// "iotest.c"

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <sys/uio.h>
#include <unistd.h>
#include <errno.h>
#include <assert.h>

#define ALIGNMENT 131072
#define BUF_SIZE ALIGNMENT

int main(int argc, char *argv[]) {
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <filename>\n", argv[0]);
        return 1;
    }
    const char *filename = argv[1];

    int fd = open(filename, O_RDONLY | O_DIRECT | O_CLOEXEC);
    if (fd < 0) {
        perror("open");
        return 1;
    }

    void *buf;
    if (posix_memalign(&buf, ALIGNMENT, BUF_SIZE) != 0) {
        perror("posix_memalign");
        close(fd);
        return 1;
    }

    memset(buf, 0, BUF_SIZE);

    struct iovec iov = {
        .iov_base = buf,
        .iov_len = BUF_SIZE
    };

    ssize_t offset = 0;
    ssize_t ret = 0;

    while (ret >= 0)
    {
        ret = syscall(SYS_preadv2, fd, &iov, 1, offset, offset >> 32, RWF_NOWAIT);

        if (ret >= 0) {
            printf("preadv2 RWF_NOWAIT succeeded, read %zd bytes\n", ret);
        } else {
            if (errno == EAGAIN) {
                printf("preadv2 RWF_NOWAIT returned EAGAIN (data not in page cache)\n");
                return 1;
            } else {
                perror("preadv2");
                return 1;
            }
        }

        if (ret == 0)
        {
            break;
        }

        offset += ret;
    }

    free(buf);
    close(fd);

    return 0;
}