Part I: FLP Format & Events#

FLP is a binary format used by Image-Line FL Studio, a music production software, to store project files. Instead of using C-style structs entirely, the FLP format has evolved from what once was a MIDI-like format to a really bad and messy combination of Type-length-value encoded “events” and structs.

Specification#

An FLP file contains of basically 2 sections or “chunks”, one is the header and other is the “data” section, which contains all the “events”.

Header chunk#

C / C++

struct {
    char magic[4];          // 'FLhd'
    uint32_t size;          // always been 6
    int16_t format;         // Internal file format
    uint16_t num_channels;  // Number of channels in channel rack
    uint16_t ppq;           // Pulses per quarter
}

Python

class Header:
    magic: str
    size: int
    format: int
    num_channels: int
    ppq: int

Data chunk#

struct {
    char magic[4];  // 'FLdt'
    uint32_t size;  // Total combined size of events
    void* events;   // Event data
}

Event#

An event can be thought of as a “flattened” dict of attributes composing a class. It can roughly be represented as:

C / C++

struct {
    uint8_t type;
    void* value;
}

Python

class Event:
    type: int
    value: object

Types#

There are basically 4 kinds of events depending on the range of type:

Event ID	Size of `value` \| Total event size
0-63	1 byte	1 + 1 = 2
64-127	2 bytes	1 + 2 = 3
128-191	4 bytes	1 + 4 = 5
192-255	Length prefixed	>= 2

Note

Length prefixed events

These events store the length of the value they contain after type in a varint. It can be considered as the only true TLV encoded event type.

struct {
    uint8_t type;     // 192-255
    uint8_t* length;  // varint
    void* value;      // string, struct or subevent
}

It should be clearer by now how the FLP format is a misfit for the data it represents.

Representation#

Event IDs 0-191 are used for storing fixed size data like integers, floats and booleans. IDs from 192-255 are used for storing structs, subevents and strings.