Is initializing a variable in between variable declarations forbidden? - fortran

Question

I have something like
integer a
integer b
b = 0
integer c
a = 0
c = 0
which does not work with the error
"A specification statement cannot appear in the executable section."
However, changing it to
integer a
integer b
integer c
a = 0
b = 0
c = 0
works.

Answer 1

Yes, this is forbidden in Fortran. This is defined in the Fortran 2008 Standard, Cl. 2.3.2 "Statement order":
1 The syntax rules of clause 2.1 specify the statement order within program units and subprograms. These rules
are illustrated in Table 2.1 [...]. Table 2.1 shows the ordering rules for statements and applies to
all program units, subprograms, and interface bodies. Vertical lines delineate varieties of statements that may
be interspersed and horizontal lines delineate varieties of statements that shall not be interspersed. [...] Between USE and CONTAINS statements in a
subprogram, nonexecutable statements generally precede executable statements [...]
(Emphasis mine)
[Slightly off-topic, but related] Please note that while
integer :: a
integer :: b = 0
integer :: c
is allowed, this has the side effect that b gets the save attribute. That is typically not what you want...

Answer 2

The error message is quite clear. Fortran programs and subprograms are divided into two parts. First the specification part where you use modules, define variables, derived types, interfaces... And then the executable part where you put the actual executable statements or control structures.
It is not possible to mix them.

Answer 3

The situation is murkier in f2008, with the BLOCK construct. That construct necessarily lies among the executable statements, but is often there for the sole purpose of adding the capability to place some specification statements after executable statements, as when an assumed length pointer is to be pointed at anonymous memory.
EDIT: Example
module anonymous
use ISO_C_BINDING
implicit none
interface
function malloc(size) bind(C,name='malloc')
import
implicit none
type(C_PTR) malloc
integer(C_SIZE_T), value :: size
end function malloc
subroutine free(ptr) bind(C,name='free')
import
implicit none
type(C_PTR), value :: ptr
end subroutine free
function strlen(str) bind(C,name='strlen')
import
implicit none
type(C_PTR), value :: str
integer(C_SIZE_T) strlen
end function strlen
end interface
contains
function hello()
type(C_PTR) hello
character(LEN=*,KIND=C_CHAR), parameter :: world = &
'Hello, world'//C_NULL_CHAR
character(LEN=len(world),KIND=kind(world)), pointer :: fptr
hello = malloc(len(world,KIND=C_SIZE_T))
call C_F_POINTER(hello,fptr)
fptr = world
end function hello
end module anonymous
program test
use anonymous
implicit none
type(C_PTR) cptr
character(LEN=:,KIND=C_CHAR), pointer :: fptr
integer(C_SIZE_T) hello_len
cptr = hello()
hello_len = strlen(cptr)
BLOCK
character(LEN=hello_len,KIND=C_CHAR), pointer :: temp
call C_F_POINTER(cptr,temp)
fptr => temp
end BLOCK
write(*,'(*(g0))') fptr(1:strlen(cptr))
end program test