Asked By: Anonymous
Does anyone know how to cast a method in C# to a void* member?
In the example below, sigaction.sa_handler is a void* member that specifies a callback function.
Environment:
- VisualStudio Professional 2019 16.9.4
- dotnet core SDK 3.1
- Ubuntu on WSL
- Tmds.Linux 0.5.0
sigaction sigact = new sigaction();
//Error CS0428 Cannot convert method group 'catcher' to non-delegate type 'void*'.Did you intend to invoke the method?
sigact.sa_handler = catcher;
//Error CS0030 Cannot convert type 'method' to 'void*'
sigact.sa_handler = (void *)catcher;
//Error CS0029 Cannot implicitly convert type 'SignalCallback' to 'void*'
SignalCallback callback = new SignalCallback(catcher);
sigact.sa_handler = callback;
//Error CS0030 Cannot convert type 'SignalCallback' to 'void*'
sigact.sa_handler = (void*)callback;
Structure of sigaction (from Tmds.Linux)
public struct sigaction
{
public sigset_t sa_mask;
public void* sa_handler { get; set; }
public void* sa_sigaction { get; set; }
public int sa_flags { readonly get; set; }
public void* sa_restorer { readonly get; set; }
}
Full source code:
using System;
using static Tmds.Linux.LibC;
using Tmds.Linux;
using System.Runtime.InteropServices;
delegate void SignalCallback(int sig);
namespace example
{
class Program
{
static void catcher(int sig)
{
Console.Write($"Signal catcher called for signal {sig}");
}
static void timestamp(string str)
{
Console.Write($"The time {str} is {DateTime.Now}");
}
static unsafe int Main(string[] args)
{
int result = 0;
sigaction sigact = new sigaction();
sigset_t waitset = new sigset_t();
siginfo_t info = new siginfo_t();
sigset_t* ptr = &sigact.sa_mask;
sigemptyset(&sigact.sa_mask);
sigact.sa_flags = 0;
//Error CS0428 Cannot convert method group 'catcher' to non-delegate type 'void*'.Did you intend to invoke the method?
sigact.sa_handler = catcher;
//Error CS0029 Cannot implicitly convert type 'SignalCallback' to 'void*'
SignalCallback callback = new SignalCallback(catcher);
sigact.sa_handler = callback;
sigaction(SIGALRM, &sigact, null);
sigemptyset(&waitset);
sigaddset(&waitset, SIGALRM);
sigprocmask(SIG_BLOCK, &waitset, null);
alarm(10);
timestamp("before sigwaitinfo()");
result = sigwaitinfo(&waitset, &info);
if (result == SIGALRM)
Console.WriteLine($"sigwaitinfo() returned for signal {info.si_signo}");
else
{
Console.WriteLine($"sigwait() returned code {result}");
Console.WriteLine($"sigwait() returned error number {errno}");
Console.WriteLine("sigwait() function failed");
}
timestamp("after sigwaitinfo()");
return result;
}
}
}
Solution
Answered By: Anonymous
To get a void*
pointer from the delegate, you need Marshal.GetFunctionPointerForDelegate
. You must make sure the delegate is not garbage-collected before its final use. You can use GC.KeepAlive
for this.
SignalCallback callback = new SignalCallback(catcher);
sigact.sa_handler = Marshal.GetFunctionPointerForDelegate(callback);
// do the rest of your stuff
// after it's last usage by unmanaged code
GC.KeepAlive(callback);
However:
You should really be relying much more on P/Invoke to do any conversion. It should in most cases be able to sort out the void*
conversions, all you need is GC.KeepAlive
.
Your current code is hugely problematic as you are casting managed pointers to void*
all over.
For example, instead of this call, which is doing a conversion from &
managed pointer to void*
sigemptyset(&waitset);
Instead you would use
sigemptyset(ref waitset);
Your structs ideally shouldn’t use properties, because it makes it harder to define attributes on the fields. I note that sigaction
doesn’t seem to be in the correct order, from what I can tell from a quick Google.
You should define sigaction
something like this
public struct sigaction
{
public SignalCallback sa_handler;
public SignalAction sa_sigaction;
public sigset_t sa_mask;
public int sa_flags;
public SignalRestorer sa_restorer;
}