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@file eet.c
@brief Eet Data Handling Library Public API Calls
These routines are used for Eet Library interaction
@mainpage Eet Library Documentation
@image html eet.png
@author Carsten Haitzler <raster\@rasterman.com>
@date 2000-2008
@section intro What is Eet?
It is a tiny library designed to write an arbitary set of chunks of data
to a file and optionally compress each chunk (very much like a zip file)
and allow fast random-access reading of the file later on. It does not
do zip as a zip itself has more complexity than is needed, and it was much
simpler to impliment this once here.
Eet is extremely fast, small and simple. Eet files can be very small and
highly compressed, making them very optimal for just sending across the
internet without having to archive, compress or decompress and install them.
They allow for lightning-fast random-acess reads once created, making them
perfect for storing data that is written once (or rarely) and read many
times, but the program does not want to have to read it all in at once.
It also can encode and decode data structures in memory, as well as image
data for saving to Eet files or sending across the network to other
machines, or just writing to arbitary files on the system. All data is
encoded in a platform independant way and can be written and read by any
@section example A simple example on using Eet
Here is a simple example on how to use Eet to save a series of strings to a
file and load them again. The advantage of using Eet over just fprintf() and
fscanf() is that not only can these entries be strings, they need no special
parsing to handle delimiter characters or escaping, they can be binary data,
image data, data structures containing integers, strings, other data
structures, linked lists and much more, without the programmer having to
worry about parsing, and best of all, Eet is very fast.
#include <Eet.h>
main(int argc, char **argv)
Eet_File *ef;
int i;
char buf[32];
char *ret;
int size;
char **entries =
"Entry 1",
"Big text string here compared to others",
"Eet is cool"
// blindly open an file for output and write strings with their NUL char
ef = eet_open("test.eet", EET_FILE_MODE_WRITE);
eet_write(ef, "Entry 1", entries[0], strlen(entries[0]) + 1, 0);
eet_write(ef, "Entry 2", entries[1], strlen(entries[1]) + 1, 1);
eet_write(ef, "Entry 3", entries[2], strlen(entries[2]) + 1, 0);
// open the file again and blindly get the entries we wrote
ef = eet_open("test.eet", EET_FILE_MODE_READ);
ret = eet_read(ef, "Entry 1", &size);
printf("%s\n", ret);
ret = eet_read(ef, "Entry 2", &size);
printf("%s\n", ret);
ret = eet_read(ef, "Entry 3", &size);
printf("%s\n", ret);
@section format What does an Eet file look like?
The file format is very simple. There is a directory block at the start of
the file listing entries and offsets into the file where they are stored,
their sizes, compression flags etc. followed by all the entry data strung one
element after the other.
All Eet files start with t a 4 byte magic number. It is written using network
byte-order (big endian, or from most significant byte first to least
significant byte last) and is 0x1ee7ff00 (or byte by byte 0:1e 1:e7 2:ff
3:00). The next 4 bytes are an integer (in big endian notation) indicating
how many entries are stored in the Eet file. 0 indicates it is empty. This is
a signed integer and thus values less than 0 are invalid, limiting the number
of entries in an Eet file to 0x7fffffff entries at most. The next 4 bytes is
the size of the directory table, in bytes, encoded in big-endian format. This
is a signed integer and cannot be less than 0.
The directory table for the file follows immediately, with a continuous list
of all entries in the Eet file, their offset in the file etc. The order of
these entries is not important, but convention would have them be from first
to last entry in the file. Each directory entry consiste of 5 integers, one
after the other, each stored as a signed, big endian integer. The first is
the offset in the file that the data for this entry is stored at (based from
the very start of the file, not relative to the end of the directory block).
The second integer holds flags for the entry. currently only the least
significant bit (bit 0) holds any useful information, and it is set to 1 if
the entry is compressed using zlib compression calls, or 0 if it is not
compressed. The next integer is the size of the entry in bytes stored in the
file. The next integer is the size of the data when decompressed (if it was
compressed) in bytes. This may be the same as the previous integer if the
entry was not compressed. The final integer is the number of bytes used by
the string identifier for the entry, without the NUL byte terminator, which
is not stored. The next series of bytes is the string name of the entry, with
the number of bytes being the same as specified in the last integer above.
This list of entries continues until there are no more entries left to list.
To read an entry from an Eet file, simply find the appropriate entry in the
directory table, find it's offset and size, and read it into memory. If it is
compressed, decompress it using zlib and then use that data.
Here is a data map of an Eet file. All integers are encoded using big-endian
notation (most significant byte first) and are signed. There is no alignment
of data, so all data types follow immediately on, one after the other. All
compressed data is compressed using the zlib compress2() function, and
decompressed using the zlib uncompress() function. Please see zlib
documentation for more information as to the encoding of compressed data.
[INT] Magic number (0x1ee7ff00)
[INT] Number of entries in the directory table
[INT] The size of the directory table, in bytes
DIRECTORY TABLE ENTRIES (as many as specified in the header):
[INT] Offest from file start at which entry is stored (in bytes)
[INT] Entry flags (1 = compressed, 0 = not compressed)
[INT] Size of data chunk in file (in bytes)
[INT] Size of the data chunk once decompressed (or the same as above, if not)
[INT] The length of the string itendifier, in bytes, without NUL terminator
[STR] Series of bytes for the string identifier, no NUL terminator
... more directory entries
... more data chunks
The contents of each entry in an Eet file has no defined format as such. It
is an opaque chunk of data, that is up to the application to deocde, unless
it is an image, ecoded by Eet, or a data structure encoded by Eet. The data
itself for these entries can be encoded and decoded by Eet with extra helper
functions in Eet. eet_data_image_read() and eet_data_image_write() are used
to handle reading and writing image data from a known Eet file entry name.
eet_data_read() and eet_data_write() are used to decode and encode program
data structures from an Eet file, making the loading and saving of program
information stored in data structures a simple 1 function call process.
Please see src/lib/eet_data.c for information on the format of these
specially encoded data entries in an Eet file (for now).
@todo Add hash table, fixed and variable array encode/decode support.
@todo Document data format for images and data structures.