fsetpos

fsetpos

Reposition a File

Portability: ISO/ANSI C conforming

SYNOPSIS
DESCRIPTION
RETURN VALUE
CAUTIONS
IMPLEMENTATION
EXAMPLE
RELATED FUNCTIONS
SEE ALSO

SYNOPSIS

#include <stdio.h>

int fsetpos(FILE *f, const fpos_t *pos);

DESCRIPTION

fsetpos positions the stream associated with the FILE object addressed by f to the position specified by the object pointed to by pos . This object is of type fpos_t , which is defined in stdio.h .

The value of the object addressed by pos may have been stored by a previous call to fgetpos for the same stream, or it may have been constructed by some other mechanism. The use of values that have not been generated by fgetpos is nonportable. See File positioning with fgetpos and fsetpos for information on the interpretation of file-position values.

The fsetpos function can be used with most files, accessed either as text or binary. Note that it may be used to reposition files that fseek cannot process, including files accessed as a binary stream using the "seq" access method. See OS/390 Files with Restricted Positioning and CMS Files with Restricted Positioning for file types that do not fully support fsetpos .

After a call to fsetpos on a stream that permits both reading and writing, the next file operation may be input or output.

RETURN VALUE

If successful, fsetpos returns 0. If it fails, fsetpos returns a nonzero value and stores an appropriate error code in errno . See the list of errno values in The errno Variable. Calls to fsetpos with an invalid pos may not be detected immediately, but such calls will probably cause errors whenever the file is next read or written.

CAUTIONS

If output occurs after a call to fsetpos , characters following the point of output may be erased from the file. This occurs when trunc=yes is in effect. Therefore, when trunc=yes is in effect, you are unable to return to a previous file position if output has been performed before that point.

See Opening Files for more information on the trunc amparm.

A program that makes direct use of the components of an fpos_t value is not portable.

IMPLEMENTATION

See File positioning with fgetpos and fsetpos for information on the implementation of fgetpos and the structure of fpos_t values.

EXAMPLE

This example illustrates how to use fsetpos and fgetpos to build and use an index file for access to individual lines of a file:

#include <stdio.h>
#include <string.h>
#define KEYLEN 10              /* size of key in record       */
#define DATALEN 70             /* size of data area in record */
#define TABSIZE 100            /* maximum number of records   */

struct  {
   char keyval[KEYLEN];
   fpos_t location;
   }  keytable[TABSIZE];

struct record {
   char keyval[KEYLEN];
   char data[DATALEN];
   };

int filesize;

void bldtable(FILE *fileptr);
int findrec(FILE *fileptr, char keyval[KEYLEN], struct record *input);

main()
{
   FILE *fileptr;
   struct record output;
   char key[KEYLEN] = "PAR-94412M";      /* key to be found */

      /* Open data file and build index file.               */
      /* Example of information in data file:               */

      /* PAR-97612MPearl & Black            $325.00         */
      /* PAR-94412MMarbled Green            $275.00         */

   if ((fileptr = fopen("ddn:DATA", "rb")) != NULL)
     bldtable(fileptr);
   else{
     puts("Unable to open input file.");
     exit(99);
   }

      /* Find desired key.                                  */
   if ( !findrec(fileptr, key, &output) )
     printf("Data area associated with key %.*s is: %.*s\n",
             KEYLEN, key, DATALEN, output.data);
   else
     puts("Unable to find matching key.");
   }

   /* Build the table for key and record addresses.         */
void bldtable(FILE *fileptr)
{
   struct record input;
   int index = 0;

   for (;;){

         /* Store file pointer location.                    */
      fgetpos(fileptr, &keytable[index].location);

         /* Read one record.                                */
      fread(&input, sizeof(struct record), 1, fileptr);
      if (feof(fileptr) || ferror(fileptr))
         break;

         /* Save the keyval.                                */
      memcpy(keytable[index].keyval, input.keyval, KEYLEN);
      ++index;
   }

   filesize = index;
   return;
}

   /* Find the key.                                         */
int findrec(FILE *fileptr, char keyval[KEYLEN],
    struct record *input)                                 
{
   int index;

      /* Search keytable for specified keyval.              */
   for(index = 0; index < filesize; ++index)
   if (memcmp(keyval, keytable[index].keyval,KEYLEN) == 0)
      break;

      /* Was the key found?                                 */
   if (index >= filesize)
      return -1;

      /* If found, read complete record from file.          */
   fsetpos(fileptr, &keytable[index].location);
   fread(input, sizeof(struct record), 1, fileptr);
   return 0;

RELATED FUNCTIONS

fgetpos , fseek , lseek