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POmemory corruption passing string from c++ dll to mono (C#)
 

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  1. POmemory corruption passing string from c++ dll to mono (C#)
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    <p>I am writing a C# GUI for a C++ application of mine. I retrieve the details of a plugin by enumerating the available DLLs, dynamically loading them one by one, and calling a function called "getdescstring" which returns the plugin description concatenated into a string.</p> <p>This works flawlessly on Windows. However, when I try to run it on a Linux box (I have tried it on a Linux Mint 64-bit (Mono 3.0.6), and a Xubuntu 32-bit (Mono 2.10.8) virtual machine, g++-4.7 on both) most of the returned strings are corrupted like this:</p> <pre><code>"grown|Barabasi-Albert grown network.|1,000|1.0|1.0|1.0||n|Number of nodes|numeric|k|Degree of neA\0\0…" </code></pre> <p>If I call the function from a standalone C++ test program, it works. No memory leak or corruption with Valgrind. When I try to run the whole program with Mono through Valgrind, it crashes on initialization, so I can't report on that.</p> <p>So I suspect some memory corruption somewhere between Mono and the DLL, but I cannot identify the location.</p> <p><strong>Update:</strong> My intuition is that calling conventions get mixed up somehow. Since 64-bit programs have unique calling conventions, maybe Mono uses ms_abi, which may conflict with sysv_abi in Unix. However, there are no calling convention flags for these, so even if this it the problem, I cannot fix it. I can set the CC to stdcall in Mono, but g++ ignores any CC attibute on 64-bit CPUs. <strong>No. I tried setting the conventions to stdcall at both ends in a 32-bit virtual machine, but no change</strong></p> <p>Here is the code called in the DLL (the function "description" returns a std::vector of strings, but passing a std::vector directly to C# seemed quite nasty)</p> <pre><code>extern "C" const char* getdescstring() { vector&lt;vector&lt;string&gt; &gt; descvec=description(); string descstr; for(unsigned i=0;i&lt;descvec.size();i++) { for(unsigned j=0;j&lt;descvec[i].size();j++) { descstr+=descvec[i][j]; descstr+="|"; } descstr+="|"; } return descstr.c_str(); } </code></pre> <p>And this is the receiving function which transforms the given string back:</p> <pre><code>public static List&lt;List&lt;string&gt;&gt; GetPluginDesc(string plugin) { List&lt;List&lt;string&gt;&gt; des = new List&lt;List&lt;string&gt;&gt;(); DllLoadUtils dl; if (OS == platform.Windows) dl = new DllLoadUtilsWindows(); else dl = new DllLoadUtilsLinux(); IntPtr dllh = dl.LoadLibrary(plugin); if (dllh == IntPtr.Zero) return null; IntPtr proc = dl.GetProcAddress(dllh, "getdescstring"); if (proc == IntPtr.Zero) return null; dsc descr = (dsc)Marshal.GetDelegateForFunctionPointer(proc, typeof(dsc)); String descstr = Marshal.PtrToStringAnsi(descr()); string[] descelem = descstr.Split('|'); List&lt;string&gt; ls = new List&lt;string&gt;(); foreach (string s in descelem) { //double pipe means EOL if (s == "") { des.Add(ls); ls = new List&lt;string&gt;(); } else ls.Add(s); } if (ls.Count &gt; 0) des.Add(ls); dl.FreeLibrary(dllh); return des; } </code></pre> <p>I have also tried this approach (modifying the delegate and the function in the DLL to accept a char* and a length as a parameter), but the results were even worse, no sensible data was transmitted:</p> <pre><code>dsc descr = (dsc)Marshal.GetDelegateForFunctionPointer (proc, typeof(dsc)); IntPtr sf = Marshal.AllocHGlobal(1024); descr (sf,1024); String descstr = Marshal.PtrToStringAnsi(sf); Marshal.FreeHGlobal(sf); </code></pre> <p>I would appreciate any help. Thanks in advance!</p> <p>I have also copied the DLL loaders and the delegate declaration here, if it can help:</p> <pre><code>interface DllLoadUtils { IntPtr LoadLibrary(string fileName); void FreeLibrary(IntPtr handle); IntPtr GetProcAddress(IntPtr dllHandle, string name); } public class DllLoadUtilsWindows : DllLoadUtils { void DllLoadUtils.FreeLibrary(IntPtr handle) { FreeLibrary(handle); } IntPtr DllLoadUtils.GetProcAddress(IntPtr dllHandle, string name) { return GetProcAddress(dllHandle, name); } IntPtr DllLoadUtils.LoadLibrary(string fileName) { return LoadLibrary(fileName); } [DllImport("kernel32")] private static extern IntPtr LoadLibrary(string fileName); [DllImport("kernel32.dll")] private static extern int FreeLibrary(IntPtr handle); [DllImport("kernel32.dll")] private static extern IntPtr GetProcAddress (IntPtr handle, string procedureName); } internal class DllLoadUtilsLinux : DllLoadUtils { public IntPtr LoadLibrary(string fileName) { return dlopen(fileName, RTLD_NOW); } public void FreeLibrary(IntPtr handle) { dlclose(handle); } public IntPtr GetProcAddress(IntPtr dllHandle, string name) { // clear previous errors if any dlerror(); var res = dlsym(dllHandle, name); var errPtr = dlerror(); if (errPtr != IntPtr.Zero) { MessageBox.Show("dlsym: " + Marshal.PtrToStringAnsi(errPtr)); } return res; } const int RTLD_NOW = 2; [DllImport("libdl.so")] private static extern IntPtr dlopen(String fileName, int flags); [DllImport("libdl.so")] private static extern IntPtr dlsym(IntPtr handle, String symbol); [DllImport("libdl.so")] private static extern int dlclose(IntPtr handle); [DllImport("libdl.so")] private static extern IntPtr dlerror(); } </code></pre> <p>the delegate is simply:</p> <pre><code>public delegate IntPtr dsc(); </code></pre>
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