Variables can have either shared or private context in a parallel environment. Variables in shared context are visible to all threads running in associated parallel loops. Variables in private context are hidden from other threads. Each thread has its own private copy of the variable, and modifications made by a thread to its copy are not visible to other threads.
The default context of a variable is determined by the following rules:
int E1; /* shared static */
void main (argvc,...) { /* argvc is shared */
int i; /* shared automatic */
void *p = malloc(...); /* memory allocated by malloc */
/* is accessible by all threads */
/* and cannot be privatized */
#pragma omp parallel firstprivate (p)
{
int b; /* private automatic */
static int s; /* shared static */
#pragma omp for
for (i =0;...) {
b = 1; /* b is still private here ! */
foo (i); /* i is private here because it */
/* is an iteration variable */
}
#pragma omp parallel
{
b = 1; /* b is shared here because it */
/* is another parallel region */
}
}
}
int E2; /*shared static */
void foo (int x) { /* x is private for the parallel */
/* region it was called from */
int c; /* the same */
... }
| Data scope attribute clause | Description |
|---|---|
| private | The private clause declares the variables in the list to be private to each thread in a team. |
| firstprivate | The firstprivate clause provides a superset of the functionality provided by the private clause. |
| lastprivate | The lastprivate clause provides a superset of the functionality provided by the private clause. |
| shared | The shared clause shares variables that appear in the list among all the threads in a team. All threads within a team access the same storage area for shared variables. |
| reduction | The reduction clause performs a reduction on the scalar variables that appear in the list, with a specified operator. |
| default | The default clause allows the user to affect the data scope attributes of variables. |
For more information, see the OpenMP directive descriptions in Pragma directives for parallel processing in the XL C/C++ Compiler Reference. You can also refer to the OpenMP Application Program Interface Language Specification, which is available at http://www.openmp.org.