Author: Matti Picus <[email protected]>
Branch: doc
Changeset: r3139:bb2d9d0e810b
Date: 2018-08-13 19:38 -0700
http://bitbucket.org/cffi/cffi/changeset/bb2d9d0e810b/
Log: edit overview
diff --git a/doc/source/overview.rst b/doc/source/overview.rst
--- a/doc/source/overview.rst
+++ b/doc/source/overview.rst
@@ -5,10 +5,11 @@
.. contents::
-This document starts, in the first section, with a simple working
-example of using CFFI to call a C function from Python. CFFI is
+The first section presents a simple working
+example of using CFFI to call a C function in a compiled shared objecti
+from Python. CFFI is
flexible and covers several other use cases presented in the second
-section. Then, the next section shows how to export Python functions
+section. The third section shows how to export Python functions
to a Python interpreter embedded in a C or C++ application. The last
two sections delve deeper in the CFFI library.
@@ -24,12 +25,13 @@
------------------
The main way to use CFFI is as an interface to some already-compiled
-library which is provided by other means. Imagine that you have a
-system-installed library called ``piapprox.dll`` (Windows) or
+shared object which is provided by other means. Imagine that you have a
+system-installed shared object called ``piapprox.dll`` (Windows) or
``libpiapprox.so`` (Linux and others) or ``libpiapprox.dylib`` (OS X),
-containing a function ``float pi_approx(int n);`` that computes some
-approximation of pi given a number of iterations. You want to call
-this function from Python.
+exporting a function ``float pi_approx(int n);`` that computes some
+approximation of pi given a number of iterations. You want to call
+this function from Python. Note this method works equally as well with a
+library ``piapprox.lib`` (Windows) or ``libpiapprox.a``.
Create the file ``piapprox_build.py``:
@@ -38,13 +40,15 @@
from cffi import FFI
ffibuilder = FFI()
- # cdef() expects a string listing the C types, functions and
- # globals needed from Python. The string follows the C syntax.
+ # cdef() expects a string declaring the C types, functions and
+ # globals needed to use the shared object. It must be in valid C syntax.
ffibuilder.cdef("""
float pi_approx(int n);
""")
- # This describes the extension module "_pi_cffi" to produce.
+ # set_source() names the python extension module "_pi_cffi" to produce
+ # and the C source code as a string that once compiled will call the
+ # function.
ffibuilder.set_source("_pi_cffi",
"""
#include "pi.h" // the C header of the library
@@ -57,7 +61,7 @@
Execute this script. If everything is OK, it should produce
``_pi_cffi.c``, and then invoke the compiler on it. The produced
``_pi_cffi.c`` contains a copy of the string given in ``set_source()``,
-in this example the ``#include "pi.h"``; then it contains some glue code
+in this example the ``#include "pi.h"``. It also contains some glue code
for all the functions declared in the ``cdef()`` above.
At runtime, you use the extension module like this:
@@ -113,6 +117,8 @@
Simple example (ABI level, in-line)
+++++++++++++++++++++++++++++++++++
+May look familiar to those who have used ctypes_.
+
.. code-block:: python
>>> from cffi import FFI
@@ -121,15 +127,14 @@
... int printf(const char *format, ...); // copy-pasted from the man
page
... """)
>>> C = ffi.dlopen(None) # loads the entire C namespace
- >>> arg = ffi.new("char[]", "world") # equivalent to C code: char
arg[] = "world";
- >>> C.printf("hi there, %s.\n", arg) # call printf
+ >>> arg = ffi.new("char[]", b"world") # equivalent to C code: char
arg[] = "world";
+ >>> C.printf(b"hi there, %s.\n", arg) # call printf
hi there, world.
17 # this is the return value
>>>
-Note that on Python 3 you need to pass byte strings to ``char *``
-arguments. In the above example it would be ``b"world"`` and ``b"hi
-there, %s!\n"``. In general it is ``somestring.encode(myencoding)``.
+In general you would encode python ``str`` to ``bytes`` with
+``somestring.encode(myencoding)`` on Python3.
*Python 3 on Windows:* ``ffi.dlopen(None)`` does not work. This problem
is messy and not really fixable. The problem does not occur if you try
@@ -172,11 +177,12 @@
f.close()
This can be used as a more flexible replacement of the struct_ and
-array_ modules. You could also call ``ffi.new("pixel_t[600][800]")``
+array_ modules, and replaces ctypes_. You could also call
``ffi.new("pixel_t[600][800]")``
and get a two-dimensional array.
.. _struct: http://docs.python.org/library/struct.html
.. _array: http://docs.python.org/library/array.html
+.. _ctypes: http://docs.python.org/library/ctypes.html
*This example does not call any C compiler.*
@@ -430,6 +436,7 @@
from cffi import FFI
ffibuilder = FFI()
+ # Note that the actual source is None
ffibuilder.set_source("_simple_example", None)
ffibuilder.cdef("""
int printf(const char *format, ...);
@@ -478,13 +485,17 @@
)
+In summary, this mode is useful when you wish to declare many C structures but
+do not need fast interaction with a shared object. It is useful for parsing
+binary files, for instance.
+
In-line, API level
++++++++++++++++++
The "API level + in-line" mode combination exists but is long
deprecated. It used to be done with ``lib = ffi.verify("C header")``.
The out-of-line variant with ``set_source("modname", "C header")`` is
-preferred. It avoids a number of problems when the project grows in
+preferred and avoids a number of problems when the project grows in
size.
@@ -589,8 +600,8 @@
needs to go through the very general *libffi* library, which is slow
(and not always perfectly tested on non-standard platforms). The API
mode instead compiles a CPython C wrapper that directly invokes the
-target function. It is, comparatively, massively faster (and works
-better than libffi ever can).
+target function. It can be massively faster (and works
+better than libffi ever will).
The more fundamental reason to prefer the API mode is that *the C
libraries are typically meant to be used with a C compiler.* You are not
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