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Cray Standard C/C++ Reference Manual

to determine their tasking context. The tasking context for these variables

cannot be changed.

Note: Variables default to extern and static default to shared. Function

parameters and variables declared taskcommon, auto, and register default

to value.

Unless the defaults argument is used, all variables referenced in but declared

outside the parallel region must be listed in one of the tasking context argument

lists. The tasking context arguments can be used only with the parallel

directive or the taskloop directive for stand-alone taskloops.

In this discussion of tasking context, global refers to variables declared at a global

level (outside of any function) and local refers to variables declared at a local

block level. Not all types of identifiers can be placed in the tasking context

argument lists. Tasking context lists must not contain tags, constants, function

names, labels, expressions of any kind, or members of a struct, union,or

enum.

3.8.13.1.1 private and value Context

Variables with private or value context have an independent instance for each

of the tasks. Generally, scalar variables that are modified inside the parallel

region are declared private or value. In general, loop control variables (LCVs)

should be declared private; the LCV for tasked loops (both nonordered and

semiordered) must be value or private.

All private variables enter the parallel region undefined. All value variables

enter the parallel region initialized with the value they had just prior to the

parallel region. All private variables and modified value variables are

undefined after exiting the parallel region. An exception to this rule occurs when

the savelast argument is specified with a stand-alone taskloop, and with all

iterate variables in semiordered tasked loops.

As a debugging aid, try using the -h msglevel_2 option when compiling your

tasked program. If a private variable is used before it is initialized, a message

will be generated to alert you to this problem.

3.8.13.1.2 shared Context

Variables with shared context have a single instance that is accessible from all

of the tasking processors. Generally, variables with values that are established

outside of the parallel region and that are not modified inside of the region

are declared shared.

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Vax C90-EL SERIES Service Manual Page 100