After searching the web for a solution and not finding a library, I’m instead trying to write a systemd journal reader using csharp and not having much luck.

There’s an example written in c++ at the bottom of the following link:

https://www.freedesktop.org/software/systemd/man/latest/sd_journal_next.html

This is the code I have so far, which, when I run it, I get

$ ./LogPublisher 
success: 0
success: -10
success: -10
Success: -10 Message: 

As far as I can tell, -10 refers to No child processes as taken from https://en.wikipedia.org/wiki/Errno.h

using System.Runtime.InteropServices;
using System.Text;

class Program
{
    enum Flags
    {
        SD_JOURNAL_LOCAL_ONLY = 1 << 0,                 // 0000 0001 = 1
        SD_JOURNAL_RUNTIME_ONLY = 1 << 1,               // 0000 0010 = 2
        SD_JOURNAL_SYSTEM = 1 << 2,                     // 0000 0100 = 4
        SD_JOURNAL_CURRENT_USER = 1 << 3,               // 0000 1000 = 8
        SD_JOURNAL_OS_ROOT = 1 << 4,                    // 0001 0000 = 16
        SD_JOURNAL_ALL_NAMESPACES = 1 << 5,             // (32) Show all namespaces, not just the default or specified one */
        SD_JOURNAL_INCLUDE_DEFAULT_NAMESPACE = 1 << 6,  // (64) Show default namespace in addition to specified one */
        SD_JOURNAL_TAKE_DIRECTORY_FD = 1 << 7,          // (128) sd_journal_open_directory_fd() will take ownership of the provided file descriptor. */
        SD_JOURNAL_ASSUME_IMMUTABLE = 1 << 8,           // (256) Assume the opened journal files are immutable. Journal entries added later may be ignored. */
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
    public struct Journal
    {
        public int toplevel_fd;
        public string path;
        public string prefix;
    }

    internal static class Systemd
    {
        [DllImport("libsystemd.so.0", CharSet = CharSet.Ansi)]
        internal static extern int sd_journal_open(ref Journal sd_journal, int flags);

        [DllImport("libsystemd.so.0", CharSet = CharSet.Ansi)]
        internal static extern int sd_journal_seek_head(ref Journal sd_journal);

        [DllImport("libsystemd.so.0", CharSet = CharSet.Ansi)]
        internal static extern int sd_journal_next(ref Journal sd_journal);

        [DllImport("libsystemd.so.0", CharSet = CharSet.Ansi)]
        internal static extern int sd_journal_get_data(ref Journal sd_journal, string field, [Out] StringBuilder data, [Out] ulong length);

        [DllImport("libsystemd.so.0", CharSet = CharSet.Ansi)]
        internal static extern int sd_journal_seek_head(IntPtr sd_journal);

        [DllImport("libsystemd.so.0", CharSet = CharSet.Ansi)]
        internal static extern void sd_journal_close(ref Journal sd_journal);
    }

    static void Main()
    {
        int success;
        Journal j = new();

        success = Systemd.sd_journal_open(ref j, (int)Flags.SD_JOURNAL_LOCAL_ONLY);
        Console.WriteLine($"Success: {success}");

        success = Systemd.sd_journal_seek_head(ref j);
        Console.WriteLine($"Success: {success}");

        // success = Systemd.sd_journal_next(ref j);
        // Console.WriteLine($"Success: {success}");

        ulong length = 0;
        StringBuilder data = new(1024 * 768 * 4);

        success = Systemd.sd_journal_get_data(ref j, "MESSAGE", data, length);
        Console.WriteLine($"Success: {success} Message: {data}");

        Systemd.sd_journal_close(ref j);
    }
}

I’ve been trying to reference the source code for journal to replicate but just cant work it out.

https://github.com/systemd/systemd/blob/main/src/systemd/sd-journal.h