docs/126590: Write routine called forever in Sample Echo Pseudo-Device Driver

[Jacek Wikiera]

>Number:         126590
>Category:       docs
>Synopsis:       Write routine called forever in Sample Echo Pseudo-Device Driver
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-doc
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          doc-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sun Aug 17 13:30:01 UTC 2008
>Closed-Date:
>Last-Modified:
>Originator:     Jacek Wikiera
>Release:        n/a
>Organization:
n/a
>Environment:
n/a
>Description:
Location:
FreeBSD Architecture Handbook, 9.4 Character Devices,  example driver code.

http://www.freebsd.org/doc/en_US.ISO8859-1/books/arch-handbook/driverbasics-char.html

Description:
The function echo_write() is incorrect - it does not zero an important variable and makes the example a bad example.
If code is built from the example, the function echo_write() does not zeroe uio->uio_resid and thus fails to tell the OS that all the data has been consumed.

The function is called again and again forever (until the caller is terminated, I guess).


>How-To-Repeat:
Build the example code, and type at the command prompt:
cat > /dev/echo

Next type a line followed by return. This will cause the file /dev/echo to be open, and continuous calls to echo_write subroutine.

The cat program will hang now and not able to read Ctrl-D from the terminal. Ctrl-C is needed.
>Fix:
In the function echo_write() add the following line:

uio->uio_resid = 0; 

This will tell the OS that all the data has been consumed.
See the source of the null device for this hint.



Patch attached with submission follows:

--- driverbasics-char.html      2008-08-16 06:08:26.000000000 +0200
+++ driverbasics-char_fix.html  2008-08-17 15:10:37.531250000 +0200
@@ -200,6 +200,8 @@
     *(echomsg->msg + MIN(uio->uio_iov->iov_len, BUFFERSIZE - 1)) = 0;
     echomsg->len = MIN(uio->uio_iov->iov_len, BUFFERSIZE);
 
+    uio->uio_resid = 0; /* signall that all data is consumed */
+
     if (err != 0) {
         uprintf("Write failed: bad address!\n");
     }


>Release-Note:
>Audit-Trail:
>Unformatted: