如何在 C# 中從 libc 調用 getpwnam()？

Question

讓我們從文檔開始： https : //man7.org/linux/man-pages/man3/getpwnam.3.html

有了這個，我做了以下 C# 代碼：

using System;
using System.Runtime.InteropServices;

if (args.Length < 1) {
    Console.Error.WriteLine("Provide user name.");
    Environment.Exit(-1);
}

var name = args[0];

if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux)) {
    Syscall.getpwnam(name, out var passwd);
    Console.WriteLine($"User = {name}, UID = {passwd.Uid}, GID = {passwd.Gid}");
    passwd = GetPasswd(name);
    Console.WriteLine($"User = {name}, UID = {passwd.Uid}, GID = {passwd.Gid}");
}
else {
    Console.WriteLine("It supposed to be run on Linux.");
}

static Passwd GetPasswd(string name) {
    var bufsize = 16384;
    var buf = new byte[bufsize];
    var passwd = new Passwd();
    Syscall.getpwnam_r(name, passwd, buf, (uint)bufsize, out var result);
    return result;
}

public struct Passwd {
    public string Name;
    public string Password;
    public uint Uid;
    public uint Gid;
    public string Gecos;
    public string Directory;
    public string Shell;
}

static class Syscall {

    [DllImport("libc", SetLastError = true)]
    public static extern void getpwnam(string name, out Passwd passwd);

    [DllImport("libc", SetLastError = true)]
    public static extern void getpwnam_r(string name, Passwd passwd, byte[] buf, uint bufsize, out Passwd result);

}

它不起作用。

這是我得到的：

User = service, UID = 0, GID = 0
Segmentation fault (core dumped)

我究竟做錯了什么？

我應該如何調用它以獲得實際結構？ 我對返回的字符串不感興趣。 我只關心Uid和Gid值。

Answer 1

正如鏈接文檔所提到的 - 此函數接受一個參數 - 名稱，並返回指向帶有數據的結構的指針。 所以簽名應該是：

[DllImport("libc", SetLastError = true)]
public static extern IntPtr getpwnam(string name);

進而：

// we have pointer here
var passwdPtr = Syscall.getpwnam(name);
// don't forget to check if pointer is not IntPtr.Zero.
// interpret data at pointer as structure
var passwd = Marshal.PtrToStructure<Passwd>(passwdPtr);
Console.WriteLine($"User = {passwd.Name}, UID = {passwd.Uid}, GID = {passwd.Gid}");