RFIO_PRESEEK(3) 					      Rfio Library Functions						   RFIO_PRESEEK(3)

NAME

       rfio_preseek - prefetch chunks of data from a file

SYNOPSIS

       #include <sys/types.h>
       #include "rfio_api.h"

       int rfio_preseek (int s, const struct iovec *iov, int iovnb);

       Under Linux, for large files:
       #define _LARGEFILE64_SOURCE
       #include <sys/types.h>
       #include "rfio_api.h"

       int rfio_preseek64 (int s, const struct iovec64 *iov, int iovnb);

       For large files, under other systems:
       #include <sys/types.h>
       #include "rfio_api.h"

       int rfio_preseek64 (int s, const struct iovec64 *iov, int iovnb);

DESCRIPTION

       rfio_preseek prefetches chunks of data at given offsets into an internal buffer (on the client side), using the descriptor s generated by a
       previous rfio_open.  The actual offset values and the lengths are given in the array of structures iov.	The number of chunks is  specified
       by iovnb.

       rfio_preseek64 does the prefetch for large files, using an array of structures of type iovec64 rather than an array of iovec.

       RFIO_READOPT must be set to RFIO_READBUF, which is the default.	The default internal buffer size is 128 kB, but the buffer size can be set
       with an entry RFIO IOBUFSIZE in shift.conf.  Then rfio_read gets the data from that buffer.

EXAMPLES

       /*   tpreseek - write NBRECORDS_TOWRITE records and
		 read back NBRECORDS_TOREAD using the rfio_preseek function */

       #include <fcntl.h>
       #include <stdio.h>
       #if defined(_WIN32)
       #include <winsock2.h>
       #endif
       #include "rfio_api.h"
       #define NBRECORDS_TOREAD 5
       #define NBRECORDS_TOWRITE 10

       main(argc, argv)
       int argc;
       char **argv;
       {
	    char buf[65536];
	    int errflg = 0;
	    int fd;
	    int i;
	    struct iovec iov[NBRECORDS_TOREAD];
	    int iovnb = NBRECORDS_TOREAD;
	    int j;
	    static int lengths[NBRECORDS_TOWRITE] = {4096, 32768, 16384, 8192,
			      65536, 32768, 16384, 4096, 65536, 8192};
	    static int records_toread[NBRECORDS_TOREAD] = {2, 4, 5, 8, 9};
       #if defined(_WIN32)
	    WSADATA wsadata;
       #endif

	    if (argc != 2) {
		 fprintf (stderr, "usage: tpreseek pathname
");
		 exit (1);
	    }
       #if defined(_WIN32)
	    if (WSAStartup (MAKEWORD (2, 0), &wsadata)) {
		 fprintf (stderr, "WSAStartup unsuccessful
");
		 exit (2);
	    }
       #endif
	    while (! errflg) {

		 /* Write variable length records.
		  * Each record is filled with the record index
		  */

		 if ((fd = rfio_open (argv[1],
			  O_WRONLY|O_CREAT|O_TRUNC, 0644)) < 0) {
		      rfio_perror ("rfio_open");
		      errflg++;
		      break;
		 }
		 for (j = 0; j < NBRECORDS_TOWRITE; j++) {
		      for (i = 0; i < lengths[j]; i++)
			   buf[i] = j;
		      if (rfio_write (fd, buf, lengths[j]) < 0) {
			   rfio_perror ("rfio_write");
			   errflg++;
			   break;
		      }
		 }
		 (void)rfio_close (fd);
		 if (errflg) break;

		 /* Prefetch a few records: the actual offsets and lengths
		  * of the records is set in the array of iov structures
		  */

		 if ((fd = rfio_open (argv[1], O_RDONLY)) < 0) {
		      rfio_perror ("rfio_open");
		      errflg++;
		      break;
		 }
		 for (j = 0; j < NBRECORDS_TOREAD; j++) {
		      /* compute record offset */
		      iov[j].iov_base = 0;
		      for (i = 0; i < records_toread[j]; i++)
			   iov[j].iov_base = (char *) iov[j].iov_base +
			       lengths[i];
		      /* set length */
		      iov[j].iov_len = lengths[records_toread[j]];
		 }
		 if (rfio_preseek (fd, iov, iovnb) < 0) {
		      rfio_perror ("rfio_preseek");
		      errflg++;
		      break;
		 }

		 /* Read back the records and check their cpntents */

		 for (j = 0; j < NBRECORDS_TOREAD; j++) {
		      if (rfio_lseek (fd, (off_t) iov[j].iov_base,
			  SEEK_SET) < 0) {
			   rfio_perror ("rfio_lseek");
			   errflg++;
			   break;
		      }
		      if (rfio_read (fd, buf, iov[j].iov_len) < 0) {
			   rfio_perror ("rfio_read");
			   errflg++;
			   break;
		      }
		      for (i = 0; i < iov[j].iov_len; i++) {
			   if (buf[i] != records_toread[j]) {
				fprintf (stderr,
				    "incorrect data read, record %d
",
				    records_toread[j]);
				errflg++;
				break;
			   }
		      }
		      if (errflg) break;
		 }
		 (void) rfio_close (fd);
		 break;
	    }
	    if (rfio_unlink (argv[1]) < 0) {
		 rfio_perror ("rfio_unlink");
		 errflg++;
	    }
       #if defined(_WIN32)
	    WSACleanup();
       #endif
	    exit (errflg ? 1 : 0);
       }

RETURN VALUE

       This routine returns 0 if the operation was successful or -1 if the operation failed. In the latter case, serrno is set appropriately.

ERRORS

       EBADF	    s is not a valid descriptor.

       EINVAL	    RFIO_READOPT is not set to RFIO_READBUF.

       SENOSHOST    Host unknown.

       SENOSSERV    Service unknown.

       SETIMEDOUT   Timed out.

       SEBADVERSION Version ID mismatch.

       SECONNDROP   Connection closed by remote end.

       SECOMERR     Communication error.

       SENORCODE    Host did not return error number.

SEE ALSO

       rfio_lseek(3), rfio_open(3), rfio_read(3)

AUTHOR

       LCG Grid Deployment Team

LCG
							   $Date: 2005/03/31 13:13:03 $ 					   RFIO_PRESEEK(3)