NET C＃在SNMP项目上的编组错误

Question

I have to construct an SNMP project to communicate with various devices. Sadly I can't use any libraries other than the windows dlls for SNMP (mgmtapi.dll, snmpapi.dll, wsnmp32.dll). I have made the wrappers for those libraries and all the structures needed by the functions of thos libraries, but after the cycle:

• SnmpMgrOpen (to initialize and get the session).
• SnmpVarBindList creation (with the SnmpMgrStrToOid, SnmpUtilMemAlloc and other marshallings) to create the GET request.
• SnmpMgrRequest to make the request and populate the SnmpVarBindList structure.
• SnmpMgrClose to close the session.

When I want to see what's inside the SnmpVarBind inside the SnmpVarBindList using

Marshal.PtrToStringAnsi(varBind.value.asnValue.text.stream, (int)varBind.value.asnValue.text.length + 1)

I'm getting the error: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

The structures translated from the header snmp.h are:

[StructLayout(LayoutKind.Sequential)]
public struct AsnOctetString
{
    public uint length;
    public int dynamic;
    public IntPtr stream;
}

[StructLayout(LayoutKind.Sequential)]
public struct AsnObjectIdentifier
{
    public uint idLength;
    public IntPtr ids;
}

[StructLayout(LayoutKind.Explicit)]
public struct AsnUnion
{
    [FieldOffset(0)]
    public int number;
    [FieldOffset(0)]
    public uint unsigned32;
    [FieldOffset(0)]
    public ulong counter64;
    [FieldOffset(0)]
    public AsnOctetString text;
    [FieldOffset(0)]
    public AsnOctetString bits;
    [FieldOffset(0)]
    public AsnObjectIdentifier objectID;
    [FieldOffset(0)]
    public AsnOctetString sequence;
    [FieldOffset(0)]
    public AsnOctetString address;
    [FieldOffset(0)]
    public uint counter;
    [FieldOffset(0)]
    public uint gauge;
    [FieldOffset(0)]
    public uint ticks;
    [FieldOffset(0)]
    public AsnOctetString arbitrary;
}

[StructLayout(LayoutKind.Sequential)]
public struct AsnAny
{
    public byte asnType;
    public AsnUnion asnValue;
}

[StructLayout(LayoutKind.Sequential)]
public struct SnmpVarBind
{
    public AsnObjectIdentifier name;
    public AsnAny value;
}

[StructLayout(LayoutKind.Sequential)]
public struct SnmpVarBindList
{
    public IntPtr list;
    public uint len;
}

Any thoughts? I can put the code that you want/need.

Answer 1

I know this answer is very late, but I've recently run into marshaling problems with the SNMP API. I had assumed that the structures are packed on byte boundaries, but that is wrong. If you look at Snmp.h , you will see #include <pshpack4.h> which turns on packing on 4 byte boundaries. This means that the field offsets are different. This is how my AsnAny looks like:

[StructLayout(LayoutKind.Explicit)]
    private struct AsnAny
    {
        [FieldOffset(0)]
        public readonly byte Type;

        [FieldOffset(4)]
        public readonly Int32 Number;

        [FieldOffset(4)]
        public readonly UInt32 Unsigned32;

        [FieldOffset(4)]
        public readonly UInt64 Counter64;

        [FieldOffset(4)]
        public readonly AsnOctetString String;

        [FieldOffset(4)]
        public readonly AsnOctetString Bits;

        [FieldOffset(4)]
        public readonly AsnObjectIdentifier Object;

        [FieldOffset(4)]
        public readonly AsnOctetString Sequence;

        [FieldOffset(4)]
        public readonly AsnOctetString IPAddress;

        [FieldOffset(4)]
        public readonly UInt32 Counter32;

        [FieldOffset(4)]
        public readonly UInt32 Gauge32;

        [FieldOffset(4)]
        public readonly UInt32 Ticks;

        [FieldOffset(4)]
        public readonly AsnOctetString Arbitrary;
    }