Base64 Encoding Padding in C - c

Question

I am writting a base64 encoder and decoder and have it almost completly functional, I just need to be able to pad the encoded data with equal signs if the number of input bytes does not round out to a multiple of 3. I am relativly new to C and am not sure how I would detect the number of bytes or pad the output accordingly. This is what I have so far
void encode(char* src, char* dest) {
char base64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
unsigned char first = (src[0] >> 2);
unsigned char second = (src[0] << 4) | (src[1] >> 4);
second = (second << 2);
second = (second >> 2);
unsigned char third = (src[1] << 2) | (src[2] >> 6);
third = (third << 2);
third = (third >> 2);
unsigned char fourth = (src[2]);
fourth = (fourth << 2);
fourth = (fourth >> 2);
dest[0] = base64[first];
dest[1] = base64[second];
dest[2] = base64[third];
dest[3] = base64[fourth];
}
And my decoder method...
void decode(char* src, char* dest) {
char base64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
int i;
int j;
int index;
for(i = 0; i < 4; i++) {
index = 0;
for(j = 0; j < 64; j++) {
if (src[i] == base64[j]) {
src[i] = index;
} else {
index++;
}
}
}
char first = (src[0] << 2) | (src[1] >> 4);
char second = (src[1] << 4) | (src[2] >> 2);
char third = (src[2] << 6) | (src[3]);
dest[0] = first;
dest[1] = second;
dest[2] = third;
}
If this is not enough and I need to also provide the rest of my code I can do so.