ffseek

Date:: 10-20-2011

NAME

ffseek, ffbksp, ffseekf, ffbkspf - Repositions a flexible file I/O

SYNOPSIS

#include <ffio.h>

#include <unistd.h>

off_t ffseek (int fd, off_t pos, int whence);

int ffbksp (int fd);

off_t ffseekf (int fd, off_t pos, int whence, struct ffsw *stat);

int ffbkspf (int fd, struct ffsw *stat);

IMPLEMENTATION

Cray Linux Environment (CLE)

DESCRIPTION

The ffseek function provides flexible file I/O (FFIO) positioning capability similar to that of lseek(2). In addition to the functionality of lseek, ffseek provides access to limited positioning on blocked files and record-oriented files. Specifying ffseek(fd, 0, 0) always rewinds a file to its initial point, regardless of whether the file is streamed or blocked.

This function allows programs to be written so that their I/O can be controlled by the assign(1) command. Both native and foreign record-oriented I/O, multifile datasets, and performance-oriented layers (such as memory-resident layers) are available with this mechanism.

Function ffbksp allows you to perform a backspace operation on those FFIO layers that support it. Currently, this is limited to cos, f77, and text.

Arguments are as follows:

fd

Number returned by ffopen(3c).

pos

Byte position requested.

whence

Specifies one of the following values (defined in header file stdio.h):

0 or SEEK_SET: Sets the pointer to the value of pos. pos must be a non-negative integer. (Special case: ffseek(fd, 0, 0) = rewind)
1 or SEEK_CUR: Sets the pointer to the current position, plus or minus pos. This is supported only in layers that are not record-oriented; layers are specified to assign -F as follows: syscall, mr (memory resident), cache, cachea, bufa.
2 or SEEK_END: Sets the pointer to the end of the file, minus pos. pos must be a non-negative integer. Not all layers support this option. (Special case: ffseek(fd, 0, 2) = position just in front of the EOD.)

stat

Pointer to the ffsw status return structure.

RETURN VALUES

Upon successful completion, a non-negative value is returned. For ffseek/ffseekf this is the current position in the file. Otherwise, -1 is returned, and, if the stat parameter is passed, the error value is found in stat.sw_error. If the stat parameter is not provided, the error code is found in errno.

EXAMPLES

An example program using ffseek is shown below:

#include <stdlib.h>
#include <fcntl.h>
#include <stdio.h>
#include <ffio.h>

main()
{
     size_t ret;
     int fd, i, j;
     off_t fret;

     fd = ffopen("data", O_RDWR | O_CREAT, 0666);

     for (i = 0 ; i < 1000 ; i++)
     {
          ret = ffwrite(fd, &i, sizeof(i));
          if (ret < 0) abort();
     }

     for (i = 0 ; i < 10 ; i++)
     {
          fret = ffseek(fd, i * sizeof(j) * 100, SEEK_SET);
          if (fret < 0) abort();
          ret = ffread(fd, &j, sizeof(j));
          if (ret < 0) abort();
          printf("Value is %d at word %d\n", j, i*100);
     }
}

Output from the previous program is as follows:

     Value is 0 at word 0
     Value is 100 at word 100
     Value is 200 at word 200
     Value is 300 at word 300
     Value is 400 at word 400
     Value is 500 at word 500
     Value is 600 at word 600
     Value is 700 at word 700
     Value is 800 at word 800
     Value is 900 at word 900