Commit 571bff88 authored by Liguo Kong's avatar Liguo Kong

modified example for calling a C function with a pointer (to array) in the signature.

parent dd8ac390
...@@ -77,3 +77,37 @@ Just like the ``sin()`` function from the math library, it is possible ...@@ -77,3 +77,37 @@ Just like the ``sin()`` function from the math library, it is possible
to declare and call into any C library as long as the module that to declare and call into any C library as long as the module that
Cython generates is properly linked against the shared or static Cython generates is properly linked against the shared or static
library. library.
Since use of pointers in C is ubiquitous, here we give a quick example of how
to call C functions whose arguments contain pointers. Suppose you want to
manage an array (allocate and deallocate) with Numpy, but its data are
computed by an external C function declared in :file:`C_func_file.h`::
void C_func(double * CPointer, unsigned int N);
where CPointer points to the array and N is its size.
You can call the function in a Cython file in the following way::
cdef extern from "C_func_file.h":
void C_func(double *, unsigned int)
import cython
import numpy as np
cimport numpy as np
def f(arr): # 'arr' is a one-dimensional array of size N
# Before calling the external function, we need to check whether the
# memory for 'arr' is contiguous or not; if not, we store the computed
# data in an contiguous array and then copy the data from that array.
np.ndarray[np.double_t, ndim=1, mode="c"] contig_arr
if arr.flags.c_contiguous:
contig_arr = arr
else:
contig_arr = arr.copy('C')
C_func(<cython.double *> contig_arr.data, contig_arr.size)
if contig_arr is not arr:
arr[...] = contig_arr
return
This way, you can have access the function more or less as a regular
Python function while its data and associated memory gracefully managed
by Numpy.
\ No newline at end of file
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