c++ reading argv into unsigned char fixed size: Segmentation fault - c++

Question

I am trying to read command line argument into a fixed size unsigned char array. I get segmentation fault.
My code:
#include <stdio.h>
#include <iostream>
#include <stdlib.h>
#include <memory.h>
unsigned char key[16]={};
int main(int argc, char** argv){
std::cout << "Hello!" << std::endl;
long a = atol(argv[1]);
std::cout << a << std::endl;
memcpy(key, (unsigned char*) a, sizeof key);
// std::cout << sizeof key << std::endl;
// for (int i = 0; i < 16; i++)
// std::cout << (int) (key[i]) << std::endl;
return 0;
}
What am I doing wrong?
To call the program:
compile: g++ main.cpp
Execute: ./a.out 128

Answer 1

You get SEGV because your address is wrong: you convert a value to an address. Plus the size is the one of the destination, should be the size of the source
The compiler issues a warning, that's never good, you should take it into account because that was exactly your error:
xxx.c:12:38: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
memcpy(key, (unsigned char*) a, sizeof key);
^
fix that like this:
memcpy(key, &a, sizeof(a));
BTW you don't have to declare key with 16 bytes. It would be safer to allocate it like this:
unsigned char key[sizeof(long)];
and when you print the bytes, iterate until sizeof(long) too, or you'll just print trash bytes in the end.
Here's a fix proposal using uint64_t (unsigned 64-bit integer from stdint.h which gives exact control on the size), zero initialization for your key and parsing using strtoll:
#include <stdio.h>
#include <iostream>
#include <stdlib.h>
#include <memory.h>
#include <stdint.h>
unsigned char key[sizeof(uint64_t)]={0};
int main(int argc, char** argv){
std::cout << "Hello!" << std::endl;
uint64_t a = strtoll(argv[1],NULL,10);
memcpy(key, &a, sizeof a);
for (int i = 0; i < sizeof(key); i++)
std::cout << (int) (key[i]) << std::endl;
return 0;
}
(if you want to handle signed, just change to int64_t)
Test on a little endian architecture:
% a 10000000000000
Hello!
0
160
114
78
24
9
0
0

Answer 2

Looks like you are copying too much data.
I also added a &a for the memcpy.
#include <stdio.h>
#include <iostream>
#include <stdlib.h>
#include <memory.h>
unsigned char key[16]={};
int main(int argc, char** argv)
{
memset(key,0x0, sizeof(key));
std::cout << "Hello!" << std::endl;
long a = atol(argv[1]);
std::cout << a << std::endl;
// the size parameter needs to be the size of a
// or the lesser of the size of key and a
memcpy(key,(void *) &a, sizeof(a));
std::cout << "size of key " << sizeof(key) << "\n";
std::cout << "key " << key << "\n";
for (int i = 0; i < 16; i++)
std::cout << " " << i << " '" << ((int) key[i]) << "'\n";
return 0;
}