Path: utzoo!attcan!uunet!munnari.oz.au!uniwa!bilby.cs.uwa.oz.au!kowari!kim
From: kim@bilby.cs.uwa.oz.au (Kim Shearer)
Newsgroups: comp.lang.c
Subject: ftell fseek
Summary: Problem updating files
Keywords: ftell fseek r+
Message-ID: <kim.660990813@kowari>
Date: 12 Dec 90 08:33:33 GMT
Organization: Dept. Computer Science, University of Western Australia.
Lines: 61


 I have been trying to come to terms with file processing in C.
 After reading the manuals etc... I have still not solved my problem

 What I am trying to do is read %s %d pairs from a file, and update
 the %d part if the %s matches a certain value. To do this I use
 ftell to keep track of where I wish to put the number.
 I realise that if things arn't the same size.... but the big problem is
 this :

    the ftell returns 3 (see example file below) and is stored
    in a long, yet the fseek imediately following resets the
    file to the start. I even read the fine print sating that in
    an update opened file you must seperate in and out operations
    by resets or fseeks.

 If anyone can tell me what is wrong or that this is incorrect use 
 of the "r+" opened file and why, I would be very grateful.

EXAMPLE FILE
-----------------
bob 2
fred 2
-----------------
CODE 
-----------------
#include <stdio.h>

#define MAX_NAMELEN 64

int     my_index, my_ndepts = 0;
FILE    *my_file_p;

main ()
{
   int  dept_no, hours_wrkd, i, old_hours, i_own_it = 0;
   char update_name[MAX_NAMELEN], name[MAX_NAMELEN];
   char filename[MAX_NAMELEN];
   int err_ret;
   long marker;

   sprintf (filename, "department%d", 2);
   my_file_p = fopen (filename, "r+");
   if (my_file_p == NULL) {
      printf (" %d could not open file %s\n", my_index, filename);
      perror (filename);
      exit (0);
   }
   fseek (my_file_p, 0L, 0);
   while (fscanf (my_file_p, "%s", name) == 1) {
      marker = ftell (my_file_p);
      fscanf (my_file_p, "%d", &old_hours);
      if (strcmp ("bob", name) == 0) {
         err_ret = fseek (my_file_p, marker, 0);
         printf (" fseek returns : %d\n", err_ret);
         fprintf (my_file_p[i], " %d", old_hours + hours_wrkd);
         err_ret = fseek (my_file_p, marker, 0);
         fscanf (my_file_p[i], "%d", &old_hours);
      }
   }
}