A string that specifies the name and extension of the file to be opened. If you do not specify the full path name of the file,
open assumes you are referring to the AutoCAD default drawing directory.
Indicates whether the file is open for reading, writing, or appending. Specify a string containing one of the following letters:
r Open for reading.
w Open for writing. If filename does not exist, a new file is created and opened. If filename already exists, its existing data is overwritten. Data passed to an open file is not actually written until the file is closed
with the close function.
a Open for appending. If filename does not exist, a new file is created and opened. If filename already exists, it is opened and the pointer is positioned at the end of the existing data, so new data you write to the
file is appended to the existing data.
The mode argument can be uppercase or lowercase. Note that in releases prior to AutoCAD 2000, mode had to be specified in lowercase.
If successful, open returns a file descriptor that can be used by the other I/O functions. If mode "r" is specified and filename does not exist, open returns nil.
NoteOn DOS systems, some programs and text editors write text files with an end-of-file marker (CTRL+Z, decimal ASCII code 26)
at the end of the text. When reading a text file, DOS returns an end-of-file status if a CTRL+Z marker is encountered, even
if that marker is followed by more data. If you intend to use open"a" mode to append data to files produced by another program, be certain the other program does not insert CTRL+Z markers at
the end of its text files.
Open an existing file:
Command: (setq a (open "c:/program files/Autodesk/AutoCAD/help/filelist.txt" "r"))
#<file "c:/program files/Autodesk/AutoCAD/help/filelist.txt">
The following examples issue open against files that do not exist:
Command: (setq f (open "c:\\my documents\\new.tst" "w"))
#<file "c:\\my documents\\new.tst">
Command: (setq f (open "nosuch.fil" "r"))
Command: (setq f (open "logfile" "a"))