pack_fopen_chunk(3) Allegro manual pack_fopen_chunk(3)NAMEpack_fopen_chunk - Opens a sub-chunk of a file. Allegro game program‐
ming library.
SYNOPSIS
#include <allegro.h>
PACKFILE *pack_fopen_chunk(PACKFILE *f, int pack);
DESCRIPTION
Opens a sub-chunk of a file. Chunks are primarily intended for use by
the datafile code, but they may also be useful for your own file rou‐
tines. A chunk provides a logical view of part of a file, which can be
compressed as an individual entity and will automatically insert and
check length counts to prevent reading past the end of the chunk. The
PACKFILE parameter is a previously opened file, and `pack' is a boolean
parameter which will turn compression on for the sub-chunk if it is
non-zero. Example:
PACKFILE *output = pack_fopen("out.raw", "w!");
...
/* Create a sub-chunk with compression. */
output = pack_fopen_chunk(output, 1);
if (!output)
abort_on_error("Error saving data!");
/* Write some data to the sub-chunk. */
...
/* Close the sub-chunk, recovering parent file. */
output = pack_fclose_chunk(output);
The data written to the chunk will be prefixed with two length counts
(32-bit, a.k.a. big-endian). For uncompressed chunks these will both be
set to the size of the data in the chunk. For compressed chunks (cre‐
ated by setting the `pack' flag), the first length will be the raw size
of the chunk, and the second will be the negative size of the uncom‐
pressed data.
To read the chunk, use the following code:
PACKFILE *input = pack_fopen("out.raw", "rp");
...
input = pack_fopen_chunk(input, 1);
/* Read data from the sub-chunk and close it. */
...
input = pack_fclose_chunk(input);
This sequence will read the length counts created when the chunk was
written, and automatically decompress the contents of the chunk if it
was compressed. The length will also be used to prevent reading past
the end of the chunk (Allegro will return EOF if you attempt this), and
to automatically skip past any unread chunk data when you call
pack_fclose_chunk().
Chunks can be nested inside each other by making repeated calls to
pack_fopen_chunk(). When writing a file, the compression status is
inherited from the parent file, so you only need to set the pack flag
if the parent is not compressed but you want to pack the chunk data. If
the parent file is already open in packed mode, setting the pack flag
will result in data being compressed twice: once as it is written to
the chunk, and again as the chunk passes it on to the parent file.
RETURN VALUE
Returns a pointer to the sub-chunked PACKFILE, or NULL if there was
some error (eg. you are using a custom PACKFILE vtable).
SEE ALSOpack_fclose_chunk(3), pack_fopen(3)Allegro version 4.4.2 pack_fopen_chunk(3)
