Writing User Exception Handler Functions (Fortran Programming Guide)

Fortran Programming Guide

Writing User Exception Handler Functions

The actions your exception handler takes are up to you. However, the routine must be an integer function with three arguments specified as shown:

handler_name( sig, sip, uap )

handler_name is the name of the integer function.
sig is an integer.
sip is a record that has the structure siginfo.

uap is not used.

Example: An exception handler function:

      INTEGER FUNCTION hand( sig, sip, uap ) 
      INTEGER sig, location
      STRUCTURE /fault/
           INTEGER address
           INTEGER trapno
      END STRUCTURE
      STRUCTURE /siginfo/
           INTEGER si_signo
           INTEGER si_code
           INTEGER si_errno
           RECORD /fault/ fault
      END STRUCTURE
      RECORD /siginfo/ sip
      location = sip.fault.address
      ... actions you take ...
      END

This f77 example would have to be modified to run on SPARC V9 architectures (-xarch=v9 or v9a) by replacing all INTEGER declarations within each STRUCTURE with INTEGER*8.

If the handler routine enabled by ieee_handler() is in Fortran as shown in the example, the routine should not make any reference to its first argument (sig). This first argument is passed by value to the routine and can only be referenced as loc(sig). The value is the signal number.

Detecting an Exception by Handler

The following examples show how to create handler routines to detect floating-point exceptions.

Example: Detect exception and abort:

demo% cat DetExcHan.f
      EXTERNAL myhandler                         
      REAL r / 14.2 /, s / 0.0 / 
      i = ieee_handler ('set', 'division', myhandler ) 
      t = r/s 
      END

      INTEGER FUNCTION myhandler(sig,code,context)
      INTEGER sig, code, context(5) 
      CALL abort() 
      END 
demo% f77 -silent DetExcHan.f 
demo% a.out 
abort: called 
Abort (core dumped) 
demo%

SIGFPE is generated whenever that floating-point exception occurs. When the SIGFPE is detected, control passes to the myhandler function, which immediately aborts. Compile with -g and use dbx to find the location of the exception.

Locating an Exception by Handler

Example: Locate an exception (print address) and abort:

demo% cat LocExcHan.F
#include "f77_floatingpoint.h"
      EXTERNAL Exhandler                             
      INTEGER Exhandler, i, ieee_handler
      REAL r / 14.2 /,  s / 0.0 /, t
C  Detect division by zero
      i = ieee_handler( 'set', 'division', Exhandler )
      t = r/s
      END

      INTEGER FUNCTION Exhandler( sig, sip, uap) 
      INTEGER sig 
      STRUCTURE /fault/
                  INTEGER address
      END STRUCTURE
      STRUCTURE /siginfo/
                  INTEGER si_signo
                  INTEGER si_code
                  INTEGER si_errno
                  RECORD /fault/ fault
      END STRUCTURE
      RECORD /siginfo/ sip
      WRITE (*,10)  sip.si_signo, sip.si_code, sip.fault.address
10        FORMAT('Signal ',i4,' code ',i4,' at hex address ', Z8 )
      CALL abort()                    
      END
demo%  f77 -silent -g LocExcHan.F
demo%  a.out 
Signal   8 code    3 at hex address    11230
abort: called
Abort (core dumped)
demo%

In SPARC V9 environments, replace the INTEGER declarations within each STRUCTURE with INTEGER*8, and the i4 formats with i8.

In most cases, knowing the actual address of the exception is of little use, except with dbx:

demo% dbx a.out
(dbx) stopi at 0x11230     Set breakpoint at address                                                  
(2) stopi at &MAIN+0x68
(dbx) run               Run program
Running: a.out 
(process id 18803)
stopped in MAIN at 0x11230
MAIN+0x68:      fdivs   %f3, %f2, %f2
(dbx) where           Shows the line number of the exception
=>[1] MAIN(), line 7 in "LocExcHan.F"
(dbx) list 7          Displays the source code line
    7         t = r/s
(dbx) cont            Continue after breakpoint, enter handler routine
Signal    8 code    3 at hex address    11230
abort: called
signal ABRT (Abort) in _kill at 0xef6e18a4
_kill+0x8:      bgeu    _kill+0x30
Current function is exhandler
   24         CALL abort()                    
(dbx) quit
demo%

Of course, there are easier ways to determine the source line that caused the error. However, this example does serve to show the basics of exception handling.