String buffer to be filled by native side

Micropython
#1

Several places in the httpd expect that the user first requests a length and then provides a char * buffer with matching space.

I would like to provide a bytearray or similar as the buffer. But MP expects a string when the parameter was a char *.

My current solution looks like this:

def get_header_value(req, name):
    blen = req.get_hdr_value_len(name);
    if not blen: return None
    buffer = " "*blen         # allocate a string as buffer incl. space for \0
    req.get_hdr_value_str(name, buffer, blen+1);
    return buffer;

I am not sure if this is a safe solution. Are python string buffers always 0 terminated char arrays internally so that " "*32 is actually 33 bytes? Can I just overwrite them?

#2

In Python, “string” in an immutable container, so in principle we are not supposed to change it.
Practically, this would probably still work because in Micropython strings are represented internally as C string (when not interned).

I recommend you pass bytes instead of string, to make things clearer, although bytes is still an immutable container. It’s also cleaner to initialize a buffer as bytes(len) instead of building a string of spaces.

If you really want to be on the safe side, create a wrapper that receives void * instead of char *, and pass to it a bytearray or memoryview.

If this turns up to be a problem, I could check if I can change the binding script to also accept bytearray/memoryview when handling strings.

#3

Thanks. Yes, indeed, bytes is a much better choice and would even work with encodings different than the MP internal one.

#4

I’ve added the capability to pass bytearray/memoryview in places where string (char *) is expected.

#5

Works as advertised! Thanks a lot. Being able to pass something mutable definitely makes things nicer.