Racket web server write file error - racket

Question

I write a web program to write file,and read file.And it failed.
And the program code is following:
#lang web-server/insta
(define (start request)
(index-wrap request))
(define (index-wrap request)
(define (response-generator embed/url)
(response/xexpr
`(html
(head (title "test"))
(body
(h1 "Test Write")
(form ((action ,(embed/url write-handler)))
(input ((type "submit"))))))))
(define (write-handler request)
(call-with-output-file "write_file"
#:exists 'truncate
(lambda (out) (print "hello,world" out)))
(index-wrap request))
(send/suspend/dispatch response-generator))
And when you click the button,it occurs an error:
call-with-output-file: cannot open output file
path: /usr/share/racket/pkgs/web-server-lib/web-server/default-web-root/htdocs/write_file
system error: Permission denied; errno=13
And I also write a test program to test the call-with-output-file function,and it write file successful.And my test program code are following:
#lang racket
(call-with-output-file "write_other"
#:exists 'truncate
(lambda (out) (print "hello,racket" out)))
So what's the problem about it?

Answer 1

Try
(define (write-handler request)
(displayln (list "HERE" (current-directory)))
(call-with-output-file "write_file"
#:exists 'truncate
(lambda (out) (print "hello,world" out)))
(index-wrap request))
(send/suspend/dispatch response-generator))
to see which directory you are trying to write to. I suspect the web-server has changed directory to something else than you expect.
EDIT
Now you know that the path is
/usr/share/racket/pkgs/web-server-lib/web-server/default-web-root/htdocs/
you have two options.
Change the permissions of the folder
Start the web-server somewhere else
The easiest is to change the permissions:
cd /usr/share/racket/pkgs/web-server-lib/web-server/default-web-root/
chmod +w htdocs
sudo chmod +w htdocs
Hopefully one of the two last lines work. If not you need to
google how to set file permissions to write on your OS.