# global_control¶

[scheduler.global_control]

Use this class to control certain settings or behavior of the oneTBB dynamic library.

An object of class global_control, or a “control variable”, affects one of several behavioral aspects, or parameters, of TBB. The global_control class is primarily intended for use at the application level, to control the whole application behavior.

The current set of parameters that you can modify is defined by the global_control::parameter enumeration. The parameter and the value it should take are specified as arguments to the constructor of a control variable. The impact of the control variable ends when its lifetime is complete.

Control variables can be created in different threads, and may have nested or overlapping scopes. However, at any point in time each controlled parameter has a single active value that applies to the whole process. This value is selected from all currently existing control variables by applying a parameter-specific selection rule.

// Defined in header <tbb/global_control.h>

namespace tbb {
class global_control {
public:
enum parameter {
max_allowed_parallelism,
terminate_on_exception
};

global_control(parameter p, size_t value);
~global_control();

static size_t active_value(parameter param);
};
} // namespace tbb


## Member types and constants¶

enum parameter::max_allowed_parallelism

Selection rule: minimum

Limits total number of worker threads that can be active in the task scheduler to parameter_value - 1.

Note

With max_allowed_parallelism set to 1, global_control enforces serial execution of all tasks by the application thread(s), that is, the task scheduler does not allow worker threads to run. There is one exception: if some work is submitted for execution via task_arena::enqueue, a single worker thread will still run ignoring the max_allowed_parallelism restriction.

enum parameter::thread_stack_size

Selection rule: maximum

enum parameter::terminate_on_exception

Selection rule: logical disjunction

Setting the parameter to 1 causes termination in any condition that would throw or rethrow an exception. If set to 0 (default), the parameter does not affect the implementation behavior.

## Member functions¶

global_control(parameter param, size_t value)

Constructs a global_control object with a specified control parameter and it’s value.

~global_control()

Destructs a control variable object and ends it’s impact.

static size_t active_value(parameter param)

Returns the currently active value of the setting defined by param.