t-head

The issue started when I wanted to use the W801 to detect a GPIO level change and then send a bot notification message through Telegram. However, when calling my own Telegram API reverse proxy (caddy v2.6.4 h1:2hwYqiRwk1tf3VruhMpLcYTg+11fCdr8S3jhNAdnPy8=), the serial log showed the SSL handshake error 0x7780 mentioned in the title.

The code was directly copied from the SDK example demo wm_https_demo.c. The error occurred during the handshake, specifically when calling HTTPWrapperSSLConnect, as shown:

wm_printf("step 1: ssl connect to...\r\n");
ret = HTTPWrapperSSLConnect(&ssl_p, fd, (const struct sockaddr *)&server, sizeof(server), HTTPS_DEMO_SERVER);
if (ret < 0)
{
    wm_printf("https connect error\r\n");
    close(fd);
    break;
}

At first, the serial output only showed step 1: ssl connect to…, then immediately the 0x7780 error, followed by https connect error and request failure. After studying the mbedtls library used internally, I found that more detailed debug logs were available. You need to enable the macro define MBEDTLS_DEBUG_C in SDK src/app/mbedtls/include/mbedtls/config.h (and make sure the makefile project rebuilds the lib target. If using my previous CLion IDE setup, select the lib configuration and rebuild). After enabling it, you can see error messages similar to those mentioned here: https://forums.mbed.com/t/error-0x7780-during-handshake/6883