vsync / thread / cond.h

Condition variable.

A very simple condition variable.

Example:

#include <vsync/thread/mutex.h>
#include <vsync/thread/cond.h>
#include <vsync/common/assert.h>
#include <pthread.h>
#include <stdio.h>

#define N            12
#define EXPECTED_VAL N

vcond_t g_cnd_count_non_zero;
vmutex_t g_mutex_count;
vuint32_t g_count = 0;

void
dec(void)
{
    for (vsize_t i = 0; i < N - 1; i++) {
        vmutex_acquire(&g_mutex_count);
        while (g_count == 0) {
            vcond_wait(&g_cnd_count_non_zero, &g_mutex_count);
        }
        g_count--;
        vmutex_release(&g_mutex_count);
    }
}

void
inc(void)
{
    vmutex_acquire(&g_mutex_count);
    if (g_count == 0) {
        vcond_signal(&g_cnd_count_non_zero);
    }
    g_count++;
    vmutex_release(&g_mutex_count);
}

void *
run(void *args)
{
    vsize_t id = (vsize_t)args;

    if (id == 0) {
        dec();
    } else {
        inc();
    }
    return NULL;
}

int
main(void)
{
    pthread_t threads[N];

    vcond_init(&g_cnd_count_non_zero);
    vmutex_init(&g_mutex_count);

    for (vsize_t i = 0; i < N; i++) {
        pthread_create(&threads[i], NULL, run, (void *)i);
    }

    for (vsize_t i = 0; i < N; i++) {
        pthread_join(threads[i], NULL);
    }
    ASSERT(g_count == 0);
    printf("Final value %u\n", g_count);
    return 0;
}

Note: include mutex.h from libvsync before including cond.h. Alternatively, users can implement the same interface with pthread_mutex_t or similar and include that to be used by cond.h.

Note: on linux compile with -D_GNU_SOURCE.

References:

Condition variable with futex

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Functions

Function	Description
vcond_init	Initializes the given condition variable.
vcond_wait	Waits on the given condition variable.
vcond_signal	Signals the condition variable.

Function vcond_init

static void vcond_init(vcond_t *c)

Initializes the given condition variable.

Parameters:

• c: address of vcond_t object.

Function vcond_wait

static void vcond_wait(vcond_t *c, vmutex_t *m)

Waits on the given condition variable.

Releases the mutex and waits till the condition variable is signaled, then reacquires the mutex.

Parameters:

• c: address of vcond_t object.
• m: address of vmutex_t object.

Function vcond_signal

static void vcond_signal(vcond_t *c)

Signals the condition variable.

Wakes up one sleeping thread waiting on the condition.

Parameters:

• c: address of vcond_t object.

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