Gfortran uninitialized warnings

1

I have been running into these -Wuninitialized warnings in gfortran - 12.1.0 (but also earlier versions going back to at least 9.4.0) and I was wondering if they are false positives.

program main
    implicit none
    character(len=1), allocatable :: a(:)
    a = ["a"]
    print*, a
end program main

Output

test.f90:4:13:

    4 |     a = ["a"]
      |             ^
Warning: ‘a.offset’ is used uninitialized [-Wuninitialized]
test.f90:3:41:

    3 |     character(len=1), allocatable :: a(:)
      |                                         ^
note: ‘a’ declared here
test.f90:4:13:

    4 |     a = ["a"]
      |             ^
Warning: ‘a.dim[0].lbound’ is used uninitialized [-Wuninitialized]
test.f90:3:41:

    3 |     character(len=1), allocatable :: a(:)
      |                                         ^
note: ‘a’ declared here
test.f90:4:13:

    4 |     a = ["a"]
      |             ^
Warning: ‘a.dim[0].ubound’ is used uninitialized [-Wuninitialized]
test.f90:3:41:

    3 |     character(len=1), allocatable :: a(:)
      |                                         ^
note: ‘a’ declared here
test.f90:4:13:

    4 |     a = ["a"]
      |             ^
Warning: ‘a.dim[0].lbound’ may be used uninitialized [-Wmaybe-uninitialized]
test.f90:3:41:

    3 |     character(len=1), allocatable :: a(:)
      |                                         ^
note: ‘a’ declared here
test.f90:4:13:

    4 |     a = ["a"]
      |             ^
Warning: ‘a.dim[0].ubound’ may be used uninitialized [-Wmaybe-uninitialized]
test.f90:3:41:

    3 |     character(len=1), allocatable :: a(:)
      |                                         ^
note: ‘a’ declared here
test.f90:4:13:

    4 |     a = ["a"]
      |             ^
Warning: ‘a.dim[0].ubound’ may be used uninitialized [-Wmaybe-uninitialized]
test.f90:3:41:

    3 |     character(len=1), allocatable :: a(:)
      |                                         ^
note: ‘a’ declared here
test.f90:4:13:

    4 |     a = ["a"]
      |             ^
Warning: ‘a.dim[0].lbound’ may be used uninitialized [-Wmaybe-uninitialized]
test.f90:3:41:

    3 |     character(len=1), allocatable :: a(:)
      |                                         ^
note: ‘a’ declared here

Any ideas what is going on? Is that the expected behaviour and I am missing something?

2

Yes, the -Wuninitialized option of gfortran produces so many false positives that it should usually be turned off, especially if you use the allocation-on-assignment feature of Fortran. For example, for the code

implicit none
integer, allocatable :: v(:)
v = [10,20]
print*,v
end

you get

c:\fortran\test>gfortran -Wall -Wextra xgfortran_warn.f90
xgfortran_warn.f90:3:11:

    3 | v = [10,20]
      |           ^
Warning: 'v.offset' is used uninitialized [-Wuninitialized]
xgfortran_warn.f90:2:28:

    2 | integer, allocatable :: v(:)
      |                            ^
note: 'v' declared here
xgfortran_warn.f90:3:11:

    3 | v = [10,20]
      |           ^
Warning: 'v.dim[0].lbound' is used uninitialized [-Wuninitialized]
xgfortran_warn.f90:2:28:

    2 | integer, allocatable :: v(:)
      |                            ^
note: 'v' declared here
xgfortran_warn.f90:3:11:

    3 | v = [10,20]
      |           ^
Warning: 'v.dim[0].ubound' is used uninitialized [-Wuninitialized]
xgfortran_warn.f90:2:28:

    2 | integer, allocatable :: v(:)
      |                            ^
note: 'v' declared here
xgfortran_warn.f90:3:11:

    3 | v = [10,20]
      |           ^
Warning: 'v.dim[0].lbound' may be used uninitialized [-Wmaybe-uninitialized]
xgfortran_warn.f90:2:28:

    2 | integer, allocatable :: v(:)
      |                            ^
note: 'v' declared here
xgfortran_warn.f90:3:11:

    3 | v = [10,20]
      |           ^
Warning: 'v.dim[0].ubound' may be used uninitialized [-Wmaybe-uninitialized]
xgfortran_warn.f90:2:28:

    2 | integer, allocatable :: v(:)
      |                            ^
note: 'v' declared here
xgfortran_warn.f90:3:11:

    3 | v = [10,20]
      |           ^
Warning: 'v.dim[0].ubound' may be used uninitialized [-Wmaybe-uninitialized]
xgfortran_warn.f90:2:28:

    2 | integer, allocatable :: v(:)
      |                            ^
note: 'v' declared here
xgfortran_warn.f90:3:11:

    3 | v = [10,20]
      |           ^
Warning: 'v.dim[0].lbound' may be used uninitialized [-Wmaybe-uninitialized]
xgfortran_warn.f90:2:28:

    2 | integer, allocatable :: v(:)
      |                            ^
note: 'v' declared here

for gfortran 12.0.1 20220213.

gfortran -Wall -Wextra -Wno-uninitialized xgfortran_warn.f90

is appropriately silent.

1 Like
3

I see, thanks @Beliavsky. I am curious if there is a bug report tracking this in Bugzilla; I couldn’t find anything, so maybe it would be a good idea to file one. I find the the uninitialised warnings in C++ extremely useful, so it would be great if gfortran could also improve.

4

I did a more thorough search for bug reports in GCC and I found some existing bug reports for gfortran, namely:

Full Bug List

5

I posted another bug report before seeing your latest post:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106089

7

Thanks @kargl for coming to the rescue, I appreciate it!

9

OK, but that would make the reader of the code wonder why the allocate(a(0)) statement appears. I would rather use the -Wno-uninitialized option than add such code.

ETA: gfortran -Wuninitialized is silent about uninitialized variable abc in the code below, which further suggests disabling the option.

implicit none
integer, allocatable :: v(:), abc(:)
character(len=1), allocatable :: a(:)
a = ["a"]
print*, a
v = [10,20]
print*,v
allocate (abc(1))
print*,abc
end