base64url without padding decoding (with TC)

b64.c

/**
 * How to use it
 * --
 * brew install tokyo-cabinet
 * clang  -o b64 b64.c -Wall -Werror -I/usr/local/include -L/usr/local/lib -ltokyocabinet
 * ./b64
 * ./b64 Zm9v
 */

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

#include <tcutil.h>

struct {
  const char *in;
  const char *out;
} b64_tests[] = {
  /* empty string test */
  {"", ""},
  /* '==' padding test */
  {"Zg", "f"},
  /* '=' padding test */
  {"Zm8", "fo"},
  /* no padding test */
  {"Zm9v", "foo"},
  /* misc test */
  {"eyJmb28iOiJiYXIifQ", "{\"foo\":\"bar\"}"},
  /* '-_' to '+/' character substitution test */
  {"NSUyKzMtMT0zICJZZXMvTm8iIDxhJmI-", "5%2+3-1=3 \"Yes/No\" <a&b>"}
};

/* base64url (aka URL safe) without padding decoding */
char *b64urldecode(const char *str, int *sp);

int main(int argc, char **argv) {
  int siz;
  char *buf;
  if (argc > 2) {
    fprintf(stderr, "Usage: %s [str]\n", argv[0]);
    exit(1);
  }
  if (argc == 2) {
    printf("IN: %s\n", argv[1]);
    buf = b64urldecode(argv[1], &siz);
    printf("OUT: %s\n", buf);
    free(buf);
  }
  else {
    int ntests = (sizeof(b64_tests)/sizeof(b64_tests[0]));
    for (int i = 0; i < ntests; i++) {
      buf = b64urldecode(b64_tests[i].in, &siz);
      if (siz == strlen(b64_tests[i].out) && !strcmp(buf, b64_tests[i].out))
        fprintf(stdout, "test #%d\t[OK]\n", i + 1);
      else {
        fprintf(stderr, "test #%d\t[KO]", i + 1);
        fprintf(stderr, " got `%s', expected `%s'\n", buf, b64_tests[i].out);
      }
      free(buf);
    }
  }
  return 0;
}

char *b64urldecode(const char *str, int *sp) {
  assert(str && sp);
  int len = strlen(str);
  char *buf = malloc(len + 3);
  memcpy(buf, str, len);
  while ((len % 4) >= 2) buf[len++] = '=';
  buf[len] = '\0';
  tcstrsubchr(buf, "-_", "+/");
  char *obuf = tcbasedecode(buf, sp);
  free(buf);
  return obuf;
}