[components/libc/posix]add comments for condition variable APIs.

components/libc/posix/pthreads/pthread_cond.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2006-2021, RT-Thread Development Team
+ * Copyright (c) 2006-2024 RT-Thread Development Team
  *
  * SPDX-License-Identifier: Apache-2.0
  *
@@ -72,6 +72,30 @@ int pthread_condattr_setpshared(pthread_condattr_t *attr, int pshared)
 }
 RTM_EXPORT(pthread_condattr_setpshared);
 
+/**
+ * @brief Initializes a condition variable.
+ *
+ * This function initializes the condition variable pointed to by `cond` with the attributes
+ * specified by `attr`. If `attr` is NULL, the condition variable is initialized with the
+ * default attributes.
+ *
+ * @param cond A pointer to the condition variable to be initialized.
+ *             Must point to valid memory.
+ * @param attr A pointer to the condition variable attributes object.
+ *             If NULL, default attributes are used.
+ *
+ * @return
+ * - `0` on success.
+ * - A non-zero error code on failure, including:
+ *   - `EINVAL`: Invalid attributes, invalid condition variable pointer, or semaphore init failed.
+ *
+ * @note
+ * - The condition variable must not be used until it has been initialized.
+ * - Each condition variable must be destroyed using `pthread_cond_destroy()`
+ *   once it is no longer needed.
+ *
+ * @see pthread_cond_destroy, pthread_cond_wait, pthread_cond_signal, pthread_cond_broadcast
+ */
 int pthread_cond_init(pthread_cond_t *cond, const pthread_condattr_t *attr)
 {
     rt_err_t result;
@@ -110,6 +134,30 @@ int pthread_cond_init(pthread_cond_t *cond, const pthread_condattr_t *attr)
 }
 RTM_EXPORT(pthread_cond_init);
 
+/**
+ * @brief Destroys a condition variable.
+ *
+ * This function destroys the condition variable pointed to by `cond`. After a condition
+ * variable is destroyed, it must not be used until it is reinitialized with
+ * `pthread_cond_init`.
+ *
+ * @param cond A pointer to the condition variable to be destroyed.
+ *             Must point to a valid, previously initialized condition variable.
+ *
+ * @return
+ * - `0` on success.
+ * - A non-zero error code on failure, including:
+ *   - `EBUSY`: The condition variable is currently in use by other threads.
+ *   - `EINVAL`: The condition variable is invalid or uninitialized.
+ *
+ * @note
+ * - The condition variable must not be destroyed while it is being used by other threads
+ *   (e.g., in `pthread_cond_wait` or `pthread_cond_timedwait`).
+ * - Attempting to destroy a condition variable that has not been initialized results in
+ *   undefined behavior.
+ *
+ * @see pthread_cond_init, pthread_cond_wait, pthread_cond_signal, pthread_cond_broadcast
+ */
 int pthread_cond_destroy(pthread_cond_t *cond)
 {
     rt_err_t result;
@@ -143,6 +191,30 @@ __retry:
 }
 RTM_EXPORT(pthread_cond_destroy);
 
+/**
+ * @brief Unblocks all threads waiting on the specified condition variable.
+ *
+ * This function wakes up all threads that are currently blocked on the condition variable
+ * pointed to by `cond`. The condition variable must be associated with a mutex, and
+ * threads waiting on the condition variable should recheck the condition after being
+ * unblocked.
+ *
+ * @param cond A pointer to the condition variable.
+ *             Must point to a valid, initialized condition variable.
+ *
+ * @return
+ * - `0` on success.
+ * - A non-zero error code on failure, including:
+ *   - `EINVAL`: The condition variable is invalid or uninitialized.
+ *
+ * @note
+ * - Calling this function does not release the associated mutex.
+ * - Waking up threads does not guarantee that any specific thread will acquire the
+ *   mutex immediately, as thread scheduling depends on the system.
+ * - Typically used when the condition might allow multiple waiting threads to proceed.
+ *
+ * @see pthread_cond_signal, pthread_cond_wait, pthread_cond_init, pthread_cond_destroy
+ */
 int pthread_cond_broadcast(pthread_cond_t *cond)
 {
     rt_err_t result;
@@ -177,6 +249,30 @@ int pthread_cond_broadcast(pthread_cond_t *cond)
 }
 RTM_EXPORT(pthread_cond_broadcast);
 
+/**
+ * @brief Wakes up one thread waiting on the specified condition variable.
+ *
+ * This function unblocks one thread that is currently waiting on the
+ * condition variable `cond`. If multiple threads are waiting, the thread to wake
+ * up is determined by the system's scheduling policies.
+ *
+ * @param cond A pointer to the condition variable to signal.
+ *             Must point to a valid and initialized condition variable.
+ *
+ * @return
+ * - `0` on success.
+ * - A non-zero error code on failure, including:
+ *   - `EINVAL`: The condition variable is invalid or uninitialized.
+ *
+ * @note
+ * - This function does not release the associated mutex.
+ * - If no threads are currently waiting on the condition variable, the call has no effect.
+ * - The awakened thread will not run until it can reacquire the associated mutex and
+ *   re-evaluate the waiting condition.
+ * - It is typically used when only one waiting thread should be allowed to proceed.
+ *
+ * @see pthread_cond_broadcast, pthread_cond_wait, pthread_cond_init, pthread_cond_destroy
+ */
 int pthread_cond_signal(pthread_cond_t *cond)
 {
     rt_base_t temp;
@@ -210,6 +306,35 @@ int pthread_cond_signal(pthread_cond_t *cond)
 }
 RTM_EXPORT(pthread_cond_signal);
 
+/**
+ * @brief Waits on a condition variable with a timeout.
+ *
+ * This function causes the calling thread to block on the condition variable `cond`,
+ * releasing the associated mutex `mutex`. The thread will remain blocked until
+ * one of the following occurs:
+ * - It is signaled or broadcast using `pthread_cond_signal` or `pthread_cond_broadcast`.
+ * - The specified timeout expires.
+ *
+ * @param cond A pointer to the condition variable to wait on.
+ *             Must point to a valid, initialized condition variable.
+ * @param mutex A pointer to the mutex associated with the condition variable.
+ *              Must be locked by the calling thread before invoking this function.
+ * @param timeout The timeout duration in milliseconds. A value of `RT_WAITING_FOREVER`
+ *                indicates the thread will wait indefinitely.
+ *
+ * @return
+ * - `RT_EOK` on successful wakeup (signaled or broadcast).
+ * - `-RT_ETIMEOUT` if the timeout expires before the condition variable is signaled.
+ * - `-RT_ERROR` if an error occurs (e.g., invalid parameters).
+ *
+ * @note
+ * - The mutex is automatically released while the thread waits and re-acquired before
+ *   the function returns.
+ * - If `timeout` is 0, the function behaves as a non-blocking check.
+ * - Ensure the condition variable and mutex are properly initialized before use.
+ *
+ * @see pthread_cond_signal, pthread_cond_broadcast, pthread_mutex_lock, pthread_mutex_unlock
+ */
 rt_err_t _pthread_cond_timedwait(pthread_cond_t *cond,
                                  pthread_mutex_t *mutex,
                                  rt_int32_t timeout)
@@ -327,6 +452,33 @@ rt_err_t _pthread_cond_timedwait(pthread_cond_t *cond,
 }
 RTM_EXPORT(_pthread_cond_timedwait);
 
+/**
+ * @brief Waits on a condition variable.
+ *
+ * This function blocks the calling thread on the condition variable `cond` and releases
+ * the associated mutex `mutex`. The thread remains blocked until it is signaled or
+ * broadcast using `pthread_cond_signal` or `pthread_cond_broadcast`. When the thread
+ * is awakened, it re-acquires the mutex and resumes execution.
+ *
+ * @param cond A pointer to the condition variable to wait on.
+ *             Must point to a valid, initialized condition variable.
+ * @param mutex A pointer to the mutex associated with the condition variable.
+ *              Must be locked by the calling thread before invoking this function.
+ *
+ * @return
+ * - `0` on success.
+ * - A non-zero error code on failure, including:
+ *   - `EINVAL`: The condition variable or mutex is invalid or uninitialized.
+ *
+ * @note
+ * - The mutex must be locked before calling this function.
+ * - Upon returning, the mutex is locked again by the calling thread.
+ * - Spurious wakeups may occur, so the thread should always recheck the waiting
+ *   condition upon wakeup.
+ * - This function may block indefinitely unless the condition is signaled or broadcast.
+ *
+ * @see pthread_cond_signal, pthread_cond_broadcast, pthread_cond_timedwait, pthread_mutex_lock
+ */
 int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex)
 {
     rt_err_t result;
@@ -349,6 +501,37 @@ __retry:
 }
 RTM_EXPORT(pthread_cond_wait);
 
+/**
+ * @brief Waits on a condition variable with a timeout.
+ *
+ * This function blocks the calling thread on the condition variable `cond`, releasing
+ * the associated mutex `mutex`. The thread remains blocked until one of the following occurs:
+ * - The condition variable is signaled or broadcast using `pthread_cond_signal` or `pthread_cond_broadcast`.
+ * - The specified absolute timeout `abstime` is reached.
+ * - A spurious wakeup occurs (requiring the thread to recheck the condition).
+ *
+ * @param cond A pointer to the condition variable to wait on.
+ *             Must point to a valid, initialized condition variable.
+ * @param mutex A pointer to the mutex associated with the condition variable.
+ *              Must be locked by the calling thread before invoking this function.
+ * @param abstime A pointer to a `struct timespec` specifying the absolute timeout (in seconds and nanoseconds
+ *                since the Epoch). If the time specified is already reached, the function immediately returns.
+ *
+ * @return
+ * - `0` on successful wakeup (signaled or broadcast).
+ * - `ETIMEDOUT` if the timeout expires before the condition variable is signaled.
+ * - A non-zero error code on failure, including:
+ *   - `EINVAL`: The condition variable, mutex, or `abstime` is invalid.
+ *   - `EPERM`: The mutex is not owned by the calling thread.
+ *
+ * @note
+ * - The mutex is released while the thread is waiting and re-acquired before the function returns.
+ * - Spurious wakeups may occur, so the thread must always recheck the waiting condition upon wakeup.
+ * - The timeout is specified in absolute time, not relative duration.
+ * - Ensure the condition variable and mutex are properly initialized before use.
+ *
+ * @see pthread_cond_wait, pthread_cond_signal, pthread_cond_broadcast, pthread_mutex_lock
+ */
 int pthread_cond_timedwait(pthread_cond_t *cond,
                            pthread_mutex_t *mutex,
                            const struct timespec *abstime)