CODEWITHSUNDEEP
lispcommon-lisp
DNess007
DNess007

Reputation: 41

Practical common lisp - chapter 3

I am new to programming and started with the practical common lisp book. My challenge is in the third chapter.

The function

(defun save-db (filename)
  (with-open-file (out filename
                   :direction :output
                   :if-exists :supersede)
    (with-standard-io-syntax)
      (print *db* out))))

When i go to the REPL and type (save-db "c:/my-cds.db"), I get the following:

Permission denied : "c:/my-cds.db" Condition of type CCL::SIMPLE-FILE-ERROR

Could someone help me with the stage as I have been stuck for 2 days trying to understand what's wrong.

Thank you.

Upvotes: 2

Views: 113

Answers (2)

Kaz
Kaz

Reputation: 58578

I use CCL on Windows. The Tankan application for learning Japanese characters is built using CCL.

This complete program writes the file successfully:

(defvar *db* 42)

(defun save-db (filename)
  (with-open-file (out filename
                   :direction :output
                   :if-exists :supersede)
    (with-standard-io-syntax)
      (print *db* out)))

(save-db "c:\\Users\\kaz\\dbfile.lisp")

I get a file containing 42.

It also works with single forward slashes: "c:/Users/...".

With single backslashes, I get that problem that CCL's parse-namestring function tries to interpret the path as having a logical host name. Same error message that you saw.

I also get a permission error if I just use "c:/dbfile.lisp".

Note that the doubled backslashes are just an escaping notation:

CCL is free software.  It is distributed under the terms of the Apache
Licence, Version 2.0.
? (length "\\")
1
? (length "\\a")
2
? (length "/a")
2

We can read a normal-looking path from the user, e.g. using read-line. When CCL prints that back to us as a string object, it adds the escapes:

> (read-line)
c:\Users\kaz\dbfile.lisp
"c:\\Users\\kaz\\dbfile.lisp"
NIL

They are not part of the data, just like the " quote characters are not.

Upvotes: 2

Silvio Mayolo
Silvio Mayolo

Reputation: 70267

As discussed in the comments, this is just the operating system protecting you from accidentally damaging your system. Even on a single-user system, the OS won't let ordinary user accounts modify system directories, including the root of the main drive.

In your case, since you're just testing things, I recommend simply working in your Documents folder, which is probably C:\Users\<YourName>\Documents, and you shouldn't run into permissions issues.

If you really do need to modify C:\, you'll need to run your program as an administrator. On Linux, that's just sudo. On Windows, I believe you just run the command line as administrator by right-clicking it and hitting "Run as Administrator".

Upvotes: 3

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