There are situations where a number of threads have to all complete their work before any of the threads can start on the next task.

**Barriers**

There are situations where a number of threads have to all complete
their work before any of the threads can start on the next task. In these
situations, it is useful to have a bar-rier where the threads will wait until
all are present.

One common example of using a barrier arises when
there is a dependence between different sections of code. For example, suppose
a number of threads compute the values stored in a matrix. The variable total needs to be calculated using the values stored in the matrix. A barrier
can be used to ensure that all the threads complete their computa-tion of the matrix
before the variable total is
calculated. Listing 4.10 shows a situation using a barrier to separate the
calculation of a variable from its use.

Listing 4.10 Using
a Barrier to Order Computation

Compute_values_held_in_matrix();

**Barrier();**

total = Calculate_value_from_matrix();

The variable total can be computed only when all threads have reached the barrier. This
avoids the situation where one of the threads is still completing its
computations while the other threads start using the results of the calculations.
Notice that another barrier could well be needed after the computation of the
value for total if that
value is then used in further calculations. Listing 4.11 shows this use of
multiple barriers.

Listing 4.11 Use
of Multiple Barriers

Compute_values_held_in_matrix();

**Barrier();**

total = Calculate_value_from_matrix();

**Barrier();**

Perform_next_calculation( total );

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