---
name: "rfio_preseek64"
section: "3"
platform: "linux"
package: "libdpm-dev"
version: "1.10.0-2+b3"
release: "Debian 10 (Buster)"
platforms: ["linux"]
url: "https://new.carta.tech/rfio_preseek64"
---

# rfio\_preseek64(3)

prefetch chunks of data from a file

## 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\n");
		exit (1);
	}
#if defined(_WIN32)
	if (WSAStartup (MAKEWORD (2, 0), &wsadata)) {
		fprintf (stderr, "WSAStartup unsuccessful\n");
		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\n",
					    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_lseek/3.md)**, **[rfio\_open(3)](/rfio_open/3.md)**, **[rfio\_read(3)](/rfio_read/3.md)**

## AUTHOR

**LCG Grid Deployment** Team