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A few nore words about what ctypes does.

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Document that using the wrong calling convention can also raise
'ValueError: Procedure called with the wrong number of arguments'.
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Thomas Heller 2006-08-02 11:35:31 +00:00
parent 5d32a9f188
commit 6a0ce407fb
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Doc/lib/libctypes.tex
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@ -6,13 +6,13 @@
\modulesynopsis{A foreign function library for Python.} \modulesynopsis{A foreign function library for Python.}
\versionadded{2.5} \versionadded{2.5}
\code{ctypes} is a foreign function library for Python. \code{ctypes} is a foreign function library for Python. It provides C
compatible data types, and allows to call functions in dlls/shared
libraries. It can be used to wrap these libraries in pure Python.
\subsection{ctypes tutorial\label{ctypes-ctypes-tutorial}} \subsection{ctypes tutorial\label{ctypes-ctypes-tutorial}}
This tutorial describes version 0.9.9 of \code{ctypes}.
Note: The code samples in this tutorial uses \code{doctest} to make sure Note: The code samples in this tutorial uses \code{doctest} to make sure
that they actually work. Since some code samples behave differently that they actually work. Since some code samples behave differently
under Linux, Windows, or Mac OS X, they contain doctest directives in under Linux, Windows, or Mac OS X, they contain doctest directives in
@ -150,8 +150,10 @@ be used as the NULL pointer):
\end{verbatim} \end{verbatim}
\code{ctypes} tries to protect you from calling functions with the wrong \code{ctypes} tries to protect you from calling functions with the wrong
number of arguments. Unfortunately this only works on Windows. It number of arguments or the wrong calling convention. Unfortunately
does this by examining the stack after the function returns: this only works on Windows, for \code{stdcall} functions. It does this
by examining the stack after the function returns, so although an
error is raised the function \emph{has} been called:
\begin{verbatim} \begin{verbatim}
>>> windll.kernel32.GetModuleHandleA() # doctest: +WINDOWS >>> windll.kernel32.GetModuleHandleA() # doctest: +WINDOWS
Traceback (most recent call last): Traceback (most recent call last):
@ -164,6 +166,25 @@ ValueError: Procedure probably called with too many arguments (4 bytes in excess
>>> >>>
\end{verbatim} \end{verbatim}
The same exception is raised when you call an \code{stdcall} function
with the \code{cdecl} calling convention, or vice versa:
\begin{verbatim}
>>> cdll.kernel32.GetModuleHandleA(None) # doctest: +WINDOWS
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: Procedure probably called with not enough arguments (4 bytes missing)
>>>
>>> windll.msvcrt.printf("spam") # doctest: +WINDOWS
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: Procedure probably called with too many arguments (4 bytes in excess)
>>>
\end{verbatim}
To find out the correct calling convention you have to look into the C
header file or the documentation for the function you want to call.
On Windows, \code{ctypes} uses win32 structured exception handling to On Windows, \code{ctypes} uses win32 structured exception handling to
prevent crashes from general protection faults when functions are prevent crashes from general protection faults when functions are
called with invalid argument values: called with invalid argument values: