High Performance Scientific Computing   Coursera Edition

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Fortran example for Newton’s method

# Fortran debugging¶

## Compiling with various gfortran flags¶

There are a number of flags you can use when compiling your code that will make it easier to debug.

Here’s a generic set of options you might try:

$gfortran -g -W -Wall -fbounds-check -pedantic-errors \ -ffpe-trap=zero,invalid,overflow,underflow program.f90 See Useful gfortran flags or the gfortran man page <http://linux.die.net/man/1/gfortran> for more information. Most of these options indicate that the program should give warnings or die if certain bad things happen. Compiling with the -g flag indicates that information should be generated and saved during compilation that can be used to help debug the code using a debugger such as gdb or totalview. You generally have to compile with this option to use a debugger. ## The gdb debugger¶ This is the Gnu open source debugger for Gnu compilers such as gfortran. Unfortunately it often works very poorly for Fortran. You can install it on the VM using: $ sudo apt-get install gdb

And then use via, e.g.:

$cd$UWHPSC/codes/fortran $gfortran -g segfault1.f90$ gdb a.out (gdb) run

.... runs for a while and then prints
Program received signal EXC_BAD_ACCESS, Could not access memory. Tells what line it died in.

(gdb) p i \$1 = 241 (gdb) q

This at least reveals where the error happened and allows printing the value of i when it died.

## Totalview¶

Totalview is a commercial debugger that works quite well on Fortran codes together with various compilers, including gfortran. It also works with other languages, and for parallel computing.