-stackvar

Force all local variables to be allocated on the memory stack.

SPARC: 77/90 x86:77

Allocate all the local variables and arrays in a routine onto the memory stack, unless otherwise specified. This option makes them automatic, rather than static, and provides more freedom to the optimizer for parallelizing a CALL in a loop.

Use of -stackvar is recommended with any of the parallelization options.

Variables and arrays are local, unless they are:

• Arguments in a SUBROUTINE or FUNCTION statement (already on stack)

• Global items in a COMMON or SAVE, or STATIC statement

• Items initialized in a type statement or DATA statement, such as: REAL X/8.0/     or     DATA X/8.0/

Initializing a local variable in a DATA statement after an executable reference to that variable is flagged as an error when -stackvar is used:

---

demo% cat stak.f
        real x
        x =  1.
        t = 0.
        print*, t
        data x/3.0/
        print *,x+t
        end
demo% f77 -o stak -stackvar stak.f
stak.f:
 MAIN:
"stak.f", line 5: Error: attempt to initialize an automatic
 variable: x

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Putting large arrays onto the stack with -stackvar can overflow the stack causing segmentation faults. Increasing the stack size may be required.

There are two stacks:

• The whole program has a main stack.

• Each thread of a multi-threaded program has a thread stack.

The default stack size is about 8 Megabytes for the main stack and 256 KBytes for each thread stack. The limit command (with no parameters) shows the current main stack size. If you get a segmentation fault using -stackvar, you might try doubling the main stack size at least once.

Example: Show the current main stack size:

---

demo% limit
cputime         unlimited
filesize        unlimited
datasize        523256 kbytes
stacksize       8192 kbytes      <---
coredumpsize    unlimited
descriptors     64 
memorysize      unlimited
demo%

---

Example: Set the main stack size to 64 Megabytes:

---

demo% limit stacksize 65536

---

Example: Set each thread stack size to 8 Megabytes:

---

demo% setenv STACKSIZE 8192

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For further information of the use of -stackvar with parallelization, see the Parallelization chapter in the Fortran Programming Guide. See csh(1) for details on the limit command.