Common Fortran 90 debugging tips

passing a variable of one type (such as real*8) to a function that sets the variable to a different type (such as real) will result in invalid data

once an error is discovered, use copious

write(*,*) 'I got here, x = ', x

statements to see if the value of x is what you think it should be

http://www.cleanscape.net/products/fortranlint/fortran-programming_tips.html

Segmentation fault [everyone's favorite] causes

• • matrix used that was not allocated

  • divide by zero

  • variable declarations in subroutines that do not match main body dimensions

Segmentation fault isolation

Scenario: given two sets of code, "working.f90" and "desired.f90" and a compiler, pgf90. "desired.f90" has a segmentation fault.

Step 0:

pgf90 works.f90

compiles and runs, whereas

pgf90 desired.ef90

compiles, but results in segmentation fault

Step 1:

pgf90 -g desired.f90

gdb a.out

run a.out

--> see where the segfault originates

Method 1: incrementally add code from "desired.f90" to "working.f90" and compile after each addition. Eventually, a segmentation fault will occur. While incrementing, use "pgf90" (with no switches)

Method 2: incrementally comment out sections of "desired.f90" until the segmentation fault is removed

Warning (applicable to both 1,2): Just because you find a segmentation fault, don't immediately discard "working.f90" because there may be more seg faults

Reminder: "pgf90" is different from "pgf90 -g" because "-g" turns off default optimizations. Thus, "pgf90" may have segmentation faults that "pgf90 -g" does not.