I'm using the libwebsockets v2.4.
The doc seems unclear to me about what I have to do with the returned value of the lws_write() function.
If it returns -1, it's an error and I'm invited to close the connection. That's fine for me.
But when it returns a value that is strictly inferior to the buffer length I pass, should I consider that I have to write the last bytes that could not be written later (in another WRITABLE callback occurrence). Is it even possible to have this situation?
Also, should I use the lws_send_pipe_choked() before using the lws_write(), considering that I always use lws_write() in the context of a WRITABLE callback?
My understanding is that lws_write always return the asked buffer length except is an error occurs.
If you look at lws_issue_raw() (from which the result is returned by lws_write()) in output.c (https://github.com/warmcat/libwebsockets/blob/v2.4.0/lib/output.c#L157), you can see that if the length written by lws_ssl_capable_write() is less than the provided length, then the lws allocate a buffer to fill up the remaining bytes on wsi->trunc_alloc, in order for it to be sent in the future.
Concerning your second question, I think it is safe to call lws_write() in the context of a WRITABLE callback without checking if the pipe is choked. However, if you happen to loop on lws_write() in the callback, lws_send_pipe_choked() must be called in order to protect the subsequent calls to lws_write(). If you don't, you might stumble upon this assertion https://github.com/warmcat/libwebsockets/blob/v2.4.0/lib/output.c#L83 and the usercode will crash.