diff options
Diffstat (limited to 'Doc/ext')
| -rw-r--r-- | Doc/ext/ext.tex | 165 |
1 files changed, 81 insertions, 84 deletions
diff --git a/Doc/ext/ext.tex b/Doc/ext/ext.tex index d168aa6..af4a667 100644 --- a/Doc/ext/ext.tex +++ b/Doc/ext/ext.tex @@ -82,11 +82,11 @@ This function takes a null-terminated character string as argument and returns an integer. We want this function to be callable from Python as follows: -\begin{verbatim} +\bcode\begin{verbatim} >>> import spam >>> status = spam.system("ls -l") -\end{verbatim} - +\end{verbatim}\ecode +% Begin by creating a file \samp{spammodule.c}. (In general, if a module is called \samp{spam}, the C file containing its implementation is called \file{spammodule.c}; if the module name is very long, like @@ -94,10 +94,10 @@ is called \file{spammodule.c}; if the module name is very long, like The first line of our file can be: -\begin{verbatim} +\bcode\begin{verbatim} #include "Python.h" -\end{verbatim} - +\end{verbatim}\ecode +% which pulls in the Python API (you can add a comment describing the purpose of the module and a copyright notice if you like). @@ -114,7 +114,7 @@ The next thing we add to our module file is the C function that will be called when the Python expression \samp{spam.system(\var{string})} is evaluated (we'll see shortly how it ends up being called): -\begin{verbatim} +\bcode\begin{verbatim} static PyObject * spam_system(self, args) PyObject *self; @@ -127,8 +127,8 @@ is evaluated (we'll see shortly how it ends up being called): sts = system(command); return Py_BuildValue("i", sts); } -\end{verbatim} - +\end{verbatim}\ecode +% There is a straightforward translation from the argument list in Python (e.g.\ the single expression \code{"ls -l"}) to the arguments passed to the C function. The C function always has two arguments, @@ -252,15 +252,15 @@ You can also define a new exception that is unique to your module. For this, you usually declare a static object variable at the beginning of your file, e.g. -\begin{verbatim} +\bcode\begin{verbatim} static PyObject *SpamError; -\end{verbatim} - +\end{verbatim}\ecode +% and initialize it in your module's initialization function (\code{initspam()}) with a string object, e.g. (leaving out the error checking for now): -\begin{verbatim} +\bcode\begin{verbatim} void initspam() { @@ -270,8 +270,8 @@ checking for now): SpamError = PyString_FromString("spam.error"); PyDict_SetItemString(d, "error", SpamError); } -\end{verbatim} - +\end{verbatim}\ecode +% Note that the Python name for the exception object is \code{spam.error}. It is conventional for module and exception names to be spelled in lower case. It is also conventional that the @@ -284,11 +284,11 @@ the string \code{"spam.error"}. Going back to our example function, you should now be able to understand this statement: -\begin{verbatim} +\bcode\begin{verbatim} if (!PyArg_ParseTuple(args, "s", &command)) return NULL; -\end{verbatim} - +\end{verbatim}\ecode +% It returns \code{NULL} (the error indicator for functions returning object pointers) if an error is detected in the argument list, relying on the exception set by \code{PyArg_ParseTuple()}. Otherwise the @@ -301,10 +301,10 @@ to modify the string to which it points (so in Standard C, the variable The next statement is a call to the \UNIX{} function \code{system()}, passing it the string we just got from \code{PyArg_ParseTuple()}: -\begin{verbatim} +\bcode\begin{verbatim} sts = system(command); -\end{verbatim} - +\end{verbatim}\ecode +% Our \code{spam.system()} function must return the value of \code{sts} as a Python object. This is done using the function \code{Py_BuildValue()}, which is something like the inverse of @@ -312,10 +312,10 @@ as a Python object. This is done using the function number of C values, and returns a new Python object. More info on \code{Py_BuildValue()} is given later. -\begin{verbatim} +\bcode\begin{verbatim} return Py_BuildValue("i", sts); -\end{verbatim} - +\end{verbatim}\ecode +% In this case, it will return an integer object. (Yes, even integers are objects on the heap in Python!) @@ -323,11 +323,11 @@ If you have a C function that returns no useful argument (a function returning \code{void}), the corresponding Python function must return \code{None}. You need this idiom to do so: -\begin{verbatim} +\bcode\begin{verbatim} Py_INCREF(Py_None); return Py_None; -\end{verbatim} - +\end{verbatim}\ecode +% \code{Py_None} is the C name for the special Python object \code{None}. It is a genuine Python object (not a \code{NULL} pointer, which means ``error'' in most contexts, as we have seen). @@ -339,15 +339,15 @@ I promised to show how \code{spam_system()} is called from Python programs. First, we need to list its name and address in a ``method table'': -\begin{verbatim} +\bcode\begin{verbatim} static PyMethodDef SpamMethods[] = { ... {"system", spam_system, 1}, ... {NULL, NULL} /* Sentinel */ }; -\end{verbatim} - +\end{verbatim}\ecode +% Note the third entry (\samp{1}). This is a flag telling the interpreter the calling convention to be used for the C function. It should normally always be \samp{1}; a value of \samp{0} means that an @@ -357,14 +357,14 @@ The method table must be passed to the interpreter in the module's initialization function (which should be the only non-\code{static} item defined in the module file): -\begin{verbatim} +\bcode\begin{verbatim} void initspam() { (void) Py_InitModule("spam", SpamMethods); } -\end{verbatim} - +\end{verbatim}\ecode +% When the Python program imports module \code{spam} for the first time, \code{initspam()} is called. It calls \code{Py_InitModule()}, which creates a ``module object'' (which is inserted in the dictionary @@ -392,10 +392,10 @@ very simple: just place your file (\file{spammodule.c} for example) in the \file{Modules} directory, add a line to the file \file{Modules/Setup} describing your file: -\begin{verbatim} +\bcode\begin{verbatim} spam spammodule.o -\end{verbatim} - +\end{verbatim}\ecode +% and rebuild the interpreter by running \code{make} in the toplevel directory. You can also run \code{make} in the \file{Modules} subdirectory, but then you must first rebuilt the \file{Makefile} @@ -405,11 +405,10 @@ you change the \file{Setup} file.) If your module requires additional libraries to link with, these can be listed on the line in the \file{Setup} file as well, for instance: -\begin{verbatim} +\bcode\begin{verbatim} spam spammodule.o -lX11 -\end{verbatim} - - +\end{verbatim}\ecode +% \section{Calling Python Functions From C} So far we have concentrated on making C functions callable from @@ -434,7 +433,7 @@ called, save a pointer to the Python function object (be careful to For example, the following function might be part of a module definition: -\begin{verbatim} +\bcode\begin{verbatim} static PyObject *my_callback = NULL; static PyObject * @@ -448,8 +447,8 @@ definition: Py_INCREF(Py_None); return Py_None; } -\end{verbatim} - +\end{verbatim}\ecode +% The macros \code{Py_XINCREF()} and \code{Py_XDECREF()} increment/decrement the reference count of an object and are safe in the presence of \code{NULL} pointers. More info on them in the section on Reference @@ -465,7 +464,7 @@ a singleton tuple. \code{Py_BuildValue()} returns a tuple when its format string consists of zero or more format codes between parentheses. For example: -\begin{verbatim} +\bcode\begin{verbatim} int arg; PyObject *arglist; PyObject *result; @@ -476,8 +475,8 @@ parentheses. For example: arglist = Py_BuildValue("(i)", arg); result = PyEval_CallObject(my_callback, arglist); Py_DECREF(arglist); -\end{verbatim} - +\end{verbatim}\ecode +% \code{PyEval_CallObject()} returns a Python object pointer: this is the return value of the Python function. \code{PyEval_CallObject()} is ``reference-count-neutral'' with respect to its arguments. In the @@ -499,13 +498,13 @@ calling Python code can handle the exception. If this is not possible or desirable, the exception should be cleared by calling \code{PyErr_Clear()}. For example: -\begin{verbatim} +\bcode\begin{verbatim} if (result == NULL) return NULL; /* Pass error back */ ...use result... Py_DECREF(result); -\end{verbatim} - +\end{verbatim}\ecode +% Depending on the desired interface to the Python callback function, you may also have to provide an argument list to \code{PyEval_CallObject()}. In some cases the argument list is also provided by the Python @@ -516,7 +515,7 @@ tuple to pass as the argument list. The simplest way to do this is to call \code{Py_BuildValue()}. For example, if you want to pass an integral event code, you might use the following code: -\begin{verbatim} +\bcode\begin{verbatim} PyObject *arglist; ... arglist = Py_BuildValue("(l)", eventcode); @@ -526,8 +525,8 @@ event code, you might use the following code: return NULL; /* Pass error back */ /* Here maybe use the result */ Py_DECREF(result); -\end{verbatim} - +\end{verbatim}\ecode +% Note the placement of \code{Py_DECREF(argument)} immediately after the call, before the error check! Also note that strictly spoken this code is not complete: \code{Py_BuildValue()} may run out of memory, and this should @@ -538,10 +537,10 @@ be checked. The \code{PyArg_ParseTuple()} function is declared as follows: -\begin{verbatim} +\bcode\begin{verbatim} int PyArg_ParseTuple(PyObject *arg, char *format, ...); -\end{verbatim} - +\end{verbatim}\ecode +% The \var{arg} argument must be a tuple object containing an argument list passed from Python to a C function. The \var{format} argument must be a format string, whose syntax is explained below. The @@ -686,7 +685,7 @@ Clearly, \samp{:} and \samp{;} mutually exclude each other. Some example calls: -\begin{verbatim} +\bcode\begin{verbatim} int ok; int i, j; long k, l; @@ -726,18 +725,17 @@ Some example calls: /* Possible Python call: f(((0, 0), (400, 300)), (10, 10)) */ } -\end{verbatim} - - +\end{verbatim}\ecode +% \section{The {\tt Py_BuildValue()} Function} This function is the counterpart to \code{PyArg_ParseTuple()}. It is declared as follows: -\begin{verbatim} +\bcode\begin{verbatim} PyObject *Py_BuildValue(char *format, ...); -\end{verbatim} - +\end{verbatim}\ecode +% It recognizes a set of format units similar to the ones recognized by \code{PyArg_ParseTuple()}, but the arguments (which are input to the function, not output) must not be pointers, just values. It returns a @@ -839,7 +837,7 @@ If there is an error in the format string, the Examples (to the left the call, to the right the resulting Python value): -\begin{verbatim} +\bcode\begin{verbatim} Py_BuildValue("") None Py_BuildValue("i", 123) 123 Py_BuildValue("iii", 123, 456, 789) (123, 456, 789) @@ -855,9 +853,8 @@ Examples (to the left the call, to the right the resulting Python value): "abc", 123, "def", 456) {'abc': 123, 'def': 456} Py_BuildValue("((ii)(ii)) (ii)", 1, 2, 3, 4, 5, 6) (((1, 2), (3, 4)), (5, 6)) -\end{verbatim} - - +\end{verbatim}\ecode +% \section{Reference Counts} \subsection{Introduction} @@ -1026,14 +1023,14 @@ The first and most important case to know about is using \code{Py_DECREF()} on an unrelated object while borrowing a reference to a list item. For instance: -\begin{verbatim} +\bcode\begin{verbatim} bug(PyObject *list) { PyObject *item = PyList_GetItem(list, 0); PyList_SetItem(list, 1, PyInt_FromLong(0L)); PyObject_Print(item, stdout, 0); /* BUG! */ } -\end{verbatim} - +\end{verbatim}\ecode +% This function first borrows a reference to \code{list[0]}, then replaces \code{list[1]} with the value \code{0}, and finally prints the borrowed reference. Looks harmless, right? But it's not! @@ -1059,7 +1056,7 @@ The solution, once you know the source of the problem, is easy: temporarily increment the reference count. The correct version of the function reads: -\begin{verbatim} +\bcode\begin{verbatim} no_bug(PyObject *list) { PyObject *item = PyList_GetItem(list, 0); Py_INCREF(item); @@ -1067,8 +1064,8 @@ no_bug(PyObject *list) { PyObject_Print(item, stdout, 0); Py_DECREF(item); } -\end{verbatim} - +\end{verbatim}\ecode +% This is a true story. An older version of Python contained variants of this bug and someone spent a considerable amount of time in a C debugger to figure out why his \code{__del__()} methods would fail... @@ -1084,7 +1081,7 @@ calls, to let other threads use the CPU while waiting for the I/O to complete. Obviously, the following function has the same problem as the previous one: -\begin{verbatim} +\bcode\begin{verbatim} bug(PyObject *list) { PyObject *item = PyList_GetItem(list, 0); Py_BEGIN_ALLOW_THREADS @@ -1092,8 +1089,8 @@ bug(PyObject *list) { Py_END_ALLOW_THREADS PyObject_Print(item, stdout, 0); /* BUG! */ } -\end{verbatim} - +\end{verbatim}\ecode +% \subsection{NULL Pointers} In general, functions that take object references as arguments don't @@ -1300,20 +1297,20 @@ done using a special invocation of the \UNIX{} loader/linker, {\em ld}(1). Unfortunately the invocation differs slightly per system. On SunOS 4, use -\begin{verbatim} +\bcode\begin{verbatim} ld spammodule.o -o spammodule.so -\end{verbatim} - +\end{verbatim}\ecode +% On Solaris 2, use -\begin{verbatim} +\bcode\begin{verbatim} ld -G spammodule.o -o spammodule.so -\end{verbatim} - +\end{verbatim}\ecode +% On SGI IRIX 5, use -\begin{verbatim} +\bcode\begin{verbatim} ld -shared spammodule.o -o spammodule.so -\end{verbatim} - +\end{verbatim}\ecode +% On other systems, consult the manual page for \code{ld}(1) to find what flags, if any, must be used. |