source: ThirdParty/mpqc_open/src/lib/util/psi3/libpsio/volseek.cc@ 645bed

/*!
   \file volseek.c
   \ingroup (PSIO)
*/

#include <unistd.h>
#include <util/psi3/libpsio/psio.h>

namespace psi3 {
namespace libpsio {

/* This is strictly used to avoid overflow errors on lseek() calls */
#define PSIO_BIGNUM 10000

/*!
** PSIO_VOLSEEK()
**
** \ingroup (PSIO)
*/
int psio_volseek(psio_vol *vol, ULI page, ULI offset, ULI numvols)
{
  int stream, errcod;
  ULI bignum, total_offset;

  bignum = PSIO_BIGNUM*numvols;

  stream = vol->stream;

  /* Set file pointer to beginning of file */
  errcod = lseek(stream, (ULI) 0, SEEK_SET);
  if(errcod == -1) return(errcod);

  /* lseek() through large chunks of the file to avoid offset overflows */
  for(; page > bignum; page -= bignum) {
      total_offset = PSIO_BIGNUM * PSIO_PAGELEN;
      errcod = lseek(stream, total_offset, SEEK_CUR);
      if(errcod == -1) return(errcod);
    }

  /* Now compute the final offset including the page-relative term */ 
  total_offset = (ULI) page/numvols; /* This should truncate */
  total_offset *= PSIO_PAGELEN;
  total_offset += offset; /* Add the page-relative term */
  errcod = lseek(stream, total_offset, SEEK_CUR);
  if(errcod == -1) return(errcod);

  return(0);
}

}
}