Why does my char* copier return different things?

Question

Writing a simple string copier and testing it in the main() fucntion. What's odd is that sometimes the program returns

"HelloHello"

like it should, but maybe every third time I run it, the program prints out:

"Hello!Hello!▌▌▌▌▌▌▌▌▌▌▒"UòB╚"

Why is the tail of garbage data only sometimes being added to the end of my second string?

#include <iostream>
using namespace std;

int strlength(const char* c)
{
    int size = 0;
    while (*c) {
        ++c;
        ++size;
    }
    return size;
}

char* mystrdup(const char* c)
{
    int size = strlength(c);
    char* result = new char;
    copy(c, c + size, result);
    return result;      
}

void print_array(const char* c)
{
    int size = strlength(c);
    while (*c) {
        cout << *c;
        ++c;
    }
}

int main()
{
    char test[] = "Hello!";
    char* res = mystrdup(test);
    print_array(test);
    print_array(res);
}

Answer 1

The program has undefined behavior because you are allocating not enough memory for the result string.

char* mystrdup(const char* c)
{
    int size = strlength(c);
    char* result = new char;
    ^^^^^^^^^^^^^^^^^^^^^^^           
    copy(c, c + size, result);
    return result;      
}

Moreover you are not copying the terminating zero to the result string.

At least the two functions strlength and mystrdup can look the following way

size_t strlength( const char *s )
{
    size_t size = 0;

    while ( s[size] ) ++size;

    return size;
}

char * mystrdup( const char *s )
{
    size_t size = strlength( s ) + 1;

    char *result = new char[size];

    copy( s, s + size, result );

    return result;      
}

Of course instead of the standard algorithm std::copy you could use the standard C function strcpy declared in the header <cstring>.

strcpy( result, s );

And do not forget to delete the allocated array.

char* res = mystrdup(test);
//…
delete [] res;

Pay attention to that the function print_array does not use the variable size. There is no need to output a C-string character by character.

The function could be defined like

std::ostream & print_array( const char *s, std::ostream &os = std::cout )
{
    return os << s;
}

And at last the identifier c is usually used with single objects of the type char. If you deal with a string then it is better to use the identifier s.