Fortran SAVE statement

Question

I've read about the save statement in the (Intel's) language reference document, but I cannot quite grasp what it does. Could someone explain to me in simple language what it means when the save statement is included in a module ?

Solution 1

In principal when a module goes out-of-scope, the variables of that module become undefined -- unless they are declared with the SAVE attribute, or a SAVE statement is used. "Undefined" means that you are not allowed to rely on the variable having the previous value if you again use the module -- it might have the previous value when you re-access the module, or it might not -- there is no guarantee. But many compilers don't do this for module variables -- the variables probably retain their values -- it isn't worth the effort for the compiler to figure out whether a module remains in scope or not and probably module variables are treated as global variables -- but don't rely on that! To be safe, either use "save" or "use" the module from the main program so that it never goes out of scope.

"save" is also important in procedures, to store "state" across invocations of the subroutine or function (as written by @ire_and_curses) -- "first invocation" initializations, counters, etc.

subroutine my_sub (y)

integer :: var
integer, save :: counter = 0
logical, save :: FirstCall = .TRUE.

counter = counter + 1

write (*, *) counter

if (FirstCall) then
   FirstCall = .FALSE.
   ....
end if

var = ....

etc.

In this code fragment, "counter" will report the number of invocations of subroutine x. Though actually in Fortran >=90 one can omit the "save" because the initialization in the declaration implies "save".

In contrast to the module case, with modern compilers, without the save attribute or initialization-on-a-declaration, it is normal for local variables of procedures to lose their values across invocations. So if you attempt to use "var" on an later call before redefining it in that call, the value is undefined and probably won't be the value calculated on a previous invocation of the procedure.

This is different from the behavior of many FORTRAN 77 compilers, some of which retained the values of all local variables, even though this wasn't required by the language standard. Some old programs were written relying on this non-standard behavior -- these programs will fail on the newer compilers. Many compilers have an option to use the non-standard behavior and "save" all local variables.

LATER EDIT: update with a code example that shows incorrect usage of a local variable that should have the save attribute but doesn't:

module subs

contains

subroutine asub (i, control)

   implicit none

   integer, intent (in) :: i
   logical, intent (in) :: control

   integer, save :: j = 0
   integer :: k

   j = j + i
   if ( control ) k = 0
   k = k + i

   write (*, *) 'i, j, k=', i, j, k

end subroutine asub

end module subs

program test_saves

   use subs
   implicit none

   call asub ( 3, .TRUE. )
   call asub ( 4, .FALSE. )

end program test_saves

Local variable k of the subroutine is intentionally misused -- in this program it is initialized in the first call since control is TRUE, but on the second call control is FALSE, so k is not redefined. But without the save attribute k is undefined, so the using its value is illegal.

Compiling the program with gfortran, I found that k retained its value anyway:

 i, j, k=           3           3           3
 i, j, k=           4           7           7

Compiling the program with ifort and aggressive optimization options, k lost its value:

 i, j, k=           3           3           3
 i, j, k=           4           7           4

Using ifort with debugging options, the problems was detected at runtime!

 i, j, k=           3           3           3
forrtl: severe (193): Run-Time Check Failure. The variable 'subs_mp_asub_$K' is being used without being defined

Solution 2

Normally, local variables go out of scope once execution leaves the current procedure, and so have no 'memory' of their value on previous invocations. SAVE is a way of specifying that a variable in a procedure should maintain its value from one call to the next. It's useful when you want to store state in a procedure, for example to keep a running total or maintain a variable's configuration.

There's a good explanation here, with an example.

Solution 3

A short explanation could be: the attribute save says that the value of a variable must be preserved across different calls to the same subroutine/function. Otherwise normally when you return from a subroutine/function, "local" variables lose their values since the memory where those vars were stored is released. It is like static in C, if you know this language.

Comments

[M. S. B.]

Also see stackoverflow.com/questions/2582409/…

[Stefano Borini]

As a complement to the very good answers you got, my recommendation is: "never use save". It's a recipe for disaster and makes the routine non thread safe and not stateless. There may be reasons to use save, but they are very, very rare, and you can do the same using a different programming style or solution.

[Friedrich Schwartz]

Uhmm, could you give an example of that ? If I understand correctly, if I declare SAVE in a module, its variables cannot change values in subroutines ?

[ire_and_curses]

@Friedrich Schwartz: Not quite. You can still set new values, but if you were to inspect the value of the variable before setting it, you would see the last value you had set the variable to. Without SAVE, you would see 'undefined'. Working through a simple example should make it clearer.

[MasterHD]

Maybe also worth mentioning that it's like the "static" keyword in C.

[Enlico]

"use" the module from the main program so that it never goes out of scope, now I see the obviousness in why I have had no problems even writing not even a single SAVE in my program! Thank you!

[Kai]

what is the purpose of the FirstCall variable? Couldn't we equivalently just check if n is equal to 0 (or 1 if it has already been incremented)? Am I missing something?

[phenomenon]

INTEGER, PARAMETER :: k=1024 equal to INTEGER, SAVE :: k=1024? Is there any difference in these usages?

[Mansoor]

Is it known if the "save" because of the initialization in the declaration is true for both Intel Fortran and GFortran?

[MathArt]

By my understanding, SAVE property is exactly equivalent to STATIC as in C/C++. There are scenarios where they are useful for efficiency.