source: example/io/attach-block.c@ 50f3f3a

/*
This is an example that uses io_uring with attach mode.
It demonstrates the what happens if some thread submits to
the original ring and then spins, while a second thread waits
on the attached ring. This deadlocks because even if rings are
attached, they don't share a completion queue. Therefore all
completion queues must be polled for progress to be made.

It uses liburing for simplicity.
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <errno.h>
#include <fcntl.h>
#include <unistd.h>

#include <pthread.h>
#include <liburing.h>

volatile bool done = false;

void * submitter(void * arg) {
        struct io_uring * ring = (struct io_uring *)arg;

        const char * path = __FILE__;

        int fd = open(path, 0, O_RDONLY);
        if( fd < 0) {
                fprintf(stderr, "Can't open file: (%d) %s\n", errno, strerror(errno));
                exit(EXIT_FAILURE);
        }
        printf("Opened file: %d\n", fd);

        char buffer[1024];
        struct io_uring_sqe * sqe = io_uring_get_sqe(ring);
        io_uring_prep_read(sqe, fd, buffer, 1024, 0);
        io_uring_submit(ring);

        printf("Submitted read\n");

        printf("Spinning\n");
        while(!__atomic_load_n(&done, __ATOMIC_SEQ_CST));

        return NULL;
}

void * poller(void * arg) {
        struct io_uring * ring = (struct io_uring *)arg;
        struct io_uring_cqe *cqe;

        printf("Waiting for results\n");

        int ret = io_uring_wait_cqe(ring, &cqe);
        if( ret < 0) {
                fprintf(stderr, "ioring wait failed: (%d) %s\n", -ret, strerror(-ret));
                exit(EXIT_FAILURE);
        }
        io_uring_cqe_seen(ring, cqe);

        printf("Got result\n");

        __atomic_store_n(&done, true, __ATOMIC_SEQ_CST);
        return NULL;
}

int main() {
        struct io_uring base_ring;
        io_uring_queue_init(8, &base_ring, 0);
        printf("Created first ring: %d\n", base_ring.ring_fd);

        struct io_uring attached_ring;
        struct io_uring_params par;
        memset(&par, '\0', sizeof(struct io_uring_params));
        par.flags |= IORING_SETUP_ATTACH_WQ;
        par.wq_fd = base_ring.ring_fd;
        io_uring_queue_init_params(8, &attached_ring, &par);
        printf("Attached second ring: %d\n", attached_ring.ring_fd);

        printf("Forking\n");
        pthread_t s, p;

        int ret = pthread_create(&s, NULL, submitter, &base_ring);
        if( ret < 0) {
                fprintf(stderr, "pthread create 1 error: (%d) %s\n", errno, strerror(errno));
                exit(EXIT_FAILURE);
        }

        ret = pthread_create(&p, NULL, poller, &attached_ring);
        if( ret < 0) {
                fprintf(stderr, "pthread create 2 error: (%d) %s\n", errno, strerror(errno));
                exit(EXIT_FAILURE);
        }

        ret = pthread_join(p, NULL);
        if( ret < 0) {
                fprintf(stderr, "pthread join 2 error: (%d) %s\n", errno, strerror(errno));
                exit(EXIT_FAILURE);
        }

        ret = pthread_join(s, NULL);
        if( ret < 0) {
                fprintf(stderr, "pthread join 1 error: (%d) %s\n", errno, strerror(errno));
                exit(EXIT_FAILURE);
        }
}