Marshal a va_list

Question

I have the following code:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)] public delegate void PanicFuncDelegate(string str, IntPtr args); private void PanicFunc(string str, IntPtr args) { LogFunc("PANIC", str, args); } public void LogFunc(string severity, string str, IntPtr args) { vprintf($"[{severity}] "+ str,args); } [DllImport("libc.so.6")] private static extern int vprintf(string format, IntPtr args); 

Sends messages to the console that are correctly formatted. I want to get values ​​from args to use them in my own log.

If I try to get the value of each pointer from an array in args (as suggested here: Marshal va_list in a C # delegate ), I get a segmentation error.

Any suggestions?

Answer 1

Just think about how the C program gets the variables from va_list, and there is a solution:

 using System; using System.Collections.Generic; using System.Linq; using System.Runtime.InteropServices; using System.Text; namespace VaTest { class Program { static void Main(string[] args) { MarshalVaArgs(vaList => vprintf("%c%d%s", vaList), false, 'a', 123, "bc"); } [DllImport("msvcrt")] //windows //[DllImport("c")] //linux private static extern int vprintf(string format, IntPtr vaList); private static int IntSizeOf(Type t) { return (Marshal.SizeOf(t) + IntPtr.Size - 1) & ~(IntPtr.Size - 1); } public static void MarshalVaArgs(Action<IntPtr> action, bool? isUnicode, params object[] args) { var sizes = new int[args.Length]; for (var i = 0; i < args.Length; i++) { sizes[i] = args[i] is string ? IntPtr.Size : IntSizeOf(args[i].GetType()); } var allocs = new List<IntPtr>(); var offset = 0; var result = Marshal.AllocHGlobal(sizes.Sum()); allocs.Add(result); for (var i = 0; i < args.Length; i++) { if (args[i] is string) { var s = (string)args[i]; var data = default(IntPtr); if (isUnicode.HasValue) { if (isUnicode.Value) { data = Marshal.StringToHGlobalUni(s); } else { data = Marshal.StringToHGlobalAnsi(s); } } else { data = Marshal.StringToHGlobalAuto(s); } allocs.Add(data); Marshal.WriteIntPtr(result, offset, data); offset += sizes[i]; } else { Marshal.StructureToPtr(args[i], result + offset, false); offset += sizes[i]; } } action(result); foreach (var ptr in allocs) { Marshal.FreeHGlobal(ptr); } } } } 

Code written and tested with .NET Core 3.0 preview 5 , compatible with .NET Framework 4.0 and C# 3.0