source: lib/data-struct/message_queue.c @ 8c42377

#include "message_queue.h"

#include <unistd.h>
#include <stdio.h>
#include <limits.h>
#include <assert.h>

/**
 * TODO look into using eventfd instead of a pipe if we have it available XXX
 */

/** 
 * @param mq A pointer to allocated space for a libtrace message queue
 * @param message_len The size in bytes of the message item, to ensure thread safety this
 *                should be less than PIPE_BUF (normally at least 512bytes)
 *                see: man 7 pipe notes on atomic operations
 */
inline void libtrace_message_queue_init(libtrace_message_queue_t *mq, size_t message_len)
{
        assert(message_len);
        assert(pipe(mq->pipefd) != -1);
        mq->message_count = 0;
        if (message_len > PIPE_BUF)
                fprintf(stderr, "Warning message queue wont be atomic (thread safe) message_len(%zu) > PIPE_BUF(%d)\n",
                                        message_len, PIPE_BUF);
        mq->message_len = message_len;
        pthread_spin_init(&mq->spin, 0);
}

/**
 * Posts a message to the given message queue.
 * 
 * This will block if a reader is not keeping up and the underlying pipe
 * fills up.
 * 
 * @param mq A pointer to a initilised libtrace message queue structure (NOT NULL)
 * @param message A pointer to the message data you wish to send
 * @return A number representing the number of messages already in the queue,
 *         0 implies a thread was waiting and will read your message, negative
 *         numbers implies threads are still waiting. Positive implies a backlog
 *         of messages.
 */
inline int libtrace_message_queue_put(libtrace_message_queue_t *mq, const void *message)
{
        int ret;
        assert(mq->message_len);
        assert(write(mq->pipefd[1], message, mq->message_len) == (int) mq->message_len);
        // Update after we've written
        pthread_spin_lock(&mq->spin);
        ret = ++mq->message_count; // Should be CAS!
        pthread_spin_unlock(&mq->spin);
        return ret;
}

/**
 * Retrieves a message from the given message queue.
 * 
 * This will block if a reader is not keeping up and the underlying pipe
 * fills up.
 * 
 * @param mq A pointer to a initilised libtrace message queue structure (NOT NULL)
 * @param message A pointer to the message data you wish to send
 * @return The number of messages remaining in the queue less any threads waiting,
 *         0 implies a thread was waiting and will read your message, negative
 *         numbers implies threads are still waiting. Positive implies a backlog
 *         of messages.
 */
inline int libtrace_message_queue_get(libtrace_message_queue_t *mq, void *message)
{
        int ret;
        // Safely decrease count first - Yes this might make us negative, however thats ok once a write comes in everything will be fine
        pthread_spin_lock(&mq->spin);
        ret = mq->message_count--;
        pthread_spin_unlock(&mq->spin);
        assert(read(mq->pipefd[0], message, mq->message_len) == (int) mq->message_len);
        return ret;
}

/**
 * Trys to retrieve a message from the given message queue.
 * 
 * This will not block and instead returns LIBTRACE_MQ_FAILED if
 * no message is available.
 * 
 * @param mq A pointer to a initilised libtrace message queue structure (NOT NULL)
 * @param message A pointer to the message data you wish to send
 * @return The number of messages remaining in the queue less any threads waiting,
 *         0 implies a thread was waiting and will read your message, negative
 *         numbers implies threads are still waiting. Positive implies a backlog
 *         of messages.
 */
inline int libtrace_message_queue_try_get(libtrace_message_queue_t *mq, void *message)
{
        int ret;
        // Safely decrease count first - Yes this might make us negative, however thats ok once a write comes in everything will be fine
        // ->Fast path avoid the lock
        if (mq->message_count <= 0)
                return LIBTRACE_MQ_FAILED;
        // Else grab lock and confirm this is so
        pthread_spin_lock(&mq->spin);
        if (mq->message_count > 0) {
                ret = --mq->message_count;
                // :( read(...) needs to be done within the *spin* lock otherwise blocking might steal our read
                assert(read(mq->pipefd[0], message, mq->message_len) == (int) mq->message_len);
        } else {
                ret = LIBTRACE_MQ_FAILED;
        }
        pthread_spin_unlock(&mq->spin);
        return ret;
}

/**
 * May be negative if threads blocking and waiting for a message.
 */
inline int libtrace_message_queue_count(const libtrace_message_queue_t *mq)
{
        // This is only ok because we know int is atomic
        return mq->message_count;
}

inline void libtrace_message_queue_destroy(libtrace_message_queue_t *mq)
{
        mq->message_count = 0;
        mq->message_len = 0;
        close(mq->pipefd[0]);
        close(mq->pipefd[1]);
        pthread_spin_destroy(&mq->spin);
}

/**
 * @return a file descriptor for the queue, can be used with select() poll() etc.
 */
inline int libtrace_message_queue_get_fd(libtrace_message_queue_t *mq)
{
        return mq->pipefd[0];
}