Condition variables in C

A thread waits until another signals that the state it needs has arrived.

#include <pthread.h>

typedef struct {
    int ready;
    pthread_mutex_t lock;
    pthread_cond_t cond;
} Gate;

void gate_init(Gate *g) {
    g->ready = 0;
    pthread_mutex_init(&g->lock, NULL);
    pthread_cond_init(&g->cond, NULL);
}

void gate_wait(Gate *g) {
    pthread_mutex_lock(&g->lock);
    while (!g->ready) {
        pthread_cond_wait(&g->cond, &g->lock);
    }
    pthread_mutex_unlock(&g->lock);
}

void gate_open(Gate *g) {
    pthread_mutex_lock(&g->lock);
    g->ready = 1;
    pthread_cond_broadcast(&g->cond);
    pthread_mutex_unlock(&g->lock);
}

How it works

1. pthread_cond_wait releases the mutex while it waits.
2. The wait must sit inside a loop, because wakeups can be spurious.
3. signal wakes one waiter, broadcast wakes them all.

Keywords and builtins used here

• NULL
• gate_init
• gate_open
• gate_wait
• int
• struct
• typedef
• void
• while

The run, in numbers

Lines

28

Characters to type

505

Tokens

151

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 86 seconds.

Type this snippet

Step 4 of 4 in Locking, step 7 of 10 in Threads & concurrency.

← Previous Next →