Fortran program using character variable with allocatable length always shows a warning when compiling

Question

I have the following code that uses a character variable with allocatable length.

PROGRAM testprog
   IMPLICIT NONE
   CHARACTER(LEN=5) :: param
   CHARACTER(LEN=:), ALLOCATABLE :: val 
   param = '12455'
   val = param
   WRITE(*,*) val 
END PROGRAM testprog

I compile it using gfortran versions 7.5 or 8.4 with all warnings activated (option -Wall) and I get the following warning:

test.f90:6:0:

    val = param

Warning: ‘.val’ may be used uninitialized in this function [-Wmaybe-uninitialized]

The program works. However, I do not understand why this warning message appears.

score 2 · Accepted Answer · answered May 11 '20 at 11:32

This is a compiler bug. It is well-known, but not fixed in GCC yet. You can see the report at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91442

You can either ignore it, or disable the "may be used uninitialized" warnings with -Wno-maybe-uninitialized or compile with optimizations (-O1 and more).

Fortran program using character variable with allocatable length always shows a warning when compiling

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