Skip to content


Subversion checkout URL

You can clone with
Download ZIP
Browse files

rewrote the initial tutorial on calling into C libraries

  • Loading branch information...
1 parent 2a05327 commit 9be1a5271548d6ffe3bb42bbdcf16deaac2cb958 Stefan Behnel committed
Showing with 80 additions and 0 deletions.
  1. +79 −0 src/tutorial/external.rst
  2. +1 −0  src/tutorial/index.rst
79 src/tutorial/external.rst
@@ -0,0 +1,79 @@
+Calling C functions
+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 :doc:`clibraries`.
+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 Unix-like
+systems, such as Linux or MacOS-X. 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 instructs Cython to generate code that imports the ``math.h``
+header file and declares the ``sin()`` function in a way that makes it
+available to Cython code. 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
1  src/tutorial/index.rst
@@ -4,6 +4,7 @@ Tutorials
.. toctree::
:maxdepth: 2
+ external

0 comments on commit 9be1a52

Please sign in to comment.
Something went wrong with that request. Please try again.