Lifetime of Variables

A variable is "alive" when its name is associated with a memory location. A variable is "alive" during the execution of the program unit that declared it:

• variables declared in the main program live throughout the execution of our application;
• local variables (declared inside a function or subroutine and which are not formal (dummy) arguments or global variables) are automatically allocated each time the procedure is called (allocation on function stack) and automatically deallocated when the procedure returns. They are called automatic variables. Their value is destroyed when the procedure returns; each call means a new variable is created.

SAVE attribute makes the variable static. It is allocated in a static part of the memory and is never deallocated (during the execution of our program). In particular static variables preserve their value between calls. Here is an example of a subroutine which counts how many times it has been called. The variable icount is static due to the initialization.

 
subroutine icount(iarg)
implicit none
  integer :: iarg     ! an argument
  integer :: ilocal=0 !local variable, static storage
  ilocal=ilocal+1
  print*,'I have been called ',ilocal,' time(s).'
end subroutine icount

Instead of initializing it, one can make the variable ilocal static using

 
integer, save :: ilocal
save ilocal

A single SAVE statement not followed by a list of variables has the effect of declaring static (``saving'') all local entities in a program unit that can be saved (note that formal (dummy) arguments and automatic arrays cannot be saved).