Finding Exception Locations

This page contains instructions on how to find out which statement caused an exception. This is particularly useful when a get routine caused the exception, because the information printed when the exception occurs only tells you that the get caused the exception. It does not tell you at what statement the get was called. The same is true if the exception occurs in some other library routine.

The following discusses two ways to find the source of an exception. Both ways involve getting a a traceback of the exception. A traceback is a list of the routines were currently executing when the exception occurred. The traceback will include more information than you need, and so you will have to know where to look to find the source of the exception.

The traceback can be printed directly from within your program, or it can be printed from within gnatgdb, which is the gnat debugger. Instructions on printing from your program are given first.

Causing your program to print a traceback

To cause your program to print a traceback when an exception occurs, you need to do three things (as shown in this example):

Include a With statement for the packages ada.exceptions and gnat.traceback.symbolic in your program.

Include an exception handler that prints symbolic_traceback(E) where E is the exception parameter.

Compile with certain options, like this:

       gnatmake -f -g myprog.adb -bargs -E

-f forces compilation (in case executable is up to date)
-g generates debug info
-bargs introduces arguments to the linker
-E include exception traceback info

Introduction to using GDB on rucs to get a traceback

The GNU debugger gdb is used to find the statement that caused the exception. The debugger will also give you a list of the procedures that were executing when the instruction occurred. The instructions describe how to use gdb on rucs and when using AdaGIDE.

Immediately below are brief instructions on how to use gdb on rucs. The instructions may not work on other machines. Instructions for AdaGIDE follow the example below.

Instructions for using GDB on rucs

Assume that you want to debug the file myprog.adb on rucs. Execute the commands listed below.

gnatmake -g -f myprog

the -g option causes debug info to be included in the .o file, and the -f option forces recompilation (it's only required if the myprog is up to date)

gnatgdb myprog

This will start the interactive debugger and bring up the debugger prompt

start

This will initialize execution of your program by executing system startup code. It stops at the beginning of your main procedure
To redirect standard input to read from a file, you can use the command start < datafile.txt which will initialize your program and specify that standard input will read from datafile.txt

break exception

Set up a breakpoint on exceptions. This will cause the debugger to take control when an exception occurs

continue

Now continue execution of your program
If an exception occurs, your program will stop and give another debug prompt

backtrace

This will show all of the currently executing subroutines on the system stack
You should be able to find line numbers of the routines from your program on the stack
The bottom item in the list shows the number of the statement where the error occurred

Sample Run

Here is file demogdb.adb:

    with ada.integer_text_io; use ada.integer_text_io;
    procedure demogdb is
        procedure demo(result: out integer)  is
            x: integer;
        begin
            get(x);            -- We will input 0 
            result := 3 / x;   -- Exception here in line 7
        end demo;

        y: integer;
    begin
        demo(y);               -- demo called here in line 12
    end demogdb;

And here is file demogdb.dat:

Sample run:

       rucs@/home/nokie/public_html/classes/320 > gnatmake -g -f demogdb
       gcc-4.0 -c -g demogdb.adb
       gnatbind -x demogdb.ali
       gnatlink demogdb.ali -g

       rucs@/home/nokie/public_html/classes/320 > gnatgdb demogdb
       GNU gdb 6.4 for GNAT GPL 2006 (20060605)
       Copyright 2005 Free Software Foundation, Inc.
       Ada Core Technologies version of GDB for GNAT GPL
       GDB is free software, covered by the GNU General Public License, and you are
       welcome to change it and/or distribute copies of it under certain conditions.
       Type "show copying" to see the conditions.
       See your support agreement for details of warranty and support.
       If you do not have a current support agreement, then there is absolutely
       no warranty for this version of GDB.  Type "show warranty" for details.
       This GDB was configured as "i686-pc-linux-gnu"...Using host libthread_db library
       "/lib/tls/i686/cmov/libthread_db.so.1".

       (gdb) start < demogdb.dat
       Breakpoint 1 at 0x80492ea: file demogdb.adb, line 12.
       Starting program: /home/nokie/public_html/classes/320/demogdb < demogdb.dat
       demogdb () at demogdb.adb:12
       12          demo(y);

       (gdb) break exception
       Breakpoint 2 at 0xb7db5cf6

       (gdb) continue
       Continuing.
       No definition of "e.full_name" in current context.

       (gdb) backtrace
       #0  0xb7db5cf6 in __gnat_raise_nodefer_with_msg () from /usr/local/gnu/lib/libgnat-4.0.so.1
       #1  0xb7db5f12 in ada.exceptions.raise_with_location_and_msg () from
       /usr/local/gnu/lib/libgnat-4.0.so.1
       #2  0xb7db5cf2 in __gnat_raise_constraint_error_msg () from /usr/local/gnu/lib/libgnat-4.0.so.1
       #3  0xb7db6037 in __gnat_rcheck_03 () from /usr/local/gnu/lib/libgnat-4.0.so.1
       #4  0x08049323 in demogdb.demo () at demogdb.adb:7
       #5  0x080492ef in demogdb () at demogdb.adb:12

       (gdb) quit
       The program is running.  Exit anyway? (y or n) y
       rucs@/home/nokie/public_html/classes/320 >

Using GDB from AdaGIDE

After compilation, simply press the Debug button.

This will bring up a console window that contains the debugger running your program.

Follow the instructions above, starting with the command start.