Bad address in my write-function - c++

Question

I have to code this little task and can´t find my mistake. It should just read some data from a file and copy it in opposite order into another one. The first part seems to work, but the while-part gives me "Bad Address" for every time the write function is used. I´m grateful for every idea!
#include <iostream>
#include <cerrno>
#include <fcntl.h>
#include <cstdio>
#include <cstring>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#define TRY(cmd,msg) { \
if ((cmd) < 0) { \
std::cerr << (msg) << ": " << strerror(errno) << std::endl; \
} \
}
int main(int argc, char *argv[]){
int fd_in, fd_out, rest;
off_t map_size, offset, length;
char * addr;
struct stat sb;
if (argc != 3) {
std::cerr << "Usage: kopfstand in out" << std::endl;
return -1;
}
if ((fd_in = open(argv[1],O_RDONLY)) == -1){
perror("open");
return -1;
}
if ((fd_out = open(argv[2],O_WRONLY|O_CREAT|O_TRUNC,S_IRUSR|S_IWUSR)) == -1){
close(fd_in);
perror("open");
return -1;
}
fstat(fd_in, &sb);
length = sb.st_size;
map_size = sysconf(_SC_PAGESIZE);
rest = length % map_size;
offset = length -rest;
if(rest != 0){
addr = (char*)mmap(NULL, rest, PROT_READ, MAP_PRIVATE, fd_in, offset);
if(addr == MAP_FAILED){
perror("Error mmaping the File");
}
for (int off = rest-1; off >= 0; --off){
TRY(write(fd_out, addr+off, 1),"write");
}
if(munmap((char*)addr, rest)== -1){
perror("munmap");
}
}
while(offset > 0){
offset =- map_size;
addr = (char*)mmap(NULL, map_size, PROT_READ, MAP_SHARED, fd_in, offset);
if(addr == MAP_FAILED){
perror("Error mmaping the File");
}
for(int off = map_size-1; off >= 0; --off){
TRY(write(fd_out, addr+off, 1),"write");
}
if(munmap((char*)addr, map_size) == -1){
perror("munmap");
}
}
close(fd_in);
close(fd_out);
return 0;
}

Answer 1

You're going to kick yourself! (I kicked myself when I finally spotted the problem!)
This line:
offset =- map_size;
should be:
offset -= map_size;
See also What does =+ mean in C?
One of the comments to the question by Andrew Medico observes that clang identifies the problem immediately — and it does:
$ clang -O3 -g -std=c++11 -Wall -Wextra -Werror badadd.cpp -o badadd
badadd.cpp:65:16: error: use of unary operator that may be intended as compound assignment (-=)
[-Werror]
offset =- map_size;
G++ (GCC) 4.9.1 (home-built on Mac OS X 10.9 and running on 10.10.1) with the same compiler options does not identify the problem.
JFTR: I found the problem the hard way (with print statements tracing the value of offset), and only read Andrew's comment after finding and posting the answer.