View source for Hacker Guide/Access

== Description  ==

The modules of ''''access'''' capability are designed to be the first and last elements of a modules chain.

They take an [[MRL]] in, and output a bitstream, that can be fed to a [[Demuxer]].

Access [[Documentation:Hacker's Guide/Access|input]] and [[Documentation:Hacker's Guide/Access Output|output]] handles most of the basic '''I/O''' for VLC. They are usually '''protocols''' implementations (http, ftp,...) or '''devices''' access (Webcams, Capture cards). 

We will discuss about ''''input access'''' in this page.

== Write an access module  ==

To write an access module, read first the [[Documentation:Hacker's Guide/Module Writers Guide|introduction to module writing]]. 

Then, you should specify your module of being of access type: 

 set_capability( "access", 60 )  
 set_category( CAT_INPUT )                                                                                                                                                                                  
 set_subcategory( SUBCAT_INPUT_ACCESS )

=== Type ===
Your module can be of either a ''''Block'''' or a ''''Read'''' type, depending on your medium:

* if the underlying protocol returns data blocks of unknown or unpredictable size, '''Block''' is a better type,
* if you can control the size of the data requested to the underlying protocol, '''Read''' is a better type.

== Functions to implement  ==

After implementing Open() and Close() functions, you will need to implement a few majors features that will be implemented by your functions. 

As you can see {{VLCSourceFile|name=include/vlc_access.h}}, you should define: 

* Seek, as in pf_seek,
* Control, as in pf_control,
* Read or Block, depending on your module [[#Type|type]].

After implementing those functions, you should assign them to the corresponding pf_ function. 

=== Seek  ===

''Prototype'': 

 int         (*pf_seek) ( access_t *, uint64_t );

The seeking function will be called whenever a seek is requested. 

The arguments are a ''pointer'' to the module structure, and the requested '''position'''. 

'''NB:''' Seeking function can be NULL, if it isn't possible to seek in the protocol or device.

You shall set p_access->info.b_eof to false if seek worked.

''Return:''<br>

If the seek has succeeded, it should return VLC_SUCCESS, else it should return VLC_EGENERIC. <br>

=== Control  ===

''Prototype'': 

 int         (*pf_control)( access_t *, int i_query, va_list args);

Control function is quite easy, the input core will query the module using this function with: 

*A pointer to the module structure.
*An '''i_query''' parameter that can be of several type. We will cover afterward the most important on. 
*A list of arguments, that depends on the '''i_query''' type.


''Return'': 

If the query has succeeded, it should return VLC_SUCCESS. Else it should return VLC_EGENERIC (''fail''). 

==== Control Query types  ====

The first ones are request to know what the module supports. They are all of '''boolean''' type and should always succeed. 

 ACCESS_CAN_SEEK,        
 ACCESS_CAN_FASTSEEK,    
 ACCESS_CAN_PAUSE,       
 ACCESS_CAN_CONTROL_PACE,

<br> The following one is a request for the '''PTS delay'''. It must always succeed. 

 ACCESS_GET_PTS_DELAY,

<br> The following ones are for '''requesting''' various '''info''' about the input, like Metadata, Titles and Chapters or device signal strength. All of them can fail. 

 ACCESS_GET_TITLE_INFO,  
 ACCESS_GET_META,        
 ACCESS_GET_CONTENT_TYPE,
 ACCESS_GET_SIGNAL,      

<br> Depending to the answer of the CAN_ requests, the core can '''set''' a few things, like pausing or changing '''title''' or '''chapter''' 

 ACCESS_SET_PAUSE_STATE,
 ACCESS_SET_TITLE,
 ACCESS_SET_SEEKPOINT,

You can find the list of arguments corresponding to query types in the comments of access_query_e definition in vlc_access.h

=== Read  ===

''Prototype'': 

 ssize_t     (*pf_read) ( access_t *, uint8_t *, size_t );

''Return'': 

Return -1 if no data yet, 0 if no more data, else actual data read on the medium. 

=== Block  ===

''Prototype'': 

 block_t    *(*pf_block)( access_t * );

''Return:'' 

Returns a block of data in his 'natural' size. It will return NULL if not yet data or eof.

To differentiate between ''no data'' and ''eof'', you shall set p_access->info.b_eof to true in case of eof.

{{Hacker_Guide}}