Using O_DIRECT

November 25th, 2007 by admin

It is sometime useful for a user program to request that to be able to read and write directly from a storage device. All DMA operation, if any, will be performed directly into the application memory space, without being going through the kernel page cache.

Here is a small code example showing how this can accomplished if proper support is provided by the file system and storage device.

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <linux/fcntl.h>
#include <sys/mman.h>
 
static char * progname;
#define PAGE_SIZE (4096)
 
void usage(void) {
    printf("Usage: %s [filename]\n", progname);
    return;
}
 
int main(int argc, char * argv[]) {
 
    const char * filename;
    int fd, ret;
    char *buffer;
 
    progname = argv[0];
    if (argc != 2) {
        usage();
        exit(0);
    }
    filename = argv[1];
    ret = posix_memalign(&amp;buffer, 512, PAGE_SIZE);
    if(ret) {
      printf("%s: %s", progname, strerror(ret));
      exit(-5);
    }
    printf("%s: Got aligned buffer %p\n", 
       progname, buffer);
    fd = open(filename, O_RDWR|O_CREAT|O_DIRECT, 
        S_IRWXU);
    if(-1 == fd) {
        perror(progname);
        exit(-1);
    }
    strcpy(buffer, "testing testing 1 2 3!");
    ret = write(fd, buffer, PAGE_SIZE);
 
    if(-1 == ret) {
      perror(progname);
      exit(-2);
    }
 
        printf("%s: Written: %s\n", progname, buffer);
 
    lseek(fd, SEEK_SET, 0);
    ret = read(fd, buffer, PAGE_SIZE);
    if(-1 == ret) {
       perror(progname);
       exit(-2);
     }
 
    printf("%s: Got %s\n", progname, buffer);
    return 0;
}