Ruslan-B · kastwey · Jun 24, 2026 · Jun 24, 2026
diff --git a/FFmpeg.AutoGen.Abstractions.Test/ConstCharPtrMarshalerTest.cs b/FFmpeg.AutoGen.Abstractions.Test/ConstCharPtrMarshalerTest.cs
@@ -0,0 +1,99 @@
+using System.Runtime.InteropServices;
+using System.Text;
+
+namespace FFmpeg.AutoGen.Abstractions.Test
+{
+    /// <summary>
+    /// Tests for <see cref="ConstCharPtrMarshaler"/>, the custom marshaler used for
+    /// <c>const char*</c> return values coming from FFmpeg. It must decode the native
+    /// UTF-8 string without ever freeing the (borrowed/static) native memory.
+    ///
+    /// This project multi-targets net9.0 and net48 so both compile-time branches of the
+    /// marshaler are covered: net9.0 references the netstandard2.1 build
+    /// (Marshal.PtrToStringUTF8) and net48 references the netstandard2.0 build
+    /// (the manual UTF-8 decode fallback).
+    /// </summary>
+    [TestClass]
+    public class ConstCharPtrMarshalerTest
+    {
+        private readonly List<IntPtr> _allocations = new();
+
+        private static ICustomMarshaler Marshaler => ConstCharPtrMarshaler.GetInstance(null!);
+
+        [TestCleanup]
+        public void Cleanup()
+        {
+            foreach (var ptr in _allocations)
+                Marshal.FreeHGlobal(ptr);
+            _allocations.Clear();
+        }
+
+        /// <summary>Allocates a NUL-terminated UTF-8 C string in unmanaged memory.</summary>
+        private IntPtr AllocUtf8(string value)
+        {
+            var bytes = Encoding.UTF8.GetBytes(value);
+            var ptr = Marshal.AllocHGlobal(bytes.Length + 1);
+            _allocations.Add(ptr);
+            Marshal.Copy(bytes, 0, ptr, bytes.Length);
+            Marshal.WriteByte(ptr, bytes.Length, 0); // NUL terminator
+            return ptr;
+        }
+
+        [TestMethod]
+        public void GetInstance_ReturnsNonNullMarshaler()
+        {
+            Assert.IsNotNull(Marshaler);
+        }
+
+        [TestMethod]
+        public void NullPointer_ReturnsNull()
+        {
+            var result = Marshaler.MarshalNativeToManaged(IntPtr.Zero);
+            Assert.IsNull(result);
+        }
+
+        [TestMethod]
+        public void EmptyString_ReturnsEmptyString()
+        {
+            var ptr = AllocUtf8(string.Empty);
+            var result = Marshaler.MarshalNativeToManaged(ptr);
+            Assert.AreEqual(string.Empty, result);
+        }
+
+        [TestMethod]
+        public void AsciiString_RoundTrips()
+        {
+            const string expected = "libavcodec 60.31.102";
+            var ptr = AllocUtf8(expected);
+            var result = Marshaler.MarshalNativeToManaged(ptr);
+            Assert.AreEqual(expected, result);
+        }
+
+        [TestMethod]
+        public void Utf8MultiByteString_RoundTrips()
+        {
+            // Mix of 2-byte (é), 3-byte (日本語) and 4-byte (emoji) UTF-8 sequences.
+            const string expected = "café — 日本語 🎬";
+            var ptr = AllocUtf8(expected);
+            var result = Marshaler.MarshalNativeToManaged(ptr);
+            Assert.AreEqual(expected, result);
+        }
+
+        [TestMethod]
+        public void CleanUpNativeData_DoesNotInvalidatePointer()
+        {
+            const string expected = "av_version_info";
+            var ptr = AllocUtf8(expected);
+
+            // CleanUpNativeData must be a no-op for borrowed FFmpeg memory: it must not
+            // free or mutate the buffer. So decoding again after it returns the same
+            // value from the still-valid pointer.
+            var first = Marshaler.MarshalNativeToManaged(ptr);
+            Marshaler.CleanUpNativeData(ptr); // documented no-op for borrowed memory
+            var second = Marshaler.MarshalNativeToManaged(ptr);
+
+            Assert.AreEqual(expected, first);
+            Assert.AreEqual(expected, second);
+        }
+    }
+}
diff --git a/FFmpeg.AutoGen.Abstractions.Test/FFmpeg.AutoGen.Abstractions.Test.csproj b/FFmpeg.AutoGen.Abstractions.Test/FFmpeg.AutoGen.Abstractions.Test.csproj
@@ -0,0 +1,37 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <!--
+      Multi-targeted on purpose: net9.0 resolves the netstandard2.1 build of
+      FFmpeg.AutoGen.Abstractions (Marshal.PtrToStringUTF8 path) while net48
+      resolves the netstandard2.0 build (manual UTF-8 decode path). This way the
+      ConstCharPtrMarshaler tests exercise both compile-time branches.
+
+      net48 is only added on Windows: the .NET Framework test host cannot run on
+      Linux/macOS, so on non-Windows hosts we build and run net9.0 only (which
+      still covers the netstandard2.1 branch). The netstandard2.0 branch is
+      validated on Windows.
+    -->
+    <TargetFrameworks>net9.0</TargetFrameworks>
+    <TargetFrameworks Condition="'$(OS)' == 'Windows_NT'">net9.0;net48</TargetFrameworks>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <LangVersion>preview</LangVersion>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="MSTest.TestAdapter" />
+    <PackageReference Include="MSTest.TestFramework" />
+    <PackageReference Include="coverlet.collector">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\FFmpeg.AutoGen.Abstractions\FFmpeg.AutoGen.Abstractions.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/FFmpeg.AutoGen.Abstractions.Test/Usings.cs b/FFmpeg.AutoGen.Abstractions.Test/Usings.cs
@@ -0,0 +1 @@
+global using Microsoft.VisualStudio.TestTools.UnitTesting;
diff --git a/FFmpeg.AutoGen.Abstractions/ConstCharPtrMarshaler.cs b/FFmpeg.AutoGen.Abstractions/ConstCharPtrMarshaler.cs
@@ -6,7 +6,27 @@ namespace FFmpeg.AutoGen.Abstractions;
 public class ConstCharPtrMarshaler : ICustomMarshaler
 {
     private static readonly ConstCharPtrMarshaler Instance = new();
-    public object MarshalNativeToManaged(IntPtr pNativeData) => Marshal.PtrToStringAnsi(pNativeData);
+
+    public object MarshalNativeToManaged(IntPtr pNativeData)
+    {
+#if NETSTANDARD2_1_OR_GREATER
+        return Marshal.PtrToStringUTF8(pNativeData);
+#else
+        if (pNativeData == IntPtr.Zero)
+            return null;
+
+        var length = 0;
+        while (Marshal.ReadByte(pNativeData, length) != 0)
+            length++;
+
+        if (length == 0)
+            return string.Empty;
+
+        var buffer = new byte[length];
+        Marshal.Copy(pNativeData, buffer, 0, length);
+        return System.Text.Encoding.UTF8.GetString(buffer);
+#endif
+    }
 
     public IntPtr MarshalManagedToNative(object managedObj) => IntPtr.Zero;
 

diff --git a/FFmpeg.AutoGen.CppSharpUnsafeGenerator/Processing/FunctionProcessor.cs b/FFmpeg.AutoGen.CppSharpUnsafeGenerator/Processing/FunctionProcessor.cs
@@ -12,8 +12,6 @@ internal class FunctionProcessor
 {
     private const string ReturnMarshalAsConstCharPtr = "[return: MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(ConstCharPtrMarshaler))]";
 
-    private const string ReturnMarshalAsLPUTF8Str = "[return: MarshalAs(UnmanagedType.LPUTF8Str)]";
-
     private const string MarshalAsUTF8Macros =
         "    \r\n" +
         "    #if NETSTANDARD2_1_OR_GREATER\r\n" +
@@ -169,10 +167,17 @@ private TypeDefinition GetReturnType(Type type, string name)
         {
             return builtinType.Type switch
             {
+                // For const char* return values we must always use ConstCharPtrMarshaler and never
+                // [return: MarshalAs(UnmanagedType.LPUTF8Str)]. LPUTF8Str on a return value makes the
+                // CLR call CoTaskMemFree on the pointer after copying the string, but FFmpeg returns
+                // pointers to static/borrowed memory (e.g. av_version_info, avcodec_get_name,
+                // av_get_pix_fmt_name, AVClass names). Freeing that memory corrupts the native heap
+                // and leads to AccessViolationException. The NoCustomStringMarshal option only makes
+                // sense for parameters, where the CLR owns the marshalled buffer.
                 PrimitiveType.Char => new TypeDefinition
                 {
                     Name = "string",
-                    Attributes = new[] { _context.NoCustomStringMarshal ? ReturnMarshalAsLPUTF8Str : ReturnMarshalAsConstCharPtr }
+                    Attributes = new[] { ReturnMarshalAsConstCharPtr }
                 },
                 PrimitiveType.Void => new TypeDefinition
                 {

diff --git a/FFmpeg.AutoGen.sln b/FFmpeg.AutoGen.sln
@@ -38,48 +38,138 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "3. Legacy", "3. Legacy", "{
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "2. Abstractions", "2. Abstractions", "{1B9CC2D2-096F-4D35-8940-DA6344450430}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "FFmpeg.AutoGen.Abstractions.Test", "FFmpeg.AutoGen.Abstractions.Test\FFmpeg.AutoGen.Abstractions.Test.csproj", "{931DC899-6676-4884-A46C-060A4786DE78}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
+		Debug|x64 = Debug|x64
+		Debug|x86 = Debug|x86
 		Release|Any CPU = Release|Any CPU
+		Release|x64 = Release|x64
+		Release|x86 = Release|x86
 	EndGlobalSection
 	GlobalSection(ProjectConfigurationPlatforms) = postSolution
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+		{931DC899-6676-4884-A46C-060A4786DE78}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{931DC899-6676-4884-A46C-060A4786DE78}.Release|Any CPU.Build.0 = Release|Any CPU
+		{931DC899-6676-4884-A46C-060A4786DE78}.Release|x64.ActiveCfg = Release|Any CPU
+		{931DC899-6676-4884-A46C-060A4786DE78}.Release|x64.Build.0 = Release|Any CPU
+		{931DC899-6676-4884-A46C-060A4786DE78}.Release|x86.ActiveCfg = Release|Any CPU
+		{931DC899-6676-4884-A46C-060A4786DE78}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -92,6 +182,7 @@ Global
 		{07F3A6D4-5599-4D77-9197-A666E8FC8EEC} = {E9CC1A7F-ADD1-486B-BEB3-B6E388B35731}
 		{F969EEDD-27A8-434B-A438-39C48BBAEFE6} = {1B9CC2D2-096F-4D35-8940-DA6344450430}
 		{1294EC3A-9667-40A2-8189-C9EC12ECFDD6} = {1B9CC2D2-096F-4D35-8940-DA6344450430}
+		{931DC899-6676-4884-A46C-060A4786DE78} = {1B9CC2D2-096F-4D35-8940-DA6344450430}
 		{ED2E790B-9233-40BC-BA66-E3291AAA9878} = {F969EEDD-27A8-434B-A438-39C48BBAEFE6}
 		{C272CD0A-E13B-4E80-8CA2-2BEC8396D8D0} = {F969EEDD-27A8-434B-A438-39C48BBAEFE6}
 		{028113A5-39CE-40A2-9118-56E99F30B4F8} = {F969EEDD-27A8-434B-A438-39C48BBAEFE6}

diff --git a/FFmpeg.AutoGen/ConstCharPtrMarshaler.cs b/FFmpeg.AutoGen/ConstCharPtrMarshaler.cs
@@ -6,7 +6,27 @@ namespace FFmpeg.AutoGen;
 public class ConstCharPtrMarshaler : ICustomMarshaler
 {
     private static readonly ConstCharPtrMarshaler Instance = new();
-    public object MarshalNativeToManaged(IntPtr pNativeData) => Marshal.PtrToStringAnsi(pNativeData);
+
+    public object MarshalNativeToManaged(IntPtr pNativeData)
+    {
+#if NETSTANDARD2_1_OR_GREATER
+        return Marshal.PtrToStringUTF8(pNativeData);
+#else
+        if (pNativeData == IntPtr.Zero)
+            return null;
+
+        var length = 0;
+        while (Marshal.ReadByte(pNativeData, length) != 0)
+            length++;
+
+        if (length == 0)
+            return string.Empty;
+
+        var buffer = new byte[length];
+        Marshal.Copy(pNativeData, buffer, 0, length);
+        return System.Text.Encoding.UTF8.GetString(buffer);
+#endif
+    }
 
     public IntPtr MarshalManagedToNative(object managedObj) => IntPtr.Zero;
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		global using Microsoft.VisualStudio.TestTools.UnitTesting;