Add example of closing a ring fd and using it via registered index

Based on io_uring-test, modified to use io_uring_close_ring_fd.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>

examples/Makefile

@@ -11,6 +11,7 @@ include ../config-host.mak
 endif
 
 example_srcs := \
+	io_uring-close-test.c \
 	io_uring-cp.c \
 	io_uring-test.c \
 	io_uring-udp.c \

examples/io_uring-close-test.c

@@ -0,0 +1,123 @@
+/* SPDX-License-Identifier: MIT */
+/*
+ * Simple app that demonstrates how to setup an io_uring interface, and use it
+ * via a registered ring fd, without leaving the original fd open.
+ *
+ * gcc -Wall -O2 -D_GNU_SOURCE -o io_uring-close-test io_uring-close-test.c -luring
+ */
+#include <stdio.h>
+#include <fcntl.h>
+#include <string.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include "liburing.h"
+
+#define QD	4
+
+int main(int argc, char *argv[])
+{
+	struct io_uring ring;
+	int i, fd, ret, pending, done;
+	struct io_uring_sqe *sqe;
+	struct io_uring_cqe *cqe;
+	struct iovec *iovecs;
+	struct stat sb;
+	ssize_t fsize;
+	off_t offset;
+	void *buf;
+
+	if (argc < 2) {
+		printf("%s: file\n", argv[0]);
+		return 1;
+	}
+
+	ret = io_uring_queue_init(QD, &ring, 0);
+	if (ret < 0) {
+		fprintf(stderr, "queue_init: %s\n", strerror(-ret));
+		return 1;
+	}
+
+	ret = io_uring_register_ring_fd(&ring);
+	if (ret < 0) {
+		fprintf(stderr, "register_ring_fd: %s\n", strerror(-ret));
+		return 1;
+	}
+	ret = io_uring_close_ring_fd(&ring);
+	if (ret < 0) {
+		fprintf(stderr, "close_ring_fd: %s\n", strerror(-ret));
+		return 1;
+	}
+
+	fd = open(argv[1], O_RDONLY);
+	if (fd < 0) {
+		perror("open");
+		return 1;
+	}
+
+	if (fstat(fd, &sb) < 0) {
+		perror("fstat");
+		return 1;
+	}
+
+	fsize = 0;
+	iovecs = calloc(QD, sizeof(struct iovec));
+	for (i = 0; i < QD; i++) {
+		if (posix_memalign(&buf, 4096, 4096))
+			return 1;
+		iovecs[i].iov_base = buf;
+		iovecs[i].iov_len = 4096;
+		fsize += 4096;
+	}
+
+	offset = 0;
+	i = 0;
+	do {
+		sqe = io_uring_get_sqe(&ring);
+		if (!sqe)
+			break;
+		io_uring_prep_readv(sqe, fd, &iovecs[i], 1, offset);
+		offset += iovecs[i].iov_len;
+		i++;
+		if (offset > sb.st_size)
+			break;
+	} while (1);
+
+	ret = io_uring_submit(&ring);
+	if (ret < 0) {
+		fprintf(stderr, "io_uring_submit: %s\n", strerror(-ret));
+		return 1;
+	} else if (ret != i) {
+		fprintf(stderr, "io_uring_submit submitted less %d\n", ret);
+		return 1;
+	}
+
+	done = 0;
+	pending = ret;
+	fsize = 0;
+	for (i = 0; i < pending; i++) {
+		ret = io_uring_wait_cqe(&ring, &cqe);
+		if (ret < 0) {
+			fprintf(stderr, "io_uring_wait_cqe: %s\n", strerror(-ret));
+			return 1;
+		}
+
+		done++;
+		ret = 0;
+		if (cqe->res != 4096 && cqe->res + fsize != sb.st_size) {
+			fprintf(stderr, "ret=%d, wanted 4096\n", cqe->res);
+			ret = 1;
+		}
+		fsize += cqe->res;
+		io_uring_cqe_seen(&ring, cqe);
+		if (ret)
+			break;
+	}
+
+	printf("Submitted=%d, completed=%d, bytes=%lu\n", pending, done,
+						(unsigned long) fsize);
+	close(fd);
+	io_uring_queue_exit(&ring);
+	return 0;
+}