Homemade fstat to get file size, always return 0 length.

Hello, I am trying to use my own function to get the file size from a file. I'll use this to allocate memory for a data structure to hold the information on the file.

The file size function looks like this:

long fileSize(FILE *fp){
    long start;
    fflush(fp);
    rewind(fp);
    start = ftell(fp);
    return (fseek(fp, 0L, SEEK_END) - start);
}

Any ideas what I'm doing wrong here?

From stackoverflow

• Do

  fseek(fp, 0L, SEEK_END);
  return (ftell(fp) - start);
  

  instead of

  return (fseek(fp, 0L, SEEK_END) - start);
  

  because fseek return zero on success not the offset as you are expecting here.

  Fred : Thank you. *I guess I should have spent some more time reading the man page. :)

• A few comments:

  • don't call fflush() - your stream might be a read stream, for which fflush() results in undefined behaviour

  • you don't have any error checking !

  • fseek() returns 0 for success - you need to call ftell() to get the length

  Change the code to this:

  long fileSize(FILE *fp)
  {    
      fseek(fp, 0L, SEEK_END);
      return ftell(fp);
  }
  

  Fred : Good suggestions, I'll take them with me. Thank you.

• You need to call ftell after fseek. Try:

  long fileSize(FILE *fp){
    long start;
    fflush(fp);
    rewind(fp);
    start = ftell(fp);
    fseek(fp, 0L, SEEK_END);
    return ftell(fp);
  }
  

  There's no need to do a difference, so your first ftell is useless and you can get rid of it. I would use:

  long filezise(FILE *fp)
  {
    fseek(fp,OL,SEEK_END);
    // fseek(f, 0, SEEK_SET); - only if you want to seek back to the beginning
    return ftell(fp);
  }
  

  Also, make sure you open your file in binary mode.

  Fred : Thats great, I did not know you could do this. Thanks!