bslmt_condition.h

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/// @file bslmt_condition.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmt_condition.h                                                  -*-C++-*-
#ifndef INCLUDED_BSLMT_CONDITION
#define INCLUDED_BSLMT_CONDITION
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmt_condition bslmt_condition
/// @brief  Provide a portable, efficient condition variable.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmt
/// @{
/// @addtogroup bslmt_condition
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmt_condition-purpose"> Purpose</a>
/// * <a href="#bslmt_condition-classes"> Classes </a>
/// * <a href="#bslmt_condition-description"> Description </a>
///   * <a href="#bslmt_condition-supported-clock-types"> Supported Clock-Types </a>
///   * <a href="#bslmt_condition-usage"> Usage </a>
///     * <a href="#bslmt_condition-example-1-basic-usage"> Example 1: Basic Usage </a>
///
/// # Purpose {#bslmt_condition-purpose}
/// Provide a portable, efficient condition variable.
///
/// # Classes {#bslmt_condition-classes}
///
/// -  bslmt::Condition: portable intra-process signaling mechanism
///
/// @see  bslmt_mutex
///
/// # Description {#bslmt_condition-description}
/// The `bslmt::Condition` class provided by this component
/// implements the concept of a *condition* *variable*, enabling multiple
/// threads to communicate information about the state of shared data.  A
/// condition variable is a signaling mechanism associated with a mutex, which
/// in turn protects a data invariant.  A condition variable enables threads to
/// wait for a predicate (i.e., logical expression) to become true, and to
/// communicate to other threads that the predicate might be true.
///
/// One or more threads can wait efficiently on a condition variable, either
/// indefinitely or until some *absolute* time, by invoking one of the following
/// methods of `bslmt::Condition`:
/// @code
/// int wait(bslmt::Mutex *mutex);
/// int timedWait(bslmt::Mutex *mutex, const bsls::TimeInterval& absTime);
/// @endcode
/// The caller must lock the mutex before invoking these functions.  The
/// `bslmt::Condition` atomically releases the lock and waits, thereby
/// preventing other threads from changing the predicate after the lock is
/// released, but before the thread begins to wait.  The `bslmt` package
/// guarantees that this lock will be reacquired before returning from a call to
/// the `wait` and `timedWait` methods, unless an error occurs.
///
/// When invoking the `timedWait` method, clients must specify, via the
/// parameter `absTime`, a timeout after which the call will return even if the
/// condition is not signaled.  `absTime` is expressed as a `bsls::TimeInterval`
/// object that holds an *absolute* time according to the clock type the
/// `bslmt::Condition` object is constructed with (the default clock is
/// `bsls::SystemClockType::e_REALTIME`).  Clients should use the
/// `bsls::SystemTime::now(clockType)` utility method to obtain the current
/// time.
///
/// Other threads can indicate that the predicate is true by signaling or
/// broadcasting the same `bslmt::Condition` object.  A broadcast wakes up all
/// waiting threads, whereas a signal wakes only one thread.  The client has no
/// control over which thread will be signaled if multiple threads are waiting:
/// @code
/// void signal();
/// void broadcast();
/// @endcode
/// A thread waiting on a condition variable may be signaled (i.e., the thread
/// may wake up without an error), but find that the predicate is still false.
/// This situation can arise for a few reasons: spurious wakeups produced by the
/// operating system, intercepted wakeups, and loose predicates.  Therefore, a
/// waiting thread should always check the predicate *after* (as well as before)
/// the call to the `wait` function.
///
/// ## Supported Clock-Types {#bslmt_condition-supported-clock-types}
///
///
/// `bsls::SystemClockType` supplies the enumeration indicating the system clock
/// on which timeouts supplied to other methods should be based.  If the clock
/// type indicated at construction is `bsls::SystemClockType::e_REALTIME`, the
/// `absTime` argument passed to the `timedWait` method should be expressed as
/// an *absolute* offset since 00:00:00 UTC, January 1, 1970 (which matches the
/// epoch used in `bsls::SystemTime::now(bsls::SystemClockType::e_REALTIME)`.
/// If the clock type indicated at construction is
/// `bsls::SystemClockType::e_MONOTONIC`, the `absTime` argument passed to the
/// `timedWait` method should be expressed as an *absolute* offset since the
/// epoch of this clock (which matches the epoch used in
/// `bsls::SystemTime::now(bsls::SystemClockType::e_MONOTONIC)`.
///
/// ## Usage {#bslmt_condition-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Basic Usage {#bslmt_condition-example-1-basic-usage}
///
///
/// Suppose we have a `bslmt::Condition` object, `condition`, and a boolean
/// predicate associated with `condition` (represented here as a free function
/// that returns a `bool` value):
/// @code
/// /// Return `true` if the invariant holds for `condition`, and `false`
/// /// otherwise.
/// bool predicate()
/// {
///     return true;
/// }
/// @endcode
/// The following usage pattern should always be followed:
/// @code
/// // ...
///
/// bslmt::Condition condition;
/// bslmt::Mutex     mutex;
///
/// mutex.lock();
/// while (false == predicate()) {
///     condition.wait(&mutex);
/// }
///
/// // Modify shared resources and adjust the predicate here.
///
/// mutex.unlock();
///
/// // ...
/// @endcode
/// The usage pattern for a timed wait is similar, but has extra branches to
/// handle a timeout:
/// @code
/// // ...
///
/// enum { e_TIMED_OUT = -1 };
/// bsls::TimeInterval absTime = bsls::SystemTime::nowRealtimeClock();
///
/// // Advance 'absTime' to some delta into the future here.
///
/// mutex.lock();
/// while (false == predicate()) {
///     const int status = condition.timedWait(&mutex, absTime);
///     if (e_TIMED_OUT == status) {
///         break;
///     }
/// }
///
/// if (false == predicate()) {
///     // The wait timed out and `predicate` returned `false`.  Perform
///     // timeout logic here.
///
///     // ...
/// }
/// else {
///     // The condition variable was either signaled or timed out and
///     // `predicate` returned `true`.  Modify shared resources and adjust
///     // predicate here.
///
///     // ...
/// }
/// mutex.unlock();
///
/// // ...
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmt
 * @{
 */
/** @addtogroup bslmt_condition
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bslmt_conditionimpl_pthread.h>
#include <bslmt_conditionimpl_win32.h>
#include <bslmt_platform.h>
 
#include <bsls_assert.h>
#include <bsls_libraryfeatures.h>
#include <bsls_timeinterval.h>
#include <bsls_systemclocktype.h>
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP11_BASELINE_LIBRARY
#include <bslmt_chronoutil.h>
 
#include <bsl_chrono.h>
#endif
 
 
namespace bslmt {
 
template <class THREAD_POLICY>
class ConditionImpl;
 
class Mutex;
 
                             // ===============
                             // class Condition
                             // ===============
 
/// This `class` implements a portable inter-thread signaling primitive.
///
/// See @ref bslmt_condition 
class Condition {
 
    // DATA
    ConditionImpl<Platform::ThreadPolicy> d_imp;  // platform-specific
                                                  // implementation
 
  private:
    // NOT IMPLEMENTED
    Condition(const Condition&);
    Condition& operator=(const Condition&);
 
  public:
    // TYPES
 
    /// The value `timedWait` returns when a timeout occurs.
    enum { e_TIMED_OUT = ConditionImpl<Platform::ThreadPolicy>::e_TIMED_OUT };
 
    // CREATORS
 
    /// Create a condition variable object.  Optionally specify a
    /// `clockType` indicating the type of the system clock against which
    /// the `bsls::TimeInterval` `absTime` timeouts passed to the
    /// `timedWait` method are to be interpreted (see {Supported
    /// Clock-Types} in the component-level documentation).  If `clockType`
    /// is not specified then the realtime system clock is used.  This
    /// method does not return normally unless there are sufficient system
    /// resources to construct the object.
    explicit
    Condition(bsls::SystemClockType::Enum clockType =
                                            bsls::SystemClockType::e_REALTIME);
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP11_BASELINE_LIBRARY
    /// Create a condition variable object.  Use the realtime system clock
    /// as the clock against which the `absTime` timeouts passed to the
    /// `timedWait` methods are interpreted (see {Supported Clock-Types} in
    /// the component-level documentation).  This method does not return
    /// normally unless there are sufficient system resources to construct
    /// the object.
    explicit
    Condition(const bsl::chrono::system_clock&);
 
    /// Create a condition variable object.  Use the monotonic system clock
    /// as the clock against which the `absTime` timeouts passed to the
    /// `timedWait` methods are interpreted (see {Supported Clock-Types} in
    /// the component-level documentation).  This method does not return
    /// normally unless there are sufficient system resources to construct
    /// the object.
    explicit
    Condition(const bsl::chrono::steady_clock&);
#endif
 
    /// Destroy this condition variable object.
    ~Condition();
 
    // MANIPULATORS
 
    /// Signal this condition variable object by waking up *all* threads
    /// that are currently waiting on this condition.  If there are no
    /// threads waiting on this condition, this method has no effect.
    void broadcast();
 
    /// Signal this condition variable object by waking up a single thread
    /// that is currently waiting on this condition.  If there are no
    /// threads waiting on this condition, this method has no effect.
    void signal();
 
    /// Atomically unlock the specified `mutex` and suspend execution of the
    /// current thread until this condition object is "signaled" (i.e., one
    /// of the `signal` or `broadcast` methods is invoked on this object) or
    /// until the specified `absTime` timeout expires, then re-acquire a
    /// lock on the `mutex`.  `absTime` is an *absolute* time represented as
    /// an interval from some epoch, which is determined by the clock
    /// indicated at construction (see {Supported Clock-Types} in the
    /// component-level documentation), and is the earliest time at which
    /// the timeout may occur.  The `mutex` remains locked by the calling
    /// thread upon returning from this function.  Return 0 on success, and
    /// `e_TIMED_OUT` on timeout.  Any other value indicates that an error
    ///  has occurred.  After an error, the condition may be destroyed, but
    /// any other use has undefined behavior.  
    /// 
    /// \pre The behavior is undefined   unless `mutex` is locked by the calling thread prior to calling this method.  
    /// 
    /// \note Note that   spurious wakeups are rare but possible, i.e.,
    /// this method may succeed (return 0) and return control to the thread
    /// without the condition object being signaled.  Also note that the
    /// actual time of the timeout depends on many factors including system
    /// scheduling and system timer resolution, and may be significantly
    /// later than the time requested.
    int timedWait(Mutex *mutex, const bsls::TimeInterval& absTime);
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP11_BASELINE_LIBRARY
    /// Atomically unlock the specified `mutex` and suspend execution of the
    /// current thread until this condition object is "signaled" (i.e., one
    /// of the `signal` or `broadcast` methods is invoked on this object) or
    /// until the specified `absTime` timeout expires, then re-acquire a
    /// lock on the `mutex`.  `absTime` is an *absolute* time represented as
    /// an interval from some epoch, which is determined by the clock
    /// associated with the time point, and is the earliest time at which
    /// the timeout may occur.  The `mutex` remains locked by the calling
    /// thread upon returning from this function.  Return 0 on success, and
    /// `e_TIMED_OUT` on timeout.  Any other value indicates that an error
    /// has occurred.  After an error, the condition may be destroyed, but
    /// any other use has undefined behavior.  
    /// 
    /// \pre The behavior is undefined   unless `mutex` is locked by the calling thread prior to calling this method.  
    /// 
    /// \note Note that   spurious wakeups are rare but possible, i.e.,
    /// this method may succeed (return 0) and return control to the thread
    /// without the condition object being signaled.  Also note that the
    /// actual time of the timeout depends on many factors including system
    /// scheduling and system timer resolution, and may be significantly
    /// later than the time requested.  Also note that the lock on `mutex`
    /// may be released and reacquired more than once before this method
    /// returns.
    template <class CLOCK, class DURATION>
    int timedWait(Mutex                                           *mutex,
                  const bsl::chrono::time_point<CLOCK, DURATION>&  absTime);
#endif
 
    /// Atomically unlock the specified `mutex` and suspend execution of the
    /// current thread until this condition object is "signaled" (i.e.,
    /// either `signal` or `broadcast` is invoked on this object in another
    /// thread), then re-acquire a lock on the `mutex`.  Return 0 on
    /// success, and a non-zero value otherwise.  Spurious wakeups are rare
    /// but possible; i.e., this method may succeed (return 0), and return
    /// control to the thread without the condition object being signaled. 
    /// 
    /// \pre The behavior is undefined   unless `mutex` is locked by the calling thread prior to calling this method.  
    /// 
    /// \note Note that   `mutex` remains
    /// locked by the calling thread upon return from this function.
    int wait(Mutex *mutex);
 
    // ACCESSORS
 
    /// Return the clock type used for timeouts.
    bsls::SystemClockType::Enum clockType() const;
};
 
 
// ============================================================================
//                             INLINE DEFINITIONS
// ============================================================================
 
                             // ---------------
                             // class Condition
                             // ---------------
 
// CREATORS
inline
Condition::Condition(bsls::SystemClockType::Enum clockType)
: d_imp(clockType)
{
}
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP11_BASELINE_LIBRARY
inline
Condition::Condition(const bsl::chrono::system_clock&)
: d_imp(bsls::SystemClockType::e_REALTIME)
{
}
 
inline
Condition::Condition(const bsl::chrono::steady_clock&)
: d_imp(bsls::SystemClockType::e_MONOTONIC)
{
}
#endif
 
inline
Condition::~Condition()
{
}
 
// MANIPULATORS
inline
void Condition::broadcast()
{
    d_imp.broadcast();
}
 
inline
void Condition::signal()
{
    d_imp.signal();
}
 
inline
int Condition::timedWait(Mutex                     *mutex,
                         const bsls::TimeInterval&  absTime)
{
    BSLS_ASSERT_SAFE(mutex);
 
    return d_imp.timedWait(mutex, absTime);
}
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP11_BASELINE_LIBRARY
template <class CLOCK, class DURATION>
inline
int Condition::timedWait(
                      Mutex                                           *mutex,
                      const bsl::chrono::time_point<CLOCK, DURATION>&  absTime)
{
    BSLS_ASSERT_SAFE(mutex);
 
    return ChronoUtil::timedWait(this, mutex, absTime);
}
#endif
 
inline
int Condition::wait(Mutex *mutex)
{
    BSLS_ASSERT_SAFE(mutex);
 
    return d_imp.wait(mutex);
}
 
// ACCESSORS
inline
bsls::SystemClockType::Enum Condition::clockType() const
{
    return d_imp.clockType();
}
 
}  // close package namespace
 
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2023 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------
 
/** @} */
/** @} */
/** @} */