[moving this to cython-users]
Robert Bradshaw, 03.05.2011 06:38:
On Fri, Apr 29, 2011 at 6:57 AM, Hans Terlouw wrote:
Recently I encountered a problem with Cython's automatic char* to string
conversion (Cython version 0.14.1). I'll attach two sample source files. The
first one, char2str_a.pyx prints "The quick...", just as I expected. But the
second example prints "... lazy dog.". In the original situation I had a
call to
free() instead of the call to strcpy() which I use here for illustration
purposes. Then I got unpredictable results. Apparently the Python string
object
keeps referring to the C char* a bit longer than I would expect. A previous
version (0.11.2) didn't have this problem.
This is due to type inference, in the second example, p_str is
inferred to be of type char*.
Just to make this a bit clearer:
cdef extern from "stdlib.h":
void free(void* ptr)
void* malloc(size_t size)
cdef extern from "string.h":
char *strcpy(char *dest, char *src)
def char2str():
cdef char *c_str_a =<char*>malloc(80)
cdef char *c_str_b = "The quick... "
cdef char *c_str_c = "... lazy dog. "
strcpy(c_str_a, c_str_b)
p_str = c_str_a
strcpy(c_str_a, c_str_c)
p_str = p_str.rstrip()
print p_str
In this example, p_str is assigned both a char* and a Python object, so
type inference makes it a Python object. The first assignment is therefore
a copy operation that creates a Python bytes object, and the second
operation assigns the object returned from the .rstrip() call.
cdef extern from "stdlib.h":
void free(void* ptr)
void* malloc(size_t size)
cdef extern from "string.h":
char *strcpy(char *dest, char *src)
def char2str():
cdef char *c_str_a =<char*>malloc(80)
cdef char *c_str_b = "The quick... "
cdef char *c_str_c = "... lazy dog. "
strcpy(c_str_a, c_str_b)
p_str = c_str_a
strcpy(c_str_a, c_str_c)
print p_str.rstrip()
Here, p_str is only assigned once from a pointer, so the type is inferred
as a char*, and the first assignment is a pointer assignment, not a copy
operation.
You can see the difference with "cython -a", which generates an HTML
representation of your code that highlights Python object operations.
(Click on a source line to see the C code).
Stefan
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