swap the file bytes

swapfile.c

#include <stdio.h>

#define BUFF_SIZE (4*1024)

void swap(char* buff, size_t count)
{
char* begin = buff;
char* end = buff + count - 1;
while ( begin < end )
    {
    *begin += *end;
    *end = *begin - *end;
    *begin = *begin - *end;
    ++begin;
    --end;
    }
}

int main(int argc, char* argv[])
{
if ( argc != 3 )
    {
    fprintf( stderr, "usage: %s <infile> <outfile>\n", argv[0] );
    return -1;
    }
FILE* infile = fopen( argv[1], "rb" );
if ( NULL == infile )
    {
    fprintf( stderr, "can't open input file\n" );
    return -2;
    }
FILE* outfile = fopen( argv[2], "wb+" );
if ( NULL == outfile )
    {
    fprintf( stderr, "can't create output file\n" );
    return -3;
    }
fseek( infile, 0, SEEK_END );
int size = ftell( infile );
int smallblocks = size % BUFF_SIZE;
int bigblocks = size / BUFF_SIZE - (0 == smallblocks);
char buff[BUFF_SIZE];

do
    {
    fseek( infile, bigblocks * BUFF_SIZE, SEEK_SET );
    size_t nread = fread( buff, 1, BUFF_SIZE, infile );
    swap( buff, nread );
    size_t nwrite = fwrite( buff, nread, 1, outfile );
    if ( 1 != nwrite )
        {
        fprintf( stderr, "can't write to file, disk is full?\n" );
        return -4;
        }
    }
while ( bigblocks-- );

return 0;
}