C reading binary files

Question

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“while( !feof( file ) )” is always wrong

If I write an array to the output file and I close the file, then open the file again and read everything until end-of-file is reached, although the file contains only 4 number, the program will read and print 5 numbers, why?

Program output:

a[0] = 4
a[1] = 7
a[2] = 12
a[3] = 34
a[4] = 34

save.bin (with a hex editor)

04000000 07000000 0C000000 22000000

#include <stdio.h>
#include <stdlib.h>
#define path "save.bin"

int main(void)
{
  FILE *f=NULL;
  int a[]={4,7,12,34},i,n=4,k;
  f=fopen(path,"wb");
  if(f==NULL)
  {
    perror("Error");
    exit(1);
  }
  for(i=0;i<n;i++)  // or I could use fwrite(a,sizeof(int),n,f);
    fwrite(&a[i],sizeof(int),1,f);
  fclose(f);
  f=fopen(path,"rb");
  if(f==NULL)
  {
    perror("Error");
    exit(1);
  }
  i=0;
  while(!feof(f))
  {
    fread(&k,sizeof(int),1,f);
    printf("a[%d] = %d\n",i,k);
    i++;
  }
  printf("\n");
  fclose(f);
  return 0;
}

score 11 · Accepted Answer · edited May 23 '17 at 11:46

11

feof(fp) becomes false (i.e. non-zero value) only if you tried to read past the end of file. That should explain why the loop is entered one more than what you expect.

From the documentation:

  The function feof() tests the end-of-file indicator for the stream
  pointed to by stream, returning nonzero if it is set.  The end-of-
  file indicator can be cleared only by the function clearerr().

Also read the post: Why is “while ( !feof (file) )” always wrong?

edited May 23 '17 at 11:46

Community

1
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answered Aug 23 '12 at 15:22

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4

+1 for explaining how `feof()` is set. – hmjd Aug 23 '12 at 15:23

hmjd · Answer 2 · 2012-08-23T15:27:56.217

while(!feof(f))
{
    fread(&k,sizeof(int),1,f);
    printf("a[%d] = %d\n",i,k);  /** THIS ASSUMES fread() WAS SUCCESSFUL. **/
    i++;
}

Check for end of file immedately after the call to fread() or check the return value of fread(), which returns the number of items read which in this case should be 1. A possible restructuring of the loop:

while(1 == fread(&k,sizeof(int),1,f))
{
    printf("a[%d] = %d\n",i,k);
    i++;
}

After the loop check feof() to ensure EOF was reached and loop did not end due to some other failure.

score 1 · Answer 3 · answered Aug 23 '12 at 15:31

I have the same problem as well. Try this instead:

i=0;
  while(true)
  {
    fread(&k,sizeof(int),1,f);
    if(feof(f))
        break;

    printf("a[%d] = %d\n",i,k);
    i++;
  }

Apparently feof doesnt return true unless you've read past the end of the file. What i do when writing arrays to binary files is to write the size first, so i know the exact amount of data i'm about to read when reading it back.

C reading binary files

3 Answers3