Fseek doesn't sets the file position of the stream in C

Question

i make this simple C program whose purpose is to count the characters that are in a text file which is given through the second command line argument. The problem i face is that fseek shows not responds with a result to have an infinite loop (while(!feof(fp))) in function "Counter".By replacing the fseek with a fgetc the programm works fine.My my question is what is going wrong with the fseek. Thanks in advance.

#include <stdio.h>

int Counter (FILE * fp);

int main(int argc, char* argv[])
{
   int cntr;
   FILE * fpc;
   fpc = fopen(argv[1],"r");
   cntr = Counter(fpc);
   fclose(fpc);
   printf("%i\n",cntr);
}

int Counter (FILE * fp)
{
    int cntr = 0;
    while (!feof(fp))
    {
        cntr++;
        fseek(fp,1,1);
    }
    return cntr;
}

Use constant `SEEK_CUR` instead of 1, it makes the code clearer. — Matthieu, Apr 30 '16 at 16:27
See [Why is `while(!feof(file)) always wrong?](http://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong). — John Bollinger, Apr 30 '16 at 16:28
Of course, because `SEEK_CUR` is defined as `1` :) What does `ftell(fp)` tell you in the middle of the `while` loop? — Matthieu, Apr 30 '16 at 16:30
Consider using `fseek(fp, 0, SEEK_END)` combined with `ftell()`. — John Bollinger, Apr 30 '16 at 16:32
Alternatively, use `fgetc()` if you feel obligated to count bytes one by one. — John Bollinger, Apr 30 '16 at 16:33
Yes, `fseek(fp,0,SEEK_END); cntr=ftell();` will give you what you're expecting. — Matthieu, Apr 30 '16 at 16:34
I am geting the same result with this fseek(fp,0,SEEK_END); cntr=ftell(); — Spyreto, Apr 30 '16 at 16:54
"Why is `while(!feof(file)) always wrong?" was very usefull thanks for your time. — Spyreto, Apr 30 '16 at 17:21

alk · Accepted Answer · 2016-04-30T16:33:37.650

3

It is allowed and well defined behaviour to seek beyond the end of a file.

That's why fseek() does not set the end-of-file indicator, it even more unsets it on success.

From the C Standard:

5 After determining the new position, a successful call to the fseek function [...] clears the end-of-file indicator for the stream, and then establishes the new position.

edited Apr 30 '16 at 16:33

answered Apr 30 '16 at 16:30

alk

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Good catch! I have seen code where you can write a byte, `fseek` 1 GB further and write another byte, leading to the file system seeing a 1 GB file fitting on a 256 MB disk. – Matthieu Apr 30 '16 at 16:33

Fseek doesn't sets the file position of the stream in C

1 Answers1