Include/pyfpe.h

   1 #ifndef Py_PYFPE_H
   2 #define Py_PYFPE_H
   3 #ifdef __cplusplus
   4 extern "C" {
   5 #endif
   6 /*
   7      ---------------------------------------------------------------------
   8     /                       Copyright (c) 1996.                           \
   9    |          The Regents of the University of California.                 |
  10    |                        All rights reserved.                           |
  11    |                                                                       |
  12    |   Permission to use, copy, modify, and distribute this software for   |
  13    |   any purpose without fee is hereby granted, provided that this en-   |
  14    |   tire notice is included in all copies of any software which is or   |
  15    |   includes  a  copy  or  modification  of  this software and in all   |
  16    |   copies of the supporting documentation for such software.           |
  17    |                                                                       |
  18    |   This  work was produced at the University of California, Lawrence   |
  19    |   Livermore National Laboratory under  contract  no.  W-7405-ENG-48   |
  20    |   between  the  U.S.  Department  of  Energy and The Regents of the   |
  21    |   University of California for the operation of UC LLNL.              |
  22    |                                                                       |
  23    |                              DISCLAIMER                               |
  24    |                                                                       |
  25    |   This  software was prepared as an account of work sponsored by an   |
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  30    |   usefulness of any information,  apparatus,  product,  or  process   |
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  40     \  endorsement purposes.                                              /
  41      ---------------------------------------------------------------------
  42 */
  43
  44 /*
  45  *       Define macros for handling SIGFPE.
  46  *       Lee Busby, LLNL, November, 1996
  47  *       busby1@llnl.gov
  48  *
  49  *********************************************
  50  * Overview of the system for handling SIGFPE:
  51  *
  52  * This file (Include/pyfpe.h) defines a couple of "wrapper" macros for
  53  * insertion into your Python C code of choice. Their proper use is
  54  * discussed below. The file Python/pyfpe.c defines a pair of global
  55  * variables PyFPE_jbuf and PyFPE_counter which are used by the signal
  56  * handler for SIGFPE to decide if a particular exception was protected
  57  * by the macros. The signal handler itself, and code for enabling the
  58  * generation of SIGFPE in the first place, is in a (new) Python module
  59  * named fpectl. This module is standard in every respect. It can be loaded
  60  * either statically or dynamically as you choose, and like any other
  61  * Python module, has no effect until you import it.
  62  *
  63  * In the general case, there are three steps toward handling SIGFPE in any
  64  * Python code:
  65  *
  66  * 1) Add the *_PROTECT macros to your C code as required to protect
  67  *    dangerous floating point sections.
  68  *
  69  * 2) Turn on the inclusion of the code by adding the ``--with-fpectl''
  70  *    flag at the time you run configure.  If the fpectl or other modules
  71  *    which use the *_PROTECT macros are to be dynamically loaded, be
  72  *    sure they are compiled with WANT_SIGFPE_HANDLER defined.
  73  *
  74  * 3) When python is built and running, import fpectl, and execute
  75  *    fpectl.turnon_sigfpe(). This sets up the signal handler and enables
  76  *    generation of SIGFPE whenever an exception occurs. From this point
  77  *    on, any properly trapped SIGFPE should result in the Python
  78  *    FloatingPointError exception.
  79  *
  80  * Step 1 has been done already for the Python kernel code, and should be
  81  * done soon for the NumPy array package.  Step 2 is usually done once at
  82  * python install time. Python's behavior with respect to SIGFPE is not
  83  * changed unless you also do step 3. Thus you can control this new
  84  * facility at compile time, or run time, or both.
  85  *
  86  ********************************
  87  * Using the macros in your code:
  88  *
  89  * static PyObject *foobar(PyObject *self,PyObject *args)
  90  * {
  91  *     ....
  92  *     PyFPE_START_PROTECT("Error in foobar", return 0)
  93  *     result = dangerous_op(somearg1, somearg2, ...);
  94  *     PyFPE_END_PROTECT(result)
  95  *     ....
  96  * }
  97  *
  98  * If a floating point error occurs in dangerous_op, foobar returns 0 (NULL),
  99  * after setting the associated value of the FloatingPointError exception to
 100  * "Error in foobar". ``Dangerous_op'' can be a single operation, or a block
 101  * of code, function calls, or any combination, so long as no alternate
 102  * return is possible before the PyFPE_END_PROTECT macro is reached.
 103  *
 104  * The macros can only be used in a function context where an error return
 105  * can be recognized as signaling a Python exception. (Generally, most
 106  * functions that return a PyObject * will qualify.)
 107  *
 108  * Guido's original design suggestion for PyFPE_START_PROTECT and
 109  * PyFPE_END_PROTECT had them open and close a local block, with a locally
 110  * defined jmp_buf and jmp_buf pointer. This would allow recursive nesting
 111  * of the macros. The Ansi C standard makes it clear that such local
 112  * variables need to be declared with the "volatile" type qualifier to keep
 113  * setjmp from corrupting their values. Some current implementations seem
 114  * to be more restrictive. For example, the HPUX man page for setjmp says
 115  *
 116  *   Upon the return from a setjmp() call caused by a longjmp(), the
 117  *   values of any non-static local variables belonging to the routine
 118  *   from which setjmp() was called are undefined. Code which depends on
 119  *   such values is not guaranteed to be portable.
 120  *
 121  * I therefore decided on a more limited form of nesting, using a counter
 122  * variable (PyFPE_counter) to keep track of any recursion.  If an exception
 123  * occurs in an ``inner'' pair of macros, the return will apparently
 124  * come from the outermost level.
 125  *
 126  */
 127
 128 #ifdef WANT_SIGFPE_HANDLER
 129 #include <signal.h>
 130 #include <setjmp.h>
 131 #include <math.h>
 132 extern jmp_buf PyFPE_jbuf;
 133 extern int PyFPE_counter;
 134 extern double PyFPE_dummy(void *);
 135
 136 #define PyFPE_START_PROTECT(err_string, leave_stmt) \
 137 if (!PyFPE_counter++ && setjmp(PyFPE_jbuf)) { \
 138         PyErr_SetString(PyExc_FloatingPointError, err_string); \
 139         PyFPE_counter = 0; \
 140         leave_stmt; \
 141 }
 142
 143 /*
 144  * This (following) is a heck of a way to decrement a counter. However,
 145  * unless the macro argument is provided, code optimizers will sometimes move
 146  * this statement so that it gets executed *before* the unsafe expression
 147  * which we're trying to protect.  That pretty well messes things up,
 148  * of course.
 149  *
 150  * If the expression(s) you're trying to protect don't happen to return a
 151  * value, you will need to manufacture a dummy result just to preserve the
 152  * correct ordering of statements.  Note that the macro passes the address
 153  * of its argument (so you need to give it something which is addressable).
 154  * If your expression returns multiple results, pass the last such result
 155  * to PyFPE_END_PROTECT.
 156  *
 157  * Note that PyFPE_dummy returns a double, which is cast to int.
 158  * This seeming insanity is to tickle the Floating Point Unit (FPU).
 159  * If an exception has occurred in a preceding floating point operation,
 160  * some architectures (notably Intel 80x86) will not deliver the interrupt
 161  * until the *next* floating point operation.  This is painful if you've
 162  * already decremented PyFPE_counter.
 163  */
 164 #define PyFPE_END_PROTECT(v) PyFPE_counter -= (int)PyFPE_dummy(&(v));
 165
 166 #else
 167
 168 #define PyFPE_START_PROTECT(err_string, leave_stmt)
 169 #define PyFPE_END_PROTECT(v)
 170
 171 #endif
 172
 173 #ifdef __cplusplus
 174 }
 175 #endif
 176 #endif /* !Py_PYFPE_H */