Using memset and memcpy correctly to initialize a character array in C++

Question

I wanted to initialize a character array with the data from a character pointer. I wrote the following code for this:

(kindly excuse what I am doing with the structure and all .. actually this code is supposed to fit into something bigger and hence the strangeness of that structure and its use)

#include <iostream>
#include <string>

struct ABC 
{
    char a;
    char b;
    char c[16];
};

int main(int argc, char const *argv[])
{
    struct ABC** abc;
    std::string _r = "Ritwik";
    const char* r = _r.c_str();

    if (_r.length() <= sizeof((*abc)->c))
    {
        int padding = sizeof((*abc)->c) - _r.length();

        std::cout<<"Size of `c` variable is : "<<sizeof((*abc)->c)<<std::endl;
        std::cout<<"Value of padding is calculated to be : "<<padding<<std::endl;

        char segment_listing[ sizeof((*abc)->c)]; 

        std::cout<<"sizeof segment_listing is "<<sizeof(segment_listing)<<std::endl;

        memcpy(segment_listing, r, _r.length());
        memset( (segment_listing + _r.length()), ' ', padding);

        std::cout<<segment_listing<<std::endl;

    }
    return 0;
}

However, when I run my code I get these wierd characters at the end of my string:

(rh4dev01:~/rough) rghosh> ./crptr
Size of `c` variable is : 16
Value of padding is calculated to be : 10
sizeof segment_listing is 16
Ritwik          °×
(rh4dev01:~/rough) rghosh> ./crptr
Size of `c` variable is : 16
Value of padding is calculated to be : 10
sizeof segment_listing is 16
Ritwik           Ñ
(rh4dev01:~/rough) rghosh> ./crptr
Size of `c` variable is : 16
Value of padding is calculated to be : 10
sizeof segment_listing is 16
Ritwik          g
(rh4dev01:~/rough) rghosh> ./crptr
Size of `c` variable is : 16
Value of padding is calculated to be : 10
sizeof segment_listing is 16
Ritwik          pô
(rh4dev01:~/rough) rghosh> ./crptr
Size of `c` variable is : 16
Value of padding is calculated to be : 10
sizeof segment_listing is 16
Ritwik
(rh4dev01:~/rough) rghosh> ./crptr
Size of `c` variable is : 16
Value of padding is calculated to be : 10
sizeof segment_listing is 16
Ritwik          àå
(rh4dev01:~/rough) rghosh> ./crptr
Size of `c` variable is : 16
Value of padding is calculated to be : 10
sizeof segment_listing is 16
Ritwik           »
(rh4dev01:~/rough) rghosh> ./crptr
Size of `c` variable is : 16
Value of padding is calculated to be : 10
sizeof segment_listing is 16
Ritwik          pZ

Can you please explain why that is happening ? Since I am printing only a character array taht is only 16 characters in length, shouldn't only 16 characters get printed ? Where are those two (sometimes zero and sometimes one) characters coming from ?

More importantly, am I corrupting any memory (that does not belong to my character array c) by my padding ?

Answer 1

Your string needs to be NUL terminated.

    memcpy(segment_listing, r, _r.length());
    memset( (segment_listing + _r.length()), ' ', padding-1);
    segment_listing[_r.length() + padding - 1] = '\0';

Perhaps you would be better served by using snprintf(), which will add the terminator for you:

    snprintf(segment_listing, sizeof(segment_listing), "%-*s",
             (int)sizeof(segment_listing)-1, r);

Answer 2

A C string is terminated by a 0 byte, which you havn't accounted for anywhere. You need to terminate your string with the value 0, and you'll have to take that extra byte in account in all your calculations.