kim@bilby.cs.uwa.oz.au (Kim Shearer) (12/12/90)
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);
}
}
}henry@zoo.toronto.edu (Henry Spencer) (12/13/90)
In article <kim.660990813@kowari> kim@bilby.cs.uwa.oz.au (Kim Shearer) writes: > fprintf (my_file_p[i], " %d", old_hours + hours_wrkd); What is the value of `i', and why on Earth are you subscripting the file pointer with it? -- "The average pointer, statistically, |Henry Spencer at U of Toronto Zoology points somewhere in X." -Hugh Redelmeier| henry@zoo.toronto.edu utzoo!henry
ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) (12/13/90)
In article <kim.660990813@kowari>, kim@bilby.cs.uwa.oz.au (Kim Shearer) writes: --> while (fscanf (my_file_p, "%s", name) == 1) { --> marker = ftell (my_file_p); > fscanf (my_file_p, "%d", &old_hours); There's at least one of your problems. You're getting the position of the file JUST AFTER a string. What you want is the position BEFORE. In short: while (marker = ftell(my_file_p), 2 == fscanf(my_file_p, "% s%d", name, &old_hours) ) { /* at this point, fseek()ing back to 'marker' will get you to the point BEFORE the string, so that another fscanf() like the one above will read the right stuff. */ } A word of warning: it would be as well to protect yourself against overflow. char format[80]; char check; sprintf(format, "%%%us%%c%%d", (sizeof name) - 1); ... 3 == fscanf(my_file_p, format, name, &check, &old_hours) && isspace(check) ... to read no more characters into name[] than will fit, and check that reading into name stopped because there was no more of the strig to read, not because name[] ran out of space. (The *scanf() functions are enough to make a C programmer think about getting f2c ...) -- The Marxists have merely _interpreted_ Marxism in various ways; the point, however, is to _change_ it. -- R. Hochhuth.