Fortran program producing different errors with the same input parameters

Question

This question may seem slightly open ended, but it's been troubling me for some time so I thought I would post it here in the hope of discussion or advice.

I am a physics PhD student running a fairly large computation on a reasonably complex fortran program. The program involves a large number of particles (~1000) that interact with each other by local potentials and move according to overdamped langevin dynamics.

Recently the program has begun to behave quite strangely. I'm not sure what changed, but it seems that different things are happening when the program is run with the same input parameters. Sometimes the program will run to completion. Other times it will produce a seg fault - at varying points within the computation. Occasionally it seems to simply grind to a halt without producing any errors, and one a couple of occasions has caused my computer to display warnings about running out of program memory.

The thing that confuses me here is why the program should be behaving differently for the same input. I'm really just hoping for suggestions of what might be going on here. Currently my only idea is some kind of memory management problem. The computer I'm running on is a 2013 iMac with 8GB of RAM, a 2.7GHz quad core i5 processor and OSX Mavericks. Not the most powerful in the world but I'm fairly sure I've run bigger computations on it without having these problems.

Answer 1

A seg fault indicates that either your program is running out of memory or that your program has an error. The most common errors in Fortran that cause seg faults are array subscript errors and disagreement between arguments in the call and the procedure (dummy arguments). For the first, turn on your compiler's option for run-time subscript checking. For the second, place all procedures into a module (or modules) and use that module (or modules). This will enable the compiler to check argument consistency.

What compiler are you using?

UPDATE: if you are using gfortran, try these compiler options: -O2 -fimplicit-none -Wall -Wline-truncation -Wcharacter-truncation -Wsurprising -Waliasing -Wimplicit-interface -Wunused -parameter -fwhole-file -fcheck=all -std=f2008 -pedantic -fbacktrace