Calling C functionsΒΆ
This tutorial describes shortly what you need to know in order to call C library functions from Cython code. For a longer and more comprehensive tutorial about using external C libraries, wrapping them and handling errors, see Using C libraries.
For simplicity, let’s start with a function from the standard C library. This does not add any dependencies to your code, and it has the additional advantage that Cython already defines many such functions for you. So you can just cimport and use them.
For example, let’s say you need a low-level way to parse a number from
a char*
value. You could use the atoi()
function, as defined
by the stdlib.h
header file. This can be done as follows:
from libc.stdlib cimport atoi
cdef parse_charptr_to_py_int(char* s):
assert s is not NULL, "byte string value is NULL"
return atoi(s) # note: atoi() has no error detection!
You can find a complete list of these standard cimport files in
Cython’s source package Cython/Includes/
. It also has a complete
set of declarations for CPython’s C-API. For example, to test at C
compilation time which CPython version your code is being compiled
with, you can do this:
from cpython.version cimport PY_VERSION_HEX
print PY_VERSION_HEX >= 0x030200F0 # Python version >= 3.2 final
Cython also provides declarations for the C math library:
from libc.math cimport sin
cdef double f(double x):
return sin(x*x)
However, this is a library that is not linked by default on some Unix-like
systems, such as Linux. In addition to cimporting the
declarations, you must configure your build system to link against the
shared library m
. For distutils, it is enough to add it to the
libraries
parameter of the Extension()
setup:
from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext
ext_modules=[
Extension("demo",
["demo.pyx"],
libraries=["m"]) # Unix-like specific
]
setup(
name = "Demos",
cmdclass = {"build_ext": build_ext},
ext_modules = ext_modules
)
If you want to access C code for which Cython does not provide a ready
to use declaration, you must declare them yourself. For example, the
above sin()
function is defined as follows:
cdef extern from "math.h":
double sin(double)
This declares the sin()
function in a way that makes it available
to Cython code and instructs Cython to generate C code that includes
the math.h
header file. The C compiler will see the original
declaration in math.h
at compile time, but Cython does not parse
“math.h” and requires a separate definition.
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
Cython generates is properly linked against the shared or static
library.