cpython/Include/pyfpe.h

#ifndef Py_PYFPE_H
#define Py_PYFPE_H
#ifdef __cplusplus
extern "C" {
#endif
/*
     ---------------------------------------------------------------------  
    /                       Copyright (c) 1996.                           \ 
   |          The Regents of the University of California.                 |
   |                        All rights reserved.                           |
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   |   Permission to use, copy, modify, and distribute this software for   |
   |   any purpose without fee is hereby granted, provided that this en-   |
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   |   includes  a  copy  or  modification  of  this software and in all   |
   |   copies of the supporting documentation for such software.           |
   |                                                                       |
   |   This  work was produced at the University of California, Lawrence   |
   |   Livermore National Laboratory under  contract  no.  W-7405-ENG-48   |
   |   between  the  U.S.  Department  of  Energy and The Regents of the   |
   |   University of California for the operation of UC LLNL.              |
   |                                                                       |
   |                              DISCLAIMER                               |
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    \  endorsement purposes.                                              / 
     ---------------------------------------------------------------------  
*/

/*
 *       Define macros for handling SIGFPE.
 *       Lee Busby, LLNL, November, 1996
 *       busby1@llnl.gov
 * 
 *********************************************
 * Overview of the system for handling SIGFPE:
 * 
 * This file (Include/pyfpe.h) defines a couple of "wrapper" macros for
 * insertion into your Python C code of choice. Their proper use is
 * discussed below. The file Python/pyfpe.c defines a pair of global
 * variables PyFPE_jbuf and PyFPE_counter which are used by the signal
 * handler for SIGFPE to decide if a particular exception was protected
 * by the macros. The signal handler itself, and code for enabling the
 * generation of SIGFPE in the first place, is in a (new) Python module
 * named fpectl. This module is standard in every respect. It can be loaded
 * either statically or dynamically as you choose, and like any other
 * Python module, has no effect until you import it.
 * 
 * In the general case, there are three steps toward handling SIGFPE in any
 * Python code:
 * 
 * 1) Add the *_PROTECT macros to your C code as required to protect
 *    dangerous floating point sections.
 * 
 * 2) Turn on the inclusion of the code by #defining WANT_SIGFPE_HANDLER in
 *    config.h.in before you configure, compile, and install Python, and the
 *    fpectl module, and any other modules which may have conditional code.
 * 
 * 3) When python is built and running, import fpectl, and execute
 *    fpectl.turnon_sigfpe(). This sets up the signal handler and enables
 *    generation of SIGFPE whenever an exception occurs. From this point
 *    on, any properly trapped SIGFPE should result in the Python
 *    FloatingPointError exception.
 * 
 * Step 1 has been done already for the Python kernel code, and will be
 * done soon for Hugunin's NumPy array package and my Gist graphics module.
 * Step 2 is usually done once at python install time. Python's behavior
 * with respect to SIGFPE is not changed unless you also do step 3. Thus
 * you can control this new facility at compile time, or run time, or both.
 * 
 ******************************** 
 * Using the macros in your code:
 * 
 * static PyObject *foobar(PyObject *self,PyObject *args)
 * {
 *     ....
 *     PyFPE_START_PROTECT("Error in foobar", return 0)
 *     dangerous_op(somearg1, somearg2, ...);
 *     PyFPE_END_PROTECT
 *     ....
 * }
 * 
 * If a floating point error occurs in dangerous_op, foobar returns 0
 * (NULL), after setting the associated value of the FloatingPointError
 * exception to "Error in foobar". ``Dangerous_op'' can be a single
 * operation, or a block, or function calls, or any combination, so long as
 * no alternate return is possible before the PyFPE_END_PROTECT macro is
 * reached.
 * 
 * The macros can only be used in a function context where an error return
 * can be recognized as signaling a Python exception. (Generally, most
 * functions that return a PyObject * will qualify.)
 * 
 * Guido's original design suggestion for PyFPE_START_PROTECT and
 * PyFPE_END_PROTECT had them open and close a local block, with a locally
 * defined jmp_buf and jmp_buf pointer. This would allow recursive nesting
 * of the macros. The Ansi C standard makes it clear that such local
 * variables need to be declared with the "volatile" type qualifier to keep
 * setjmp from corrupting their values. Some current implementations seem
 * to be more restrictive. For example, the HPUX man page for setjmp says
 * 
 *   Upon the return from a setjmp() call caused by a longjmp(), the
 *   values of any non-static local variables belonging to the routine
 *   from which setjmp() was called are undefined. Code which depends on
 *   such values is not guaranteed to be portable.
 * 
 * I therefore decided on a more limited form of nesting, using a counter
 * variable (PyFPE_counter) to keep track of any recursion.  If an exception
 * occurs in an ``inner'' pair of macros, the return will apparently
 * come from the top level.
 * 
 */

#ifdef WANT_SIGFPE_HANDLER
#include <signal.h>
#include <setjmp.h>
#include <math.h>
extern jmp_buf PyFPE_jbuf;
extern int PyFPE_counter;
extern double PyFPE_dummy(void);

#define PyFPE_START_PROTECT(err_string, leave_stmt) \
if (!PyFPE_counter++ && setjmp(PyFPE_jbuf)) { \
	PyFPE_counter = 0; \
	PyErr_SetString(PyExc_FloatingPointError, err_string); \
	leave_stmt; \
}

/*
 * This (following) is a heck of a way to decrement a counter. However,
 * code optimizers will sometimes move this statement so that it gets
 * executed *before* the unsafe expression which we're trying to protect.
 * This pretty well messes things up, of course. So the best I've been able
 * to do is to put a (hopefully fast) function call into the expression
 * which counts down PyFPE_counter, and thereby monkey wrench the overeager
 * optimizer. Better solutions are welcomed....
 */
#define PyFPE_END_PROTECT PyFPE_counter -= (int)PyFPE_dummy();

#else

#define PyFPE_START_PROTECT(err_string, leave_stmt)
#define PyFPE_END_PROTECT

#endif

#ifdef __cplusplus
}
#endif
#endif /* !Py_PYFPE_H */