Question

I have a bytes string which is passed into a cython method, and I need to pass it into a c function:

def stuff(bytes b):
    #b is b'something'
    some_c_function(b, len(b))

The pointer I passed in is saved to some struct inside C-land. By the time it’s used, b has disappeared, so I get a segmentation fault. How do I keep it alive outside of the function?

I tried something like this:

from libc.stdlib import malloc, memcpy

def stuff(bytes b):
    cdef char *data
    data_len = len(b)
    data = malloc(data_len)
    memcpy(data, b, data_len)
    some_c_function(data, data_len)

But I get an error on the malloc call: Obtaining char * from temporary Python value.

Answer 1

1. You need to cimport the malloc function, otherwise Cython handles the import as default Python import and thinks malloc is a ordinary Python function.

2. data_len should be len(b) + 1 and the last byte must be set to 0, i.e.
   data[data_len - 1] = 0. This is the way how a string is terminated in C/C++.

3. After you’ve cimport-ed malloc and Cython treats the function as a C-function, you will need to make a cast, otherwise the Cython compiler will complain.

   data = <char*> malloc(data_len)

4. What you have malloc-ed, needs to be freed by yourself. Ensure that this will happen.