How to debug Fortran programs using gdb

Previously, I thought the debugging functionality provided by gdb for Fortran program was quite limited due to experiences of a couple of failed attempts to print allocatable arrays. However, having given several another tries today, I discovered that gdb does support printing values of allocatable arrays, although it may be a little inconvenient. Now, let‘s see the details:

We have the test code as follows, in which a derived type cmp, an allocatable array a and a static array b are defined.

 program test
   implicit none

   type cmp
      integer a
      integer b
      integer, dimension(:), allocatable :: c
      integer, dimension(5) :: d
   end type cmp

   integer, dimension(:), allocatable :: a
   integer, dimension(5) :: b = (/1,2,3,4,5/)
   type(cmp) x

   allocate(a(5))
   allocate(x%c(5))

   a = (/1,2,3,4,5/)
   x%a = 5
   x%b = 10
   x%c = a
   x%d = a

   write(STDOUT,*), a
   write(STDOUT,*), b
   write(STDOUT,*), x%c

   deallocate(a)
   deallocate(x%c)
 end program test

For the static array b, there is no need to mention it because gdb has a good support whether or not it is defined inside a derived type. However, if we want to directly print the allocatable array in gdb, a null value 0 will be printed:

(gdb) p a
$1 = (0)

If we treat the allocatable array as a C pointer,

(gdb) p *a
Attempt to take contents of a non-pointer value.
(gdb) p a(1)
$2 = 0 # Note: this is a wrong value
(gdb) p a[1]
A syntax error in expression, near `[1]‘. # Note: square brackets are invalid

The correct form is:

(gdb) p *((integer *)a)
$3 = 1
(gdb) p *((integer *)a+1)
$4 = 2
# ... and so on.

The whole array can also be printed out all at once:

(gdb) p *((integer *)a)@5
$5 = (1, 2, 3, 4, 5)

For the allocatable array in the derived type cmp, we should note that the member variable along with its parent should be wrapped in brackets:

(gdb) p *((integer *)x%c)
Attempt to extract a component of a value that is not a structure. # wrong
(gdb) p *((integer *)(x%c))
$1 = 1 # right
(gdb) p *((integer *)(x%c)+1)
$2 = 2
(gdb) p *((integer *)(x%c))@5
$3 = (1, 2, 3, 4, 5)

OK, up to now, our Fortran program can be debugged efficiently, without having to insert debugging print or write subroutine any more!

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