On 24/09/14 20:13, Armin Rigo wrote:
Hi,
On 23 September 2014 14:54, Eleytherios Stamatogiannakis
<est...@gmail.com> wrote:
p = clib.getString(...) # a "char *"
length = clib.strlen(p) # the standard strlen() function from C
b = unicode(ffi.buffer(p, length), 'utf-8')
I've tried that, and the overhead of the second call is more or less equal
to the cost of the copy when using ffi.string.
You cannot have a C function returning a 'char[]'. That's why you
need to declare it returning a 'char *', and then you don't know the
length. Sorry, it's the way C works; there is nothing I can do about
that :-)
Thank you for clarifying. I thought that ffi.buffer scanned for the \0
to find the end of the string for "char[]" types.
Occasionally, we see C functions with this kind of signature:
size_t getString(xxx, char **result);
This would return the length, and use 'result' as an output parameter,
to store into '*result' a pointer to the string. If you really care
about performance, then you might want to change the C library you're
binding to in order to do that.
Unfortunately, the C library that i use (libsqlite3) does not provide a
function like that :( . It has a function that returns the size of the
string, but in my tests the overhead of doing another CFFI call (to find
the size) is greater than doing the 2nd copy (depending on the average
string size).
We are doing 100s of millions of string passing calls back and forth
from the libsqlite3 library, so any way to improve the efficiency of
this case would be more than welcome :) .
Best regards,
l.
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