// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.Marshalling;

internal static partial class Interop
{
    internal static partial class Sys
    {
        [LibraryImport(Libraries.SystemNative, EntryPoint = "SystemNative_GetHostName", SetLastError = true)]
        private static unsafe partial int GetHostName(byte* name, int nameLength);

        internal static unsafe string GetHostName()
        {
            if (OperatingSystem.IsWasi())
            {
                return "localhost";
            }

            const int HOST_NAME_MAX = 255;
            const int ArrLength = HOST_NAME_MAX + 1;

            byte* name = stackalloc byte[ArrLength];
            int err = GetHostName(name, ArrLength);
            if (err != 0)
            {
                // This should never happen.  According to the man page,
                // the only possible errno for gethostname is ENAMETOOLONG,
                // which should only happen if the buffer we supply isn't big
                // enough, and we're using a buffer size that the man page
                // says is the max for POSIX (and larger than the max for Linux).
                Debug.Fail($"GetHostName failed with error {err}");
                throw new InvalidOperationException($"{nameof(GetHostName)}: {err}");
            }

            // If the hostname is truncated, it is unspecified whether the returned buffer includes a terminating null byte.
            name[ArrLength - 1] = 0;

            return Utf8StringMarshaller.ConvertToManaged(name)!;
        }
    }
}