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#include <sys/mman.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <stdlib.h>
union key_type {
unsigned int number;
char bytes[sizeof(int)];
};
static void fill_in_some_byte(int remaining_bytes, unsigned int *data,
union key_type key)
{
char *start_data = (char*) data;
switch(remaining_bytes) {
case 3:
start_data[2] ^= key.bytes[2];
case 2:
start_data[1] ^= key.bytes[1];
case 1:
*start_data ^= key.bytes[0];
}
}
static int calculate_memory(int block_counter, int size, int pos)
{
return block_counter*size + (pos+1)*sizeof(int);
}
int main(int argc, char **argv)
{
int fd, size, pgs, offset, pos, block_counter, next_used_memory;
unsigned int *data;
union key_type key;
struct stat sb;
size = 4096;
offset = 0;
block_counter = 0;
key.number = ~ (unsigned int) strtol(argv[2], NULL, 10);
pgs = getpagesize();
size = ((size-1) / pgs+1) * pgs;
if(argc != 3) {
perror("enter only three params");
return 1;
}
fd = open(argv[1], O_RDWR);
if(fd == -1) {
perror("can't open file");
return 2;
}
fstat(fd, &sb);
while(offset < sb.st_size-1) {
data = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, offset);
if(data == MAP_FAILED) {
perror("mapping error");
return 3;
}
pos = 0;
next_used_memory = calculate_memory(block_counter, size, pos);
while(next_used_memory <= sb.st_size && next_used_memory <= size) {
data[pos] ^= key.number;
++pos;
next_used_memory = calculate_memory(block_counter, size, pos);
}
if(next_used_memory > sb.st_size && next_used_memory <= size) {
fill_in_some_byte(sizeof(int) - (next_used_memory - sb.st_size),
data+pos, key);
}
munmap(data, size);
offset += size;
++block_counter;
}
close(fd);
return 0;
}
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