filebuff/direct_read.c

/* direct_read.c

   Demonstrate the use of O_DIRECT to perform I/O bypassing the buffer cache
   ("direct I/O").

   Usage: direct_read file length [offset [alignment]]

   This program is Linux-specific.
*/
#define _GNU_SOURCE     /* Obtain O_DIRECT definition from <fcntl.h> */
#include <fcntl.h>
#include <malloc.h>
#include "tlpi_hdr.h"

int
main(int argc, char *argv[])
{
    int fd;
    ssize_t numRead;
    size_t length, alignment;
    off_t offset;
    char *buf;

    if (argc < 3 || strcmp(argv[1], "--help") == 0)
        usageErr("%s file length [offset [alignment]]\n", argv[0]);

    length = getLong(argv[2], GN_ANY_BASE, "length");
    offset = (argc > 3) ? getLong(argv[3], GN_ANY_BASE, "offset") : 0;
    alignment = (argc > 4) ? getLong(argv[4], GN_ANY_BASE, "alignment") : 4096;

    fd = open(argv[1], O_RDONLY | O_DIRECT);
    if (fd == -1)
        errExit("open");

    /* memalign() allocates a block of memory aligned on an address that
       is a multiple of its first argument. By specifying this argument as
       2 * 'alignment' and then adding 'alignment' to the returned pointer,
       we ensure that 'buf' is aligned on an odd multiple of
       'alignment'. We do this to ensure that if, for example, we ask
       for a 256-byte aligned buffer, we don't accidentally get
       a buffer that is also aligned on a 512-byte boundary. */

    buf = memalign(alignment * 2, length + alignment);
    if (buf == NULL)
        errExit("memalign");

    buf += alignment;

    if (lseek(fd, offset, SEEK_SET) == -1)
        errExit("lseek");

    numRead = read(fd, buf, length);
    if (numRead == -1)
        errExit("read");
    printf("Read %ld bytes\n", (long) numRead);

    exit(EXIT_SUCCESS);
}