Allocate local variables on the stack whenever possible.
This option makes writing recursive and re-entrant code easier and provides the optimizer more freedom when parallelizing loops.
Use of -stackvar is recommended with any of the parallelization options.
Local variables are variables that are not dummy arguments, COMMON variables, variables inherited from an outer scope, or module variables made accessible by a USE statement.
With -stackvar in effect, local variables are allocated on the stack unless they have the attributes SAVE or STATIC. Note that explicitly initialized variables are implicitly declared with the SAVE attribute. A structure variable that is not explicitly initialized but some of whose components are initialized is, by default, not implicitly declared SAVE. Also, variables equivalenced with variables that have the SAVE or STATIC attribute are implicitly SAVE or STATIC.
A statically allocated variable is implicitly initialized to zero unless the program explicitly specifies an initial value for it. Variables allocated on the stack are not implicitly initialized except that components of structure variables can be initialized by default.
Putting large arrays onto the stack with -stackvar can overflow the stack causing segmentation faults. Increasing the stack size may be required.
The initial thread executing the program has a main stack, while each slave thread of a multithreaded program has its own thread stack.
For slave threads, the default thread stack size is 4 Megabytes on 32–bit systems and 8 Megabytes on 64–bit systems. The limit command (with no parameters) shows the current main stack size. If you get a segmentation fault using -stackvar, try increasing the main and thread stack sizes.
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
You can set the size of the thread stack used by each slave thread by setting the STACKSIZE or OMP_STACKSIZE environment variable. See the OpenMP API User's Guide for more information about these environment variables.
Compile with -xcheck=stkovf to enable runtime checking for stack overflow situations. See the -xcheck option for more information.