HP Fortran Programmer's Guide (September 2007)

Controlling data storage

Automatic and static variables

Chapter 3100

Automatic and static variables

By default, HP Fortran allocates stack storage for program variables. Such variables are

called automatic variables because they are allocated at each invocation of the program

unit in which they are declared.

Static variables are allocated storage from static memory when the program is ﬁrst loaded

into memory. They remain allocated for the life of the program.

HP Fortran allocates static storage for the following variables:

• Variables speciﬁed in a COMMON or EQUIVALENCE statement.

• Variables initialized in a type declaration statement or in a DATA statement.

• Variables speciﬁed in a SAVE or STATIC statement. A SAVE statement without a variable

list speciﬁes static storage for all variables in the scoping unit.

• Variables in program ﬁles that have been compiled with the +save or +Oinitcheck

command-line option. See “Uninitialized variables” on page 238 for information about

using these options when porting.

Static variables have two characteristics that are of special interest:

• They are set to 0 or null value at load-time.

• They do not require re-initialization at each invocation of their program unit.

Static variables have several disadvantages. In Fortran programs that use recursion, static

variables can defeat one purpose of recursion—to provide a fresh set of local variables at each

recursive call. Also, the widespread use of static variables in a program can slow its

performance: static variables are ineligible for such fundamental optimizations as register

allocation, and they can limit the optimization of program units that use them.

The following example program illustrates the difference between automatic and static

variables. The program consists of a main program unit that calls a recursive internal

subroutine. The subroutine increments two variables (stat_val and auto_val), prints the

updated variables, and then calls itself recursively. Neither of the two variables is explicitly

initialized, but stat_val is declared with the SAVE attribute, which means that it is allocated

static storage and is pre-initialized to 0 by the compiler.

The program is shown below.