source: example/io/filereader.c @ f520c4c

/*
This is a file reading example that users io_uring in non-blocking mode.
It demonstrates the bare minimum needed to use io_uring.
It also optionally pre-registers the file descriptors (and a pipe, just to show it works).
It uses liburing for simplicity.
*/


#include <errno.h>
#include <fcntl.h>
#include <liburing.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char * argv[]) {
        if(argc != 3 && argc != 4) {
            printf("usage:   %s FILE TIMES [fixed] - read FILE from disk TIMES times\n", argv[0]);
            return EXIT_FAILURE;
      }

        bool fixed = false;
        if(argc == 4) {
                fixed = 0 == strcmp(argv[3], "fixed");
        }

      int times = atoi( argv[2] );
      if(times <= 0) {
            printf("Invalid number of times %d (from %s).\n", times, argv[2]);
            return EXIT_FAILURE;
      }

      int fd = open(argv[1], 0);
      if(fd < 0) {
            printf("Could not open file %s.\n", argv[1]);
            return EXIT_FAILURE;
      }

        int rfd = fd;

        /* prep the array */
      char data[100];
      struct iovec iov = { data, 100 };

        /* init liburing */
        struct io_uring ring;
      io_uring_queue_init(256, &ring, 0);

        int pipefds[2];
        if(fixed) {
                int ret = pipe(pipefds);
                if( ret < 0 ) {
                        printf("Pipe Error : %s\n", strerror( errno ));
                        return EXIT_FAILURE;
                }
                rfd = 0;
                int fds[] = {
                        fd, pipefds[0], pipefds[1]
                };
                int cnt = sizeof(fds) / sizeof(fds[0]);
                printf("Registering %d files as fixed\n", cnt);
                ret = io_uring_register_files(&ring, fds, cnt);
                if( ret < 0 ) {
                        printf("Register Error : %s\n", strerror( -ret ));
                        return EXIT_FAILURE;
                }
        }

      /* declare required structs */
        printf("Reading %s(%d) %d times\n", argv[1], fd, times);
        size_t counter = 0;
        for(int i = 0; i < times; i++) {
                /* get an sqe and fill in a READV operation */
              struct io_uring_sqe * sqe = io_uring_get_sqe(&ring);
                io_uring_prep_readv(sqe, rfd, &iov, 1, 0);
                if(fixed) {
                        sqe->flags = IOSQE_FIXED_FILE;
                }

                /* tell the kernel we have an sqe ready for consumption */
        io_uring_submit(&ring);

                /* poll the cq and count how much polling we did */
                while(true) {
                        struct io_uring_cqe * cqe = NULL;
                        /* wait for the sqe to complete */
                        int ret = io_uring_wait_cqe_nr(&ring, &cqe, 0);

                        /* read and process cqe event */
                        switch(ret) {
                        case 0:
                                if( cqe->res < 0 ) {
                                        printf("Completion Error : %s\n", strerror( -cqe->res ));
                                        return EXIT_FAILURE;
                                }
                                io_uring_cqe_seen(&ring, cqe);
                                goto LOOP;
                        case -EAGAIN:
                                counter++;
                                break;
                        default:
                                printf("Wait Error : %s\n", strerror( -ret ));
                                return EXIT_FAILURE;
                        }
                }

                LOOP:;
        }

        printf("%zu\n", counter);

      io_uring_queue_exit(&ring);

      close(fd);

        if(fixed) {
                close(pipefds[0]);
                close(pipefds[1]);
        }
}