Release Notes

Open Source Used In AsyncOS 8.8 for Cisco Web Security Appliances

When reading, two methods of handling special markers are available:

1. You can ask the library to save the contents of COM and/or APPn markers

into memory, and then examine them at your leisure afterwards.

2. You can supply your own routine to process COM and/or APPn markers

on-the-fly as they are read.

The first method is simpler to use, especially if you are using a suspending

data source; writing a marker processor that copes with input suspension is

not easy (consider what happens if the marker is longer than your available

input buffer). However, the second method conserves memory since the marker

data need not be kept around after it's been processed.

For either method, you'd normally set up marker handling after creating a

decompression object and before calling jpeg_read_header(), because the

markers of interest will typically be near the head of the file and so will

be scanned by jpeg_read_header. Once you've established a marker handling

method, it will be used for the life of that decompression object

(potentially many datastreams), unless you change it. Marker handling is

determined separately for COM markers and for each APPn marker code.

To save the contents of special markers in memory, call

jpeg_save_markers(cinfo, marker_code, length_limit)

where marker_code is the marker type to save, JPEG_COM or JPEG_APP0+n.

(To arrange to save all the special marker types, you need to call this

routine 17 times, for COM and APP0-APP15.) If the incoming marker is longer

than length_limit data bytes, only length_limit bytes will be saved; this

parameter allows you to avoid chewing up memory when you only need to see the

first few bytes of a potentially large marker. If you want to save all the

data, set length_limit to 0xFFFF; that is enough since marker lengths are only

16 bits. As a special case, setting length_limit to 0 prevents that marker

type from being saved at all. (That is the default behavior, in fact.)

After jpeg_read_header() completes, you can examine the special markers by

following the cinfo->marker_list pointer chain. All the special markers in

the file appear in this list, in order of their occurrence in the file (but

omitting any markers of types you didn't ask for). Both the original data

length and the saved data length are recorded for each list entry; the latter

will not exceed length_limit for the particular marker type. Note that these

lengths exclude the marker length word, whereas the stored representation

within the JPEG file includes it. (Hence the maximum data length is really

only 65533.)

It is possible that additional special markers appear in the file beyond the

SOS marker at which jpeg_read_header stops; if so, the marker list will be

extended during reading of the rest of the file. This is not expected to be

common, however. If you are short on memory you may want to reset the length

limit to zero for all marker types after finishing jpeg_read_header, to