error: implicitly declaring library function 'snprintf' with type 'int (char *, unsigned long, const char *, ...)' [-Werror,-Wimplicit-function-declaration]
  snprintf(context->out_buf, context->out_buf_len, "user=%s\1auth=Bearer %s\1\1", authid, token);
  ^

note: include the header <stdio.h> or explicitly provide a declaration for 'snprintf'