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DYNAMIC LINK LIBRARY : mpg123clr Project Overview

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Project Installation:

The project is part of a Visual Studio 2008 Solution.

The Visual Studio solution file is 2008clr.sln located in a folder/directory named 2008clr.

This folder/directory should be located in the mpg123/ports/MSVC++ folder to maintain file relationships

with required mpg123 library files. i.e. at mpg123/ports/MSVC++/2008clr.

The solution includes the following projects:

mpg123clr The mpg123 CLR project. This creates the CLR compatible DLL file.

scanclr Demonstration project for comparison with scan.c example.

feedseekclr Demonstration project for comparison with feedseek.c example

replacereaderclr Demonstration project, how to implement ReplaceReader in CLR (proof of concept)

libmpg123 For convenience, ensures that libmpg123.lib is available for linking

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Compiling:

Configuration Manager:

mpg123clr requires linking to a libmpg123 static library, this library must be compiled prior to

compiling the clr projects. mpg123clr uses relative folder position during linking; therefore either:-

ensure that the 2008clr solution is correctly located within the ports\msvc++ folder, or

change the "linker->input->additional dependencies" to suit your installation configuration

NOTE: x64 Active Solution Platform configurations are not supplied with this solution but have been tested.

(Effectively all that is required is to change all "include" references to account for the "x64" output locations)

NOTE: The "unsafe" context of the ReplaceReaderClr project is only required to allow the use of callbacks.

mpg123clr is written to avoid the use of "unsafe" for the majority of normal operations.

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Using mpg123clr:

Mpg123clr is a CLR ready dll. Simply add a reference to the mpg123clr.dll file in your application project.

You may also add a "using" directive in your program files. C# e.g. using mpg123clr;

The mpg123clr project is configured to create an additional Intellisense documentation file (mpg123clr.xml)

to assist usage. If you manually relocate the .dll please also copy the .xml file to ensure correct Intellisense

operation.

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Common Errors:

1) Problem: "Unhandled Exception - file not found" error when the application is executed.

Cause: Incorrect libmpg123 library build / missing libmpg123.dll.

Solution: Copy libmpg123.dll to the application output folder - possibly in a Post-Build Event (see scanclr for

an example).

Explanation: mpg123clr requires static library libmpg123.lib and NOT dynamic library libmpg123.dll.

If the libmpg123 library is built using an incorrect Project Context (such as Debug_Generic_Dll), mpg123clr

will attempt to load the libmpg123.dll at runtime even though this file is not required.

Alternatively: Use Configuration Manager and select a non-dll build context for the libmpg123 project.

(i.e. Debug_Generic not Debug_Generic_Dll)

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Developer Notes:

1) Documentation <remarks> do not appear in CLR Intellisense environment (at time of writing).

So deferred in favour of <para> within <summary>.

2) During development "off_t" parameters conflicted with intellisense (although size_t worked just fine)

Since many CLR functions that refer to off_t features (e.g. positioners) actually expect 64bit long

(or long long in C++) parameters, I've implemented off_t as a long long - with one notable exception.

This fixed intellisense and accommodates the CLR 64bit long implementation positioners: And then intellisense

started working correctly and made this a moot point - at least as far as intellisense is concerned.

3) off_t notable exception - SeekDelegate, SeekHandleDelegate

Usually 2008clr functions are called by the callee, long long (clr 64bit long) can be marshaled and passed

to libmpg123. The seek delegate is called directly by the libmpg123 seek functions - there isn't an opportunity

to marshal the offending parameters. A library compiled in 32 bit will expect the SeekDelegate to have a 32bit

position parameter but in 64bit compilation the same parameter will be 64bit. Therefore the need to be correct

overrides the need for consistency with CLR 64bit positioners.

SIDE NOTE: In early tests the SeekDelegate was defined with a long long parameter - libmpg123 called it oblivious

to the fact that the type was incorrect. The only impact was that the 32bit position and 32bit origin (whence)

were compacted to a single 64bit parameter. This is fixed, no longer an issue!!!

4) PosixSeek and PosixRead ARE ONLY INCLUDED AS PROOF OF CONCEPT - to prove that the callbacks into CLR would

function correctly with libmpg123 cdecl calls. It saved making a separate dll for the posix elements.

REPEAT AFTER ME: Not for normal consumption....

5) The __clrcall decoration of mpg123clr functions is not strictly necessary for Visual Studio 2005 and later,

but has been left in for compatibility with earlier versions. See MS documentation of "double thunking"

6) Many functions use pin_ptr to pin the return variables prior to calling the libmpg123 function, this was

considered as an alternative to avoid the use of local variables with attendant overhead of

allocate/use/copy/destroy etc. In practice pin_ptr has significant overhead, empirical tests showed very

little performance difference between either method.

7) Set pin_ptr to NULL after use, to force early GC (recommended). Carried out performance trials and found

insignificant impact. Since most functions are likely to be used repeatedly decided to follow recommended usage.

If code analysis proves this to be redundant - can be removed...

(1.9.0.1 update) "insignificant" in Debug mode, quite significant if the debug overhead is removed,

i.e. Release mode. So try/finally blocks removed in favour of allocating temporary local return variable.

8) (1.12.0.0 update) Support for mpg123_open_handle warrants further description... here it is.

a) Pinning of delegates is not required. See Microsoft: C++, How to: Marshal Callbacks and Delegates Using C++ Interop

b) Pinning of objects is an entirely different matter, so why is the open_handle object not pinned?

Answer: Because it's an object. I allowed passing of any object not just handles.

i) a pin_ptr cannot be a field member for instance scope.

ii) cannot pin the object itself because there's no object.field to pin

iii) cannot pin the object target because the referenced object may be unblittable.

iv) GCHandle.Alloc(obj, GCHandleType.Pinned) doesn't work on unblittable objects (eg managed references like BinaryReader)

Since I don't restrict which objects may be passed I'm forced to exclude pinning.

c) The generated handle doesn't need to be pinned. GCHandle.Alloc(obj) creates a potentially GC relocatable object

containing a Handle to the object. The GCHandle.ToIntPtr(userObjectHandle) creates an integer representation of the

handle that is valid irrespective of any GC relocation of either the object or the GCHandle and can therefore be

passed through unmanaged code to CLR native methods that can re-reference the original object via GCHandle.FromIntPtr

and gch.Target.

All this seems relatively efficient, certainly there is little performance difference between the ReplaceReader

and ReplaceReaderHandle sample code.

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Tested/Untested

I haven't used mpg123clr in a production project yet, although one is about to start. Therefore EVERYTHING should

be treated as though unproven. UPDATE: the scanclr, feedseekclr and replacereaderclr examples have provided a basic

functional test.

Largefile support totally untested... would appreciate a volunteer.

The majority of functions have been tried in test projects with the following exceptions.

64bit compilation. mpg123clr compiles correctly in 64bit x64 platform mode, but hasn't been tested.

ICY(icy_meta); Never had genuine ICY data to test with.

Icy2Utf8(icy_text); Never tried. Please provide sample material.

enc_from_id3(encbyte); Works on libmpg123 enums, never had real world text samples.

store_utf8(...); Works with CLR generated text, not tried with real world text samples.

mpg123str.strlen(...) Not tested with utf8 text strings. Please provide sample material.

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Documentation

mpg123clr is 90% documentation, use intellisense as your reference. If in doubt refer back to the libmpg123

documents. NOTE: You need to build the mpg123clr library to create the intellisense file.

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Error Trapping

libmpg123 is an error agnostic library - it shouldn't generate errors, but neither does it trap them.

mpg123clr is built on the same concept although some CLR generated errors have been trapped. My personal

preference is that exceptions should be avoided not relied on...

THE CALLING APPLICATION IS RESPONSIBLE FOR ERROR DETECTION, TRAPPING AND CORRECTION.

All projects are "as is" with no express or implied warranty. Demonstration projects are to demonstrate the

use of the library and are NOT considered examples of good coding practice. Use at own risk.

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Revision History

1.8.1.0 04-Aug-09 Initial release.

1.9.0.0 24-Sep-09 Function names harmonized with libmpg123 (mb)

1.9.0.0 30-Sep-09 Project config - if exists, copy libmpg123.dll to app output folder (mb)

1.9.0.0 01-Oct-09 Technical cleanup - subst nullptr for NULL (mb)

1.9.0.1 24-Nov-09 Performance update - removed try/finally (mb)

1.12.0.0 14-Apr-10 Release match - added open_handle and framebyframe support (mb)

1.13.0.0 13-Jan-11 Release match - added encsize and strlen support (mb)

Constructive feedback preferred.

Malcolm Boczek

mpg123clr original author.