Sorting array of c strings alphabetically

I have a homework problem I'm having bit of a problem with, I'm asked to sort an array of C strings alphabetically using C++, sorting algo used must be bubble sort. What I've done so-far (replicated below) can sort the array but only based on the first alphabet. How do I further sort strings the strings with the same initial alphabet ?

<snipped>@arch:~/College/OOP/Lab/W3$ cat 2.cpp

/*
 * Write a function which sorts an array of C strings in ascending order using bubble sort. The
 * number of strings in the array and the array must be passed as parameters to the function
 */

#include <iostream>
#include <cstring>

using namespace std;

void sort(char **sar, unsigned num, unsigned len)
{
    char *temp = new char[len];

    if (temp == NULL)
    {
        cout << "\nOut-Of-Memory\n";
        return;
    }

    for (unsigned a = 0; a < num-1; a++)
    {
        for (unsigned b = 0; b < ((num-a)-1); b++)
        {
            if (sar[b][0] > sar[b+1][0])
            {
                strcpy(temp, sar[b]);
                strcpy(sar[b], sar[b+1]);
                strcpy(sar[b+1], temp);
            }
        }
    }

    delete[] temp;
}

int main(int argc, char *argv[])
{
    char **sar;
    unsigned num;
    unsigned len;

    cout << "Number of Strings: ";
    cin  >> num;
    cout << "Length of Strings: ";
    cin  >> len;

    cin.ignore(); // Flush buffer to fix a bug (getline after cin).

    sar = (char **) new char*[num];
    if (sar == NULL)
    {
        cout << "\nOut-Of-Memory\n";
        return -1;
    }

    for (unsigned i = 0; i < num; i++)
    {
        sar[i] = (char *) new char[len];
        if (sar[i] == NULL)
        {
            // Let's pretend we 'know' memory management
            // because obviously modern OSs are incapable
            // of reclaiming heap from a quitting process..
            for (unsigned j = 0; j < i; j++)
                delete[] sar[j];
            cout << "\nOut-Of-Memory\n";
            return -1;
        }
    }

    for (unsigned x = 0; x < num; x++)
        cin.getline(&sar[x][0], 512);

    sort(sar, num, len);

    cout << '\n';
    for (unsigned y = 0; y < num; y++)
        cout << sar[y] << '\n';

    for (unsigned z = 0; z < num; z++)
        delete[] sar[z];
    delete[] sar;

    return 0;
}