b64.c

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

unsigned char base64_table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

char *b64_encode(unsigned char *s, int len) {
  int bit_len = len * 8;
  int dest_len = bit_len / 6 + (bit_len % 6 != 0);
  int res_len = dest_len + (dest_len % 4 != 0) * (4 - dest_len % 4);
  char *res = (char *)malloc(res_len + 1);
  res[res_len] = '\0';

  for (int i = 0; i < res_len; i++) {
    if (i >= dest_len) {
      res[i] = '=';
      continue;
    }

    unsigned char mask = (1 << 6) - 1;
    int offset = i * 6 % 8;
    char table_idx;
    if (offset == 0) {
      table_idx = s[i * 6 / 8] >> 2 & mask;
    } else {
      table_idx = s[i * 6 / 8] << offset;
      table_idx = table_idx >> 2 & mask;
      if (offset > 2) {
        int next = i * 6 / 8 + 1;
        int next_sz = offset - 2;
        if (len > next) {
          table_idx |= s[next] >> (10 - offset);
        }
      }
    }

    res[i] = base64_table[table_idx];
  }

  return res;
}
int main(int argc, char *argv[])
{
  unsigned char *s = "ABCDEFG";
  char *enc = b64_encode(s, strlen(s));
  printf("%s\n", enc);
  free(enc);
  return 0;
}