Release Notes

Open Source Used In AsyncOS 8.8 for Cisco Web Security Appliances

Here we revisit the JPEG compression outline given in the overview.

1. Allocate and initialize a JPEG compression object.

A JPEG compression object is a "struct jpeg_compress_struct". (It also has

a bunch of subsidiary structures which are allocated via malloc(), but the

application doesn't control those directly.) This struct can be just a local

variable in the calling routine, if a single routine is going to execute the

whole JPEG compression sequence. Otherwise it can be static or allocated

from malloc().

You will also need a structure representing a JPEG error handler. The part

of this that the library cares about is a "struct jpeg_error_mgr". If you

are providing your own error handler, you'll typically want to embed the

jpeg_error_mgr struct in a larger structure; this is discussed later under

"Error handling". For now we'll assume you are just using the default error

handler. The default error handler will print JPEG error/warning messages

on stderr, and it will call exit() if a fatal error occurs.

You must initialize the error handler structure, store a pointer to it into

the JPEG object's "err" field, and then call jpeg_create_compress() to

initialize the rest of the JPEG object.

Typical code for this step, if you are using the default error handler, is

struct jpeg_compress_struct cinfo;

struct jpeg_error_mgr jerr;

...

cinfo.err = jpeg_std_error(&jerr);

jpeg_create_compress(&cinfo);

jpeg_create_compress allocates a small amount of memory, so it could fail

if you are out of memory. In that case it will exit via the error handler;

that's why the error handler must be initialized first.

2. Specify the destination for the compressed data (eg, a file).

As previously mentioned, the JPEG library delivers compressed data to a

"data destination" module. The library includes one data destination

module which knows how to write to a stdio stream. You can use your own

destination module if you want to do something else, as discussed later.

If you use the standard destination module, you must open the target stdio

stream beforehand. Typical code for this step looks like:

FILE * outfile;