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pack_fopen_chunk - Man Page

Opens a sub-chunk of a file. Allegro game programming library.


#include <allegro.h>

PACKFILE *pack_fopen_chunk(PACKFILE *f, int pack);


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 routines. 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 (created 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 uncompressed 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 Also

pack_fclose_chunk(3), pack_fopen(3)

Referenced By

pack_fclose_chunk(3), pack_feof(3), pack_ferror(3), pack_fgets(3), PACKFILE(3), pack_fopen(3), pack_fopen_vtable(3), pack_fputs(3), pack_fread(3), pack_fseek(3), pack_fwrite(3), pack_getc(3), pack_putc(3).

version 4.4.3 Allegro manual