How can I redirect 404 to a specific html file in Mongoose?

I am trying to do this with the option url_rewrites in struct mg_serve_http_opts:

struct mg_serve_http_opts httpOptions;
httpOptions.url_rewrites = "404=/error.html";

Accordingly to Mongoose documentation, if I pass a number as uri_pattern, then "it is treated as HTTP error code, and file_or_directory_path should be an URI to redirect to"

Source: https://cpr.readthedocs.io/en/latest/opt/mongoose/docs/Options/

But I am working with mongoose 6.18, and checking the source code, the HTTP error code uri_pattern seems to have been removed. Is there any other way to do this? I can also upgrade to mongoose 7.1 if necessary, but I really wouldn't like to downgrade to mongoose 5.6.

Thanks.

I managed to find a solution to this problem for mongoose 6.18. It's not a generic solution like url_rewrites, which would have worked with any HTTP error code, but it's a solution that works well for HTTP 404.

During the event handler, I have to manually parse the URL and check if the requested file exists. It it does, I serve the HTTP page as normal; if it doesn't, I serve the error file.

This is the event handler code:

static const char *documentRoot = "/var/www/html";


static void eventHandler( struct mg_connection *connection, int event, void *eventData )
{
    char resource[4096];
    memset( resource, 0, sizeof(resource));


    switch( event )
    {
        case MG_EV_HTTP_REQUEST:
        {
            struct http_message* httpMessage = (struct http_message*)eventData;


            /*
             * Build absolute path to the requested file.
             */
            strcpy( resource, documentRoot );
            strncat( resource, httpMessage->uri.p, httpMessage->uri.len );


            /*
             * Check if the requested file exists in the filesystem.
             */
            struct stat info;
            if( stat( resource, &info ) == 0 )
            {
                struct mg_serve_http_opts httpOptions;
                memset( &httpOptions, 0, sizeof(httpOptions));

                httpOptions.document_root = documentRoot;
                mg_serve_http( connection, httpMessage, httpOptions );
            }
            else
            {
                struct mg_str mimeType;
                memset( &mimeType, 0, sizeof(mimeType));

                struct mg_str extraHeaders;
                memset( &extraHeaders, 0, sizeof(extraHeaders));


                /*
                 * Build absolute path to the 404 html file.
                 */
                strcpy( resource, documentRoot );
                strcat( resource, "/error404.html" );


                mimeType.p = "text/html";
                mimeType.len = strlen( mimeType.p );
                mg_http_serve_file( connection, httpMessage, resource, mimeType, extraHeaders );
            }
        }
        break;
        default:;
    }
}