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I'm debugging a larger numerical program that I have added on to. It is written in fortran90, compiled with gfortran (the latest version available for Mac) and I am debugging it using gdb (again the latest version available for Mac).
My additions have a bug somewhere and I am trying to locate it, which is clear as running the program does not produce the expected result. When I run it in gdb, I get the following output at the end:
Note: The following floating-point exceptions are signalling: IEEE_INVALID_FLAG IEEE_DIVIDE_BY_ZERO IEEE_UNDERFLOW_FLAG IEEE_DENORMAL
[Inferior 1 (process 83843) exited normally]
I would like to identify exactly where this FPE occurs, but it seems that a floating point exception does not cause the program to crash. I tested this by explicitly dividing by 0 in my code - it did not cause the program to stop running, but led to unexpected behavior.
What is the proper flag for either gdb or gfortran to ensure that the program stops running (ideally with a backtrace) when it reaches a floating point exception? I tried following the instructions here but it did not appear to change anything.
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Gdb is a debugger for C (and C++). It allows you to do things like run the program up to a certain point then stop and print out the values of certain variables at that point, or step through the program one line at a time and print out the values of each variable after executing each line.
GDB stands for GNU Project Debugger and is a powerful debugging tool for C(along with other languages like C++). It helps you to poke around inside your C programs while they are executing and also allows you to see what exactly happens when your program crashes.
The gfortran debug options can help us here. The -ffpe-trap compilation flag enables exception traps for the various floating point errors; we can catch these with GDB to find the exact cause of our bug. Just as for a C program, we start our Fortran executable under control of the debugger. To start debugging our program we invoke gdb PROGRAM.
This is just as we’d expect from our C / C++ debugging experience – GDB is using the debug information from gfortran to step through lines of source code, so we can see how the state is changing. We could just do that until the error occurs – but it would be nice to jump to the exact moment of the floating point error we’re expecting.
The GNU Debugger allows you to see what is going on "inside" a program while it executes - or what a program was doing at the moment it crashed. GDB supports C, C++, Java, Fortran and Assembly among other languages; it is also designed to work closely with the GNU Compiler Collection (GCC).
We are used to using GDB for debugging C and C++ but it can also be used to debug other languages including Fortran, D, Go and Ada. Debug info formats like DWARF, along with some language-specific extensions, allow GDB to support most of the commonly-used compiled languages.
Probably you need to add these flags when compiling your code:
gfortran -g -fbacktrace -ffpe-trap=zero,overflow,underflow youcode.f90 -o run.exe
Explanation for compiler flags from gfortran manual:
-g
to include debug data
-fbacktrace
Specify that, when a runtime error is encountered or a deadly signal is emitted (segmentation fault, illegal instruction, bus error or floating-point exception), the Fortran runtime library should output a backtrace of the error. This option only has influence for compilation of the Fortran main program.
-ffpe-trap=list
Specify a list of IEEE exceptions when a Floating Point Exception (FPE) should be raised. On most systems, this will result in a SIGFPE signal being sent and the program being interrupted, producing a core file useful for debugging. list is a (possibly empty) comma-separated list of the following IEEE exceptions: invalid (invalid floating point operation, such as SQRT(-1.0)), zero (division by zero), overflow (overflow in a floating point operation), underflow (underflow in a floating point operation), precision (loss of precision during operation) and denormal (operation produced a denormal value).
Some of the routines in the Fortran runtime library, like ‘CPU_TIME’, are likely to to trigger floating point exceptions when ffpe-trap=precision is used. For this reason, the use of ffpe-trap=precision is not recommended.
Take a look at these two places for more info:
https://gcc.gnu.org/onlinedocs/gcc-4.3.2/gfortran.pdf http://faculty.washington.edu/rjl/uwamath583s11/sphinx/notes/html/gfortran_flags.html
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