CSP in C. Build with `gcc -o csp csp.c -lpthread`

csp.c

#include <pthread.h>
#include <semaphore.h>
#include <stdlib.h>
#include <stdio.h>

struct channel_node {
	void                *val;
	struct channel_node *next;
	struct channel_node *prev;
};

struct channel {
	struct channel_node *head;
	struct channel_node *tail;
	sem_t                mutex;
	sem_t                sent;
	sem_t                received;
};

struct channel *
channel_init(void)
{
	struct channel *ch = malloc(sizeof(struct channel));
	ch->head = NULL;
	ch->tail = NULL;
	sem_init(&ch->mutex, 0, 1);
	sem_init(&ch->sent, 0, -1);
	sem_init(&ch->received, 0, 0);
	return ch;
}

void *
channel_recv(struct channel *ch)
{
	sem_wait(&ch->mutex);

	struct channel_node *node = ch->head;
	void *msg = node->val;
	if (ch->head == ch->tail) {
		ch->head = NULL;
		ch->tail = NULL;
	} else {
		ch->head = node->next;
	}
	free(node);

	sem_post(&ch->received);
	sem_post(&ch->mutex);

	return msg;
}

void
channel_wait(struct channel *ch, size_t count)
{
	size_t i;
	for (i = 0; i < count; ++i) {
		sem_wait(&ch->received);
		sem_wait(&ch->sent);
	}
}

void
channel_send(struct channel *ch, void *msg)
{
	sem_wait(&ch->mutex);

	struct channel_node *node = malloc(sizeof(struct channel_node));
	node->val = msg;
	node->prev = ch->tail;
	node->next = NULL;
	if (ch->head == NULL) {
		ch->head = node;
		ch->tail = node;
	} else {
		ch->tail->next = node;
		ch->tail = node;
	}

	sem_post(&ch->sent);
	sem_post(&ch->mutex);
}

size_t
channel_length(struct channel *ch)
{
	sem_wait(&ch->mutex);
	size_t len = 0;
	struct channel_node *node = ch->head;
	while (node != NULL) {
		node = node->next;
		++len;
	}
	sem_post(&ch->mutex);
	return len;
}

void
channel_free(struct channel *ch)
{
	if (ch->head != NULL) free(ch->head);
	if (ch->tail != NULL) free(ch->tail);
	sem_destroy(&ch->mutex);
	sem_destroy(&ch->sent);
	sem_destroy(&ch->received);
	free(ch);
}

void *
pinger(void *p)
{
	struct channel *ch = (struct channel *)p;
	channel_send(ch, "ping");
	channel_send(ch, "ping");
	printf("Channel length is %zd\n", channel_length(ch));
	channel_wait(ch, 2);
	char *pong = channel_recv(ch);
	printf("%s\n", pong);
	pong = channel_recv(ch);
	printf("%s\n", pong);
	printf("Channel length is %zd\n", channel_length(ch));
	pthread_exit(NULL);
	return NULL;
}

void *
ponger(void *p)
{
	struct channel *ch = (struct channel *)p;
	char *ping = channel_recv(ch);
	printf("%s\n", ping);
	ping = channel_recv(ch);
	printf("%s\n", ping);
	channel_send(ch, "pong");
	channel_send(ch, "pong");
	pthread_exit(NULL);
	return NULL;
}

int
main(void)
{
	struct channel *ch = channel_init();
	pthread_t t1, t2;
	pthread_create(&t1, NULL, pinger, ch);
	pthread_create(&t2, NULL, ponger, ch);
	pthread_join(t1, NULL);
	pthread_join(t2, NULL);
	channel_free(ch);
	return 0;
}