[Python-Dev] buffer interface for C extensions

[Dan Lenski]

Hi all,

I've written a small C extension to submit commands to SCSI devices via 
Linux's sg_io driver (for a camera hacking project).  The extension is 
just a wrapper around a couple ioctl()'s with Pythonic exception handling 
thrown in.  One of my extension methods is called like this from python:

sg.write(fd, command[, data, timeout)

Both command and data are binary strings.  I would like to be able to use 
either a regular Python string or an array('B', ...) for these read-only 
arguments.  So I tried to use the "t#" argument specifier to indicate that 
these arguments could be either strings or objects that implement the read-
only buffer interface:

if (!PyArg_ParseTuple(args, "it#|t#i:write", &sg_fd, &cmd,
                      &cmdLen, &buf, &bufLen, &timeout))
    return NULL;

Now, this works fine with strings, but when I call it with an array I get 
a TypeError:

TypeError: write() argument 2 must be string or read-only character 
buffer, not array.array

So, I then tried changing "t#" to "w#" to indicate that the arguments must 
implement the /read-write/ buffer interface.  Now the array objects work, 
but when I try a string argument, I naturally get this error:

TypeError: Cannot use string as modifiable buffer

So here's what I don't understand.  Why doesn't the "t#" argument 
specifier support read-write buffers as well as read-only buffers?  Aren't 
read-write buffers a *superset* of read-only buffers??  Is there something 
I'm doing wrong or a quick fix to get this to work appropriately?

Thanks for any help!

Dan