[SOLVED][API] what is openmpt_stream_get_file_callbacks() ?

[jseb]

Hello,

I'm learning to use the API , and when reading this:

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mod = openmpt_module_create2( openmpt_stream_get_file_callbacks(), file, &libopenmpt_example_logfunc, NULL, &libopenmpt_example_errfunc, NULL, &mod_err, &mod_err_str, NULL );


I wonder what the first argument openmpt_stream_get_file_callbacks() is doing.

Reading the doc for openmpt_module_create2, i thought at first sight it was a callback function which will be invoked every time we need to fill a buffer.
But it doesn't seem to be the case: if it was the case, it should give frequency of refill, and it should be an user defined callback.

Moreover, the buffer filling is done by this function in the rendering loop:

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count = openmpt_module_read_interleaved_stereo( mod, SAMPLERATE, BUFFERSIZE, buffer );




The help for openmpt_stream_get_file_callbacks() doesn't say much...

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◆ openmpt_stream_get_file_callbacks()
static openmpt_stream_callbacks openmpt_stream_get_file_callbacks (void)


Thank you.

[manx]

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mod = openmpt_module_create2( openmpt_stream_get_file_callbacks(), file, &libopenmpt_example_logfunc, NULL, &libopenmpt_example_errfunc, NULL, &mod_err, &mod_err_str, NULL );


I wonder what the first argument openmpt_stream_get_file_callbacks() is doing.

Reading the doc for openmpt_module_create2, i thought at first sight it was a callback function which will be invoked every time we need to fill a buffer.
But it doesn't seem to be the case: if it was the case, it should give frequency of refill, and it should be an user defined callback.

The help for openmpt_stream_get_file_callbacks() doesn't say much...

The first argument to openmpt_module_create2() is a structure containing function pointers to file i/o functions, in particular, a read, and optionally seek and tell functions. Only read is required, but file i/o will require less resources when also seek and tell are provided. These callbacks operate on the file stream which is passed as the second argument to openmpt_module_create2(). This stream pointer is in turn passed to the callback functions provided in the struct. See https://lib.openmpt.org/doc/group__libopenmpt__c.html#gaeccd341852695cc71d8a5e3ac47ff168.

openmpt_stream_get_file_callbacks() fills the callbacks structure as required for a C stdio FILE* object, which is passed as the second argument as file in the quoted example.
I.e. if you open your file via fopen(), you can just use openmpt_stream_get_file_callbacks().
If you open your file using standard unix file descriptor, you can use openmpt_stream_get_fd_callbacks().
If you use other means for file i/o, you need to provide suitable callback functions yourself.

I'll add some more documentation to openmpt_stream_get_file_callbacks() and openmpt_stream_get_fd_callbacks().

Moreover, the buffer filling is done by this function in the rendering loop:

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count = openmpt_module_read_interleaved_stereo( mod, SAMPLERATE, BUFFERSIZE, buffer );


This fills the output buffer as required by the API user. This has nothing to do with file i/o. File loadingg is finished as soon as openmpt_module_create2() returns.

[jseb]

Thank you, i understand now.