libmp4v2.dll help

[roidy]

Hi, does anyone have any experience setting up and using libmp4v2.dll from
VB.net 2008. I just carn`t figure out how to declare the correct structures
and varibles in VB.

This page describes the main MP4Tags structure:-
http://mp4v2.googlecode.com/svn/doc/trunk/api/struct_m_p4_tags__s.html

This page describles the api
functions:-http://mp4v2.googlecode.com/svn/doc/trunk/api/group__mp4__meta.html#g71f371b895fe494922900a48383a6a4f

Any help or advice to get me started would be appreciated

 

[roidy]

Ok this is what I`ve come up with so far to try out two of the api calls:-

Public Class Form1
Declare Auto Function MP4GetMetadataName Lib "e:\libmp4v2.dll" (ByVal
filehandle As Integer, ByRef val As IntPtr) As Boolean
Declare Auto Function MP4Read Lib "e:\libmp4v2.dll" (ByVal filename As
String) As Long

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles MyBase.Load
Dim filehandle As Long
Dim result As Boolean
Dim test As IntPtr

filehandle = MP4Read("e:\test.mp4")
result = MP4GetMetadataName(filehandle, test)
End Sub
End Class

The equivalent c code is as follows:-

char *value;
bool result;
MP4FileHandle mp4file = MP4Read( "e:\test.mp4");
result = MP4GetMetadataName( mp4file, &value );


However I get an "Arithmetic operation resulted in an overflow" exception on
the call to MP4GetMetadataName in the VB version. Anybody know where I`m
going wrong?

Thanks
Rob

 

[Armin Zingler]

Ok this is what I`ve come up with so far to try out two of the api
calls:-
Public Class Form1
Declare Auto Function MP4GetMetadataName Lib "e:\libmp4v2.dll"
(ByVal filehandle As Integer, ByRef val As IntPtr) As Boolean
Declare Auto Function MP4Read Lib "e:\libmp4v2.dll" (ByVal
filename As String) As Long

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles MyBase.Load
Dim filehandle As Long
Dim result As Boolean
Dim test As IntPtr

filehandle = MP4Read("e:\test.mp4")
result = MP4GetMetadataName(filehandle, test)
End Sub
End Class

The equivalent c code is as follows:-

char *value;
bool result;
MP4FileHandle mp4file = MP4Read( "e:\test.mp4");
result = MP4GetMetadataName( mp4file, &value );


However I get an "Arithmetic operation resulted in an overflow"
exception on the call to MP4GetMetadataName in the VB version.
Anybody know where I`m going wrong?

Yes, you use wrong declarations and you don't use them not correctly. Where
did you get them from?

These are the original declarations:

typedef void* MP4FileHandle;

bool MP4GetMetadataName(MP4FileHandle hFile, char** value);

MP4FileHandle MP4Read(const char* fileName, uint32_t verbosity
DEFAULT(0));


This is just an attempt to find the closest possible declarations:

Declare Auto Function MP4GetMetadataName Lib "e:\libmp4v2.dll"
(ByVal filehandle As IntPtr, ByRef val As IntPtr) As Boolean

Declare Auto Function MP4Read Lib "e:\libmp4v2.dll" (ByVal
filename As String, byval verbosity as UInteger) As IntPtr


Call:

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles MyBase.Load
Dim filehandle As IntPtr
Dim result As Boolean
Dim test As IntPtr
Dim bytes(1023) As Byte

filehandle = MP4Read("e:\test.mp4", ???) '<-- don't know what to pass

Dim gch = Runtime.InteropServices.GCHandle.Alloc(bytes(0))
result = MP4GetMetadataName(filehandle, gch.AddrOfPinnedObject())
gch.Free()

End Sub

Maybe (hopefully) there is another way to translate a char**. You have to
convert the byte array to a String using one of the System.Text.Encoding
objects.


Armin

 

[roidy]

Thanks for the help Armin.

I`ve tried the code you posted but I`m now getting a "Handle is not pinned."
exception on the call to MP4GetMetadataName and the value of filehandle is
always 0. I don`t know the first thing about pinned objects or using dll`s
in VB.

Thanks
Rob

 

[Armin Zingler]

Thanks for the help Armin.

I`ve tried the code you posted but I`m now getting a "Handle is not
pinned." exception on the call to MP4GetMetadataName and the value of
filehandle is always 0. I don`t know the first thing about pinned
objects or using dll`s in VB.

Sry, my fault. Add one parameter to the Alloc function:

Dim gch = Runtime.InteropServices.GCHandle.Alloc( _
bytes(0), _
Runtime.InteropServices.GCHandleType.Pinned _
)

But, I can't test it here so I have not clue if it works.

Armin

 

[roidy]

Thanks that cleared up the "Handle is not pinned exception.". However the
filehandle value from MP4Read still returns 0. This is the VB code I have so
far:-

Public Class Form1
Declare Auto Function MP4GetMetadataName Lib "e:\libmp4v2.dll" _
(ByVal filehandle As IntPtr, ByRef val As IntPtr) As Boolean
Declare Auto Function MP4Read Lib "e:\libmp4v2.dll" _
(ByVal filename As String, ByVal verbosity As UInteger) As
IntPtr

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As
System.EventArgs) _
Handles MyBase.Load
Dim filehandle As IntPtr
Dim result As Boolean
Dim bytes(1023) As Byte
filehandle = MP4Read("e:/test.mp4", 0)
Dim gch = Runtime.InteropServices.GCHandle.Alloc(bytes(0),
Runtime.InteropServices.GCHandleType.Pinned)
result = MP4GetMetadataName(filehandle, gch.AddrOfPinnedObject())
gch.Free()
End Sub
End Class


This is the same version written in c :-

extern "C" int main(int argc, char** argv)
{
char *value;
MP4FileHandle filehandle= MP4Read( "e:/test.mp4",0 );
MP4GetMetadataName( mp4file, &value );
fprintf( stdout, " Name: %s\n", value );
_getch();
return 0;
}

The c version works perfectly and returns a filehandle value of 0x0035e250
however the VB version always returns a filehandle value of 0 meaning that
the call to MP4Read has failed in some way. Could it be the way I`m passing
the filename string to the MP4Read function in VB?

Rob

 

[roidy]

Managed to get it working Thanks....

Rob

Thanks that cleared up the "Handle is not pinned exception.". However the
filehandle value from MP4Read still returns 0. This is the VB code I have
so far:-

Public Class Form1
Declare Auto Function MP4GetMetadataName Lib "e:\libmp4v2.dll" _
(ByVal filehandle As IntPtr, ByRef val As IntPtr) As
Boolean
Declare Auto Function MP4Read Lib "e:\libmp4v2.dll" _
(ByVal filename As String, ByVal verbosity As UInteger) As
IntPtr

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As
System.EventArgs) _
Handles MyBase.Load
Dim filehandle As IntPtr
Dim result As Boolean
Dim bytes(1023) As Byte
filehandle = MP4Read("e:/test.mp4", 0)
Dim gch = Runtime.InteropServices.GCHandle.Alloc(bytes(0),
Runtime.InteropServices.GCHandleType.Pinned)
result = MP4GetMetadataName(filehandle, gch.AddrOfPinnedObject())
gch.Free()
End Sub
End Class


This is the same version written in c :-

extern "C" int main(int argc, char** argv)
{
char *value;
MP4FileHandle filehandle= MP4Read( "e:/test.mp4",0 );
MP4GetMetadataName( mp4file, &value );
fprintf( stdout, " Name: %s\n", value );
_getch();
return 0;
}

The c version works perfectly and returns a filehandle value of 0x0035e250
however the VB version always returns a filehandle value of 0 meaning that
the call to MP4Read has failed in some way. Could it be the way I`m
passing the filename string to the MP4Read function in VB?

Rob