Base 64 Encoding in C

base64_encode.c

#include <stdio.h>
#include <stdlib.h>

#define FIRST_SIX_INDEX   (buf[0] & 0xfc) >> 2
#define SECOND_SIX_INDEX ((buf[0] & 0x03) << 4) + ((buf[1] & 0xf0) >> 4)
#define THIRD_SIX_INDEX  ((buf[1] & 0x0f) << 2) + ((buf[2] & 0xc0) >> 6)
#define FOURTH_SIX_INDEX   buf[2] & 0x3f


char base64_map[]="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";


char *
base64_encode(const unsigned char *src, size_t len) {
    char *encoded = malloc( ((len/3) * 4) + 1);
    int encoded_counter = 0, cursor = 0;
    unsigned char buf[3];

    /* Process the string 3 bytes at a time, allowing us to generate the
              corresponding 4 6-bit values for the base64 representation */
    for (; (cursor+3) < len; cursor += 3) {
      // Buffer 3 bytes
        buf[0] = *(src +  cursor      );
        buf[1] = *(src + (cursor + 1) );
        buf[2] = *(src + (cursor + 2) );

      // Use bitmask macros to find the corresponding value in the base64 map
        encoded[encoded_counter++] = base64_map[ FIRST_SIX_INDEX ];
        encoded[encoded_counter++] = base64_map[ SECOND_SIX_INDEX ];
        encoded[encoded_counter++] = base64_map[ THIRD_SIX_INDEX ];
        encoded[encoded_counter++] = base64_map[ FOURTH_SIX_INDEX ];
    }

    /* If a remainder is available, we do a very similar process - but we also
        ensure that the output string is padded to a multiple of 4 - using '=' */
    if ((len-cursor) > 0) {
      // Buffer final bytes, unused space should be nullified.
        for (int j = 0; j < 3; j++) {
            if (cursor < len) {
                buf[j] = *(src + cursor++);
                continue;
            }
            buf[j] = '\0';
        }

      // First remainder will always be valid, fallback to '=' (64 on map) on
      //  subsequent output values.
        encoded[encoded_counter++] = base64_map[ FIRST_SIX_INDEX ];
        encoded[encoded_counter++] = base64_map[ (SECOND_SIX_INDEX > 0) ? SECOND_SIX_INDEX : 64 ];
        encoded[encoded_counter++] = base64_map[ (THIRD_SIX_INDEX  > 0) ? THIRD_SIX_INDEX  : 64 ];
        encoded[encoded_counter++] = base64_map[ (FOURTH_SIX_INDEX > 0) ? FOURTH_SIX_INDEX : 64 ];
    }
    
    encoded[encoded_counter++] = '\0';
    return encoded;
}