Hacker Guide/Access
Contents
Description
The modules of 'access' capability are designed to be the first and last elements of a modules chain.
Access input and 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.
Starting to write an access module
To write an access module, read the 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 )
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 include/vlc_access.h, you should define:
- Seek, as in pf_seek
- Control, as in pf_control
- Read or Block
Your module can be of a 'Block' or a 'Read' type, depending on what you need to access.
After implementing those function, you should assign them to the corresponding pf_ function.
Seek
int (*pf_seek) ( access_t *, uint64_t );
Seeking function can be NULL, if it isn't possible to seek in the protocol or device.
Control
int (*pf_control)( access_t *, int i_query, va_list args);
Read
ssize_t (*pf_read) ( access_t *, uint8_t *, size_t ); /* Return -1 if no data yet, 0 if no more data, else real data read */
Block
block_t *(*pf_block)( access_t * ); /* return a block of data in his 'natural' size, NULL if not yet data or eof */
Functionality
Generally speaking, access input will provide Read() functions and output Write().
Both needs to provide Seek() and Control() even if the underlying architecture does not support these actions. In this case, Seek() and Control() should return VLC_EGENERIC, although Control() might need to answer ACCESS_OUT_CONTROLS_PACE requests. See modules/access_output/http.c about using a source without seeking.
