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int func(char const *var) {
  static char *oldenv;
  char const *env_format = "TEST=%s";
  size_t const len = strlen(var) + strlen(env_format);
  char *env = (char *) malloc(len);
  if (env == NULL) {
    return -1;
  }
  int rc = snprintf(env, len, env_format, var);
  if (rc < 0 || (size_t)rc >= len) {
    /* Handle Error */
  }
  if (putenv(env) != 0) {
    free(env);
    return -1;
  }
  if (oldenv != NULL)
    free(oldenv); //* avoid memory leak */
  oldenv = env;
  return 0;
}