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Let's take a deeper look at how the AutoCAD core environment is
emulated. When one of the IGES commands is entered, its command
implementing function first calls dbload(), implemented in autosim.c. This function builds a simulated AutoCAD database in
three steps.
First, ads_getvar()
is used to capture the current settings of all AutoCAD system
variables referred to by IGES. These variables are stored in static
data defined in the file autodata.h, each named the same as its
incarnation within the AutoCAD core. Other required information
accessible through ads_getvar(), including the current drawing
name and the AutoCAD version, are obtained as well.
Since ADS provides all the
facilities we need to read and write the AutoCAD database, there's no
need to prepare a local copy of it. AutoCAD core code, however,
addresses the database by means of long handles instead of the
ads_name objects used by ADS. To avoid modifying the many
places database addresses are assumed to be longs, dbload() makes a pass over the AutoCAD database and prepares a
temporary file named dbmap which, addressed by an entity index,
returns the ADS name for that entity. All entity references are
translated through the file and the global symbols entadr and
entcnt contain, respectively, an index into the file and its
current length.
Finally, the entire contents of each
of AutoCAD's symbol tables are retrieved with the ads_tblnext()
function. Each symbol table item is decoded into the format used
within AutoCAD and stored in a temporary file named after the symbol
table. Its file handle is kept in the local copy of the AutoCAD
symbol table descriptor. The symbol tables are read and the copies
prepared by symtgrab(), called from dbload().![[Footnote]](i/foot.gif)
As the block table is copied, the entities that define the block must
be stepped through and their names added to the database index file.
The index of the block definition entity is stored in the local copy
of the block symbol table item. This is not presently implemented,
and hence IGESOUT of drawings containing blocks will fail.
Next: Using The Database
Up: The AutoCAD Database
Previous: The AutoCAD Database
Editor: John Walker