interpreting backtrace error message

Question

When I compile my code using gfortran -g -fbacktrace -ffpe-trap=invalid,overflow,underflow File.f90 I get the following error:

Program received signal SIGFPE : Floating - Point exception - erroneous arithmetic operation.

Backtrace for this error:

#0 0x7f3da0768ed7 in ???
#1 0x7f3da076810d in ???
#2 0x7f3d9fe9b7ef in ???
#3 0x7f3da0230a3e in ???

My question is: how can I interpret these numbers and ???'s under "backtrace for this error:". How can I use this error message to help me find the error? Are they somehow related to the specific lines of code that are problematic? If so, how?

As of now I realize I have an erroneous arithmetic operation error but I have no idea where and this backtrace error message doesn't help at all. If I compile using just gfortran File.f90, there are no error messages at all during compilation or during running.

Answer 1

It might depend on which target you're using. The GFortran backtrace functionality depends on libbacktrace, which might not work on all targets. On Ubuntu 16.04 x86_64 for the code

program bt
  use iso_fortran_env
  implicit none
  real(real64) :: a, b = -1
  a = sqrt(b)
  print *, a
end program bt

when compiling with

gfortran -g -ffpe-trap=invalid  bt.f90

I get

Program received signal SIGFPE: Floating-point exception - erroneous arithmetic operation.

Backtrace for this error:
#0  0x7F08C38E8E08
#1  0x7F08C38E7F90
#2  0x7F08C35384AF
#3  0x4007F9 in MAIN__ at bt.f90:5
zsh: floating point exception  ./a.out

where on stack frame #3 you can see that the error occurs on line 5 in bt.f90.

Now, what are the stuff on stack frames #0-#2? Well, they are the backtracing functionality in libgfortran, the GFortran runtime library. libbacktrace for one reason or another cannot resolve symbols in a dynamic library. If I link it statically:

gfortran -g -static -ffpe-trap=invalid  bt.f90

I get

Program received signal SIGFPE: Floating-point exception - erroneous arithmetic operation.

Backtrace for this error:
#0  0x40139E in _gfortrani_backtrace
#1  0x400D00 in _gfortrani_backtrace_handler
#2  0x439B2F in gsignal
#3  0x400C01 in MAIN__ at bt.f90:5
zsh: floating point exception  ./a.out