How to print allocatable fortran int64 array in GDB

Question

I know I can print the n+1-th element of a normal integer array data in GDB as

print *((integer *)data + n)

But how can I correctly print the element out if data is an integer(INT64) allocatable array?

Answer 1

Note: some older GDB versions or branches used in some unfortunate OSs or distributions may fail to support the allocatable arrays correctly. In that case use the C syntax.

If int64_t isn't recognized by an old GDB, use long or whatever old C type corresponds to a 64-bit integer.

---

You can really just do

 print data(n+1)

Using

 print *((integer *)data + n)

is C mode GDB syntax, but in Fortran mode it is really simple.

If you really want the complicated C syntax, you can use it even in Fortran mode, it is

 print *((int64_t *)(&data) + n)

In C mode (after set langauge c), you can also use

print *((int64_t *)data + n)

this one does not work in Fortran mode (Cannot access memory at address 0x29).

---

Example:

use iso_fortran_env

integer(int64), allocatable :: data(:)

integer :: n

data = [(i, i=1, 100)]

n = 5

continue

end

gdb:

GNU gdb (GDB; openSUSE Leap 15.1) 8.3.1
...
(gdb) break int64.f90:9
Breakpoint 1 at 0x4005ec: file int64.f90, line 9.
(gdb) run
Starting program: /home/lada/f/testy/stackoverflow/a.out 

Breakpoint 1, MAIN__ () at int64.f90:9
9       n = 5
Missing separate debuginfos, use: zypper install libgcc_s1-gcc10-debuginfo-10.1.1+git68-lp151.27.1.x86_64 libquadmath0-gcc10-debuginfo-10.1.1+git68-lp151.27.1.x86_64
(gdb) step
13      end
(gdb) print data(n+1)
$1 = 6
(gdb) print *((int64_t *)(&data) + n)
$2 = 6
(gdb) set language c
Warning: the current language does not match this frame.
(gdb) print *((int64_t *)data + n)
$3 = 6
(gdb) print *((long *)data + n)
$4 = 6