// Ivan Novick
// Aug 26, 2009
// Direct IO to a device with no filesystem

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <errno.h>
#include <malloc.h>

#define BUFSIZE 4096
#define LOCATION 4096*16
#define DATA_SIZE 100

int main(int argc, char** argv) {
    int fd;
    int r;
    int i;

    // allocate blocks that are aligned for direct IO
    char* buf = memalign(BUFSIZE, BUFSIZE);
    char* readbuf = memalign(BUFSIZE, BUFSIZE);
    memset(buf, 0, BUFSIZE);
    memset(readbuf, 0, BUFSIZE);
    for(i = 0; i < DATA_SIZE ; ++i){
        buf[i] = i;
    }


    // open a device 
    fd = open("/dev/sdc", O_DIRECT|O_WRONLY, 0);
    if (fd < 0) {
        fprintf(stderr, "return from open %d\n", fd);
        return 1;
    }

    // seek to a position in the device
    r = lseek(fd, LOCATION, SEEK_SET);
    if (r < 0) {
        fprintf(stderr, "return from lseek unexpected -- %d -- %s\n", r, strerror(errno));
        return 1;
    }
    

    // write the buffer to the device
    r = write(fd, buf, BUFSIZE);
    if (r != BUFSIZE) {
        fprintf(stderr, "return from write unexpected -- %d -- %s\n", r, strerror(errno));
        return 1;
    }

    close(fd);

    // reopen the device for reading
    fd = open("/dev/sdc", O_RDONLY, 0);
    if (fd < 0) {
        fprintf(stderr, "return from open %d\n", fd);
        return 1;
    }

    // seek to the position where the data should be
    r = lseek(fd, LOCATION, SEEK_SET);
    if (r < 0) {
        fprintf(stderr, "return from lseek unexpected -- %d -- %s\n", r, strerror(errno));
        return 1;
    }


    // read the data from the device
    r = read(fd, readbuf, BUFSIZE);
    close(fd);

    printf("read %d bytes from device\n", r);
    for (i = 0; i < DATA_SIZE; ++i){
        printf("%d ", readbuf[i]);  
    }

    printf("\n");

    // free the memory
    free(buf);
    free(readbuf);
    return 0;
}