Unable to send data to libwebsocket server

Question

I'm trying to write a small plugin for X-Plane, to create a simple websocket server with libwebsocket. I'm able to connect to the websocket from Google Chrome, however, when I send data to the server, X-Plane immediately crashes.

I'm pretty certain the following code is causing the problem:

unsigned char *buf = (unsigned char*) malloc(LWS_SEND_BUFFER_PRE_PADDING + 13 + LWS_SEND_BUFFER_POST_PADDING);
buf = (unsigned char*) "Hello World!";
libwebsocket_write(wsi, buf, len, LWS_WRITE_TEXT);
free(buf);

I'm not a C programmer / specialist at all, but does the above seem to have a problem at all?

I've posted the full source on Gist: https://gist.github.com/josefvanniekerk/868432986f2f963a5583

Answer 1

With any sane interface, @iharob would be right - to be clear he/she is right about you misunderstanding string assignment.

However, libwebsockets is a bit 'special'. You need to copy the string into the malloc()'d array LWS_SEND_BUFFER_PRE_PADDING bytes in. libwebsockets then overwrites the preceding bytes.

So you want something like (assuming you are not trying to send the terminating zero on the string):

char *text = "Hello World!";
int len = strlen (text);
unsigned char *buf = malloc(LWS_SEND_BUFFER_PRE_PADDING + len + LWS_SEND_BUFFER_POST_PADDING);
/* copy string but not terminating NUL */
memcpy (buf + LWS_SEND_BUFFER_PRE_PADDING, text, len );
libwebsocket_write(wsi, buf + LWS_SEND_BUFFER_PRE_PADDING, len, LWS_WRITE_TEXT);
free(buf);

If you want to send the NUL as well:

char *text = "Hello World!";
int len = strlen (text) + 1;
unsigned char *buf = malloc(LWS_SEND_BUFFER_PRE_PADDING + len + LWS_SEND_BUFFER_POST_PADDING);
/* copy string including terminating NUL */
memcpy (buf + LWS_SEND_BUFFER_PRE_PADDING, text, len );
libwebsocket_write(wsi, buf + LWS_SEND_BUFFER_PRE_PADDING, len, LWS_WRITE_TEXT);
free(buf);

Answer 2

You need to copy the string, assignment doesn't work as you expect.

Instead of

buf = (unsigned char *)"Hello World";

you need

memcpy(buf, "Hello World", 1 + strlen("Hello World"));

when you do the assignment, you make buf point to a static string literal, and free() is not possible in this case.

Your program invokes undefined behavior because of that.

You need to include <string.h>