[Mono-dev] Howto Marshal IntPtr to Elf32_Phdr[] ?

[Quandary]

Hi Tom,

Thank you, that works.

Still I was hoping for a less-hackish method via MarshalAs.

address + j * sizeof(mystruct_t)
sounds awfully x86 specific, maybe even x86-32 specific
But then again, so is struct Elf32_Phdr the way I did it.


As a side-note to other potential users:
If I/you apply
[MarshalAs(UnmanagedType.LPStr)]
to
public System.IntPtr dlpi_name;
and instead call it
public string dlpi_name;
then I needn't marshall it in the callback.


Kind regards

Stefan


On 03/27/2011 06:23 PM, Tom Spink wrote:
> Hi Quandry,
>
> You've almost got it - you just need to do a bit of pointer arithmetic.
>
> In your for loop, you've got this:
>
> ///
> for (j = 0; j < info.dlpi_phnum; j++)
>             Console.WriteLine("\t\t header {0}: address={1}\n", j, 22);
> ///
>
> Not sure what that 22 is... but if you do this:
>
> ///
> for (int j = 0; j < info.dlpi_phnum; j++) {
>     var ptr = new IntPtr(info.dlpi_phdr.ToInt64() + (j *
> Marshal.SizeOf(typeof(Elf32_Phdr))));
>     var hdr = (Elf32_Phdr)Marshal.PtrToStructure(ptr, typeof(Elf32_Phdr));
>
>     Console.WriteLine("\t\t header {0}: address={1}: offset={2}\n", j,
> info.dlpi_phdr, hdr.p_offset);
> }
> ///
>
> Inside your for loop, you've now got the 'hdr' variable, which is the
> Elf32_Phdr struct of the 'current element' in the array.  This works
> because info.dlpi_phdr is a pointer to the base address of the array -
> where the array is just a sequential list of Elf32_Phdr structures.
> So, by taking the base address, then adding on the index into the
> array (multiplied by the size of the array's element type), you'll get
> the base address of that particular element.  Once you've got this
> base address, all you have to do is marshal that pointer, to the right
> structure type.
>
> Also, make sure you mark your structures with the following attribute:
>
> [StructLayout(LayoutKind.Sequential)]
>
> Hope this helps!
>
> -- Tom
>
> On 27 March 2011 16:43, Quandary <quandary82 at hailmail.net> wrote:
>> Hi everyone,
>>
>> I have a problem with dl_iterate_phdr.
>> (man 3 dl_iterate_phdr)
>>
>> You find the (till now unanswered) question on stackoverflow, I'm not
>> inclined to retype everything here again, so below the link:
>> http://stackoverflow.com/questions/5447282/c-howto-marshal-intptr-to-an-array-of-struct
>>
>> In a nutshell, the problem is the struct dl_phdr_info you see below.
>>
>> It seems I need to Marshal
>> public System.IntPtr dlpi_phdr;
>> to
>> public Elf32_Phdr[] dlpi_phdr;
>> somehow.
>>
>> Or maybe I did translate it to managed code the wrong way.
>> Can anybody have a look at it ?
>>
>> All necessary code/structs to get the sample running you find on the
>> stackoverflow link.
>>
>>
>> Here's the C struct:
>>
>>  struct dl_phdr_info {
>>            ElfW(Addr)        dlpi_addr;  /* Base address of object */
>>            const char       *dlpi_name;  /* (Null-terminated) name of
>>                                             object */
>>            const ElfW(Phdr) *dlpi_phdr;  /* Pointer to array of
>>                                             ELF program headers
>>                                             for this object */
>>            ElfW(Half)        dlpi_phnum; /* # of items in dlpi_phdr */
>>        };
>>
>> Here's my C# translation of the above struct:
>>
>>   public struct dl_phdr_info
>>     {
>>         public System.UInt32 dlpi_addr; /* Base address of object */
>>
>>         // TODO: String, MarshalAs Pointer
>>         public System.IntPtr dlpi_name;  /* (Null-terminated) name of
>> object*/
>>
>>         /* Pointer to array of ELF program headers for this object */
>>         public System.IntPtr dlpi_phdr; // Hackish, cannot read it, but then
>> at least the rest works
>>
>>         // This way it throws an exception at runtime.
>>
>> //[System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.LPStruct)]
>>         //public Elf32_Phdr[] dlpi_phdr;
>>
>>         public System.UInt16  dlpi_phnum; /* # of items in 'dlpi_phdr' */
>>     }
>>
>>
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>>
>>
>
>