargetLongFormat;
}

Simply call the function ReadLnkFile() to get your target file from the .lnk file:

string strFullTargetPath = ReadLnkFile("C:\\MyShortcut.lnk");

Points of Interest

Many thanks to Jesse Hager (jessehager@iname.com) for reverse engineering the shortcut file format and publishing the document 'The Windows Shortcut File Format'. Without this I would still stuck with IShellLink::GetPath() crashing my application.

History

2008-02-29, Peter Thoemmes: first release.

2008-03-01, Peter Thoemmes: description updated.