OpenMP includes a set of compiler directives and an API. Any code following the compiler directive #pragma omp parallel is identified as a parallel region and is performed by a number of threads equal to the number of processing cores in the system. The advantage of OpenMP (and similar tools) is that thread creation and management are handled by the OpenMP library and are not the responsibility of application developers
Along with its #pragma omp parallel compiler directive, OpenMP provides the compiler directive #pragma omp critical, which specifies the code region following the directive as a critical section in which only one thread may be active at a time. In this way, OpenMP provides support for ensuring that threads do not generate race conditions.
As an example of the use of the critical-section compiler directive, first assume that the shared variable counter can be modified in the update() function as follows:
void update(int value)
{
counter += value;
}
If the update() function can be part of—or invoked from—a parallel region, a race condition is possible on the variable counter
The critical-section compiler directive can be used to remedy this race condition and is coded as follows:
void update(int value)
{
#pragma omp critical
{
counter += value;
}
}
The critical-section compiler directive behaves much like a binary semaphore or mutex lock, ensuring that only one thread at a time is active in the critical section. If a thread attempts to enter a critical section when another thread is currently active in that section (that is, owns the section), the calling thread is blocked until the owner thread exits. If multiple critical sections must be used, each critical section can be assigned a separate name, and a rule can specify that no more than one thread may be active in a critical section of the same name simultaneously
An advantage of using the critical-section compiler directive in OpenMP is that it is generally considered easier to use than standard mutex locks. However, a disadvantage is that application developers must still identify possible race conditions and adequately protect shared data using the compiler directive. Additionally, because the critical-section compiler directive behaves much like a mutex lock, deadlock is still possible when two or more critical sections are identified