Lightweight C library for HTML5 websockets
Tips on debugging with lws

Problem with the library, or your code?

Because lws is only really used when already combined with user code, it can be a headache figuring out if the actual problem is inside lws or in the user code.

If it's in lws, I would really like to solve it, but if it's in your code, that's your problem. Finding out which side it's on when it involves your code is also something you need to try to resolve.

The minimal examples are useful because if they demonstrate the same problem, it's something about your platform or lws itself, I have the minimal examples so I can test it and find out if it's your platform. If I can reproduce it, it's my problem.

Debug builds

With cmake, build with -DCMAKE_BUILD_TYPE=DEBUG to build in extra logging, and use a log level bitmap of eg, 1039 or 1151 to enable the extra logs for print.

The minimal examples take a -d xxx commandline parameter so you can select the logging level when you run it.

The extra logging can be very useful to understand the sequencing of problematic actions.


If your problems involve heap corruption or use-after-free, Valgrind is indespensible. It's simple to use, if you normally run xxx, just run valgrind xxx. Your code will run slower, usually something like 2 - 4x slower but it depends on the exact code. However you will get a backtrace as soon as there is some kind of misbehaviour of either lws or your code.

lws is developed using valgrind routinely and strives to be completely valgrind-clean. So typically any problems reported are telling you about problems in user code (or my bugs).

Traffic dumping

The best place for dumping traffic, assuming you are linking against a tls library, is lws_ssl_capable_read() and lws_ssl_capable_write() in either ./lib/tls/openssl/openssl-ssl.c or ./lib/tls/mbedtls/mbedtls-ssl.c according to which tls library you are using. There are default-#if 0 sections in each function like

#if 0
* If using mbedtls type tls library, this is the earliest point for all
* paths to dump what was received as decrypted data from the tls tunnel
lwsl_notice("%s: len %d\n", __func__, len);
lwsl_hexdump_notice(buf, len);

Enable these to get hexdumps for all unencrypted data in both directions.