Write a program which reverses the order of the bytes in a binary file with a limited number of file positioning steps

Question

I need to write a program which reverses the order of the bytes in a given binary file. It accepts the file name in the command line. In addition it can use file positioning functions such as fseek no more than a fixed number of times.

Here is a code which I wrote which does not use it a fixed number of times:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(int argc, char** argv) {

    if (argc>2) {
        printf("Please enter a valid file name");
        return 1;
    } else {
        FILE* file;
        file=fopen(argv[1], "r");
        if (file==NULL) {
            printf("Please enter a valid file name");
            return 1;
        } else {
            FILE* fileBackUp;
            fileBackUp=fopen("c:\backupFile.txt", "w+");
            fseek(file, 0, SEEK_END);
            fseek(file, -1, SEEK_CUR);
            while (ftell(file)>=0) {
                int c= fgetc(file);
                fputc(c, fileBackUp);
                fseek(file, -2, SEEK_CUR);
            }

            fseek(fileBackUp, 0, SEEK_SET);

            int c;
            while (!feof(fileBackUp)) {
                c=fgetc(fileBackUp)
                fputc(c,file);
            }

            fclose(fileBackUp);
            fclose(file);
        }
    }
    return 1;
}

It uses an extra file for it. I surely believe that there's a shorter elegant way to do that with a fewer steps as requested. Any suggestions?

Another thing: It seems that the first condition is always being filled, how come?

should this be tagged 'homework'? if not, why do you have the restriction on how many fseek's you can call? — mfsiega, Jul 17 '12 at 15:50
@mfrankli: Homework on July? :) No, it's a question from a past test I'm trying to answer. — Numerator, Jul 17 '12 at 15:53
You only seem to be reversing the order of the *bytes* in the above code though ? — Paul R, Jul 17 '12 at 15:55
No - you're still just reversing the order of the bytes - if you want to reverse all the bits then you need to do a bit reversal on each byte as well. — Paul R, Jul 17 '12 at 16:00
@PaulR: Doesn't fgetc give me bit by bit considering a Binary File? — Numerator, Jul 17 '12 at 16:04
When you write out each byte you need to reverse the order of the bits within the byte - there are plenty of examples of how to do this on SO - just use the search option to find "bit reversal", e.g. http://stackoverflow.com/questions/746171/best-algorithm-for-bit-reversal-from-msb-lsb-to-lsb-msb-in-c — Paul R, Jul 17 '12 at 16:06
@PaulR : Thank you.I see, another question- Declaring "rb" in fopen function or using fread or fwrite won't change in this case? — Numerator, Jul 17 '12 at 16:08

perreal · Accepted Answer · 2012-07-18T04:36:56.380

2

#include <stdio.h>

long max(long v1, long v2) { return v1 >= v2 ? v1 : v2; }
long min(long v1, long v2) { return v1 >= v2 ? v2 : v1; }
void invert_bits(char *arr, size_t size);

int main(int argc, char *argv[]) {
  size_t  read_sz;
  FILE * infile = fopen(argv[1], "r");
  fseek(infile, 0, SEEK_END);
  long file_sz = ftell(infile);
  long offset  = file_sz;
  long total_read = 0;
  long num_seeks  = 10;
  size_t  buffer_sz = (file_sz + num_seeks - 1) / num_seeks;
  char    buffer[buffer_sz];
  while (file_sz > total_read) {
    if ((offset + num_seeks - 1) / num_seeks < buffer_sz) {
      buffer_sz = offset / num_seeks;
    }
    offset = max(0, offset - buffer_sz);
    fseek(infile, offset, SEEK_SET);
    read_sz = fread(buffer, 1,
        min(buffer_sz, file_sz - total_read), infile);
    total_read += read_sz;
    invert_bits(buffer, read_sz); 
    num_seeks--;
  }
  fclose(infile);
}

void invert_bits(char *arr, size_t size) {
  size_t i;
  int j;
  for (i = size; i > 0; i--) {
    char v = arr[i - 1];
    char o = 0;
    for (j = 0; j < 8; j++) {
      o |= v & 1;
      v >>= 1;
      o <<= 1;
    }
    printf("%c", o);
  }
}

edited Jul 18 '12 at 04:36

answered Jul 17 '12 at 16:13

perreal

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Thank you for the answer. file_sz is the number of bytes from the beginning of the file,right? why do you subtract the buffer value each time? – Numerator Jul 17 '12 at 16:37
Are you sure that " file_sz -= buffer_sz;" at the end of the while loop is correct? it doesn't work for me. – Numerator Jul 17 '12 at 17:36
even without it, it's not correct. there is a problem. we read the same bytes again. try file-size=10, buffer-size=4. – Numerator Jul 17 '12 at 17:45
3

This still calls `fseek` and `fread` a variable number of times in the while() loop, based on the file size. Instead of using a constant buffer size of 1024, use a constant fraction of the file size. I'm not sure why you'd want to do that, but it's a constraint from the question (possibly to test student's knowledge of dynamic memory allocation and file IO?) – Kevin Vermeer Jul 17 '12 at 18:04

Write a program which reverses the order of the bytes in a binary file with a limited number of file positioning steps

1 Answers1