Event loops¶

In event-driven programming, an application express interest in certain events and respond to them when they occur. The responsibility of gathering events from the operating system or monitoring other sources of events is handled by libuv, and the user can register callbacks to be invoked when an event occurs. The event-loop usually keeps running forever. In pseudocode:

while there are still events to process:
    e = get the next event
    if there is a callback associated with e:
        call the callback

Some examples of events are:

• File is ready for writing
• A socket has data ready to be read
• A timer has timed out

This event loop is encapsulated by uv_run() – the end-all function when using libuv.

The most common activity of systems programs is to deal with input and output, rather than a lot of number-crunching. The problem with using conventional input/output functions (read, fprintf, etc.) is that they are blocking. The actual write to a hard disk or reading from a network, takes a disproportionately long time compared to the speed of the processor. The functions don’t return until the task is done, so that your program is doing nothing. For programs which require high performance this is a major roadblock as other activities and other I/O operations are kept waiting.

One of the standard solutions is to use threads. Each blocking I/O operation is started in a separate thread (or in a thread pool). When the blocking function gets invoked in the thread, the processor can schedule another thread to run, which actually needs the CPU.

The approach followed by libuv uses another style, which is the asynchronous, non-blocking style. Most modern operating systems provide event notification subsystems. For example, a normal read call on a socket would block until the sender actually sent something. Instead, the application can request the operating system to watch the socket and put an event notification in the queue. The application can inspect the events at its convenience (perhaps doing some number crunching before to use the processor to the maximum) and grab the data. It is asynchronous because the application expressed interest at one point, then used the data at another point (in time and space). It is non-blocking because the application process was free to do other tasks. This fits in well with libuv’s event-loop approach, since the operating system events can be treated as just another libuv event. The non-blocking ensures that other events can continue to be handled as fast they come in [1].

Hello World¶

With the basics out of the way, lets write our first libuv program. It does nothing, except start a loop which will exit immediately.

helloworld/main.c

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11

#include <stdio.h>
#include <uv.h>

int main() {
    uv_loop_t *loop = uv_loop_new();

    printf("Now quitting.\n");
    uv_run(loop);

    return 0;
}

This program quits immediately because it has no events to process. A libuv event loop has to be told to watch out for events using the various API functions.

Watchers¶

Watchers are how users of libuv express interest in particular events. Watchers are opaque structs named as uv_TYPE_t where type signifies what the watcher is used for. A full list of watchers supported by libuv is:

libuv watchers

typedef struct uv_loop_s uv_loop_t;
typedef struct uv_ares_task_s uv_ares_task_t;
typedef struct uv_err_s uv_err_t;
typedef struct uv_handle_s uv_handle_t;
typedef struct uv_stream_s uv_stream_t;
typedef struct uv_tcp_s uv_tcp_t;
typedef struct uv_udp_s uv_udp_t;
typedef struct uv_pipe_s uv_pipe_t;
typedef struct uv_tty_s uv_tty_t;
typedef struct uv_poll_s uv_poll_t;
typedef struct uv_timer_s uv_timer_t;
typedef struct uv_prepare_s uv_prepare_t;
typedef struct uv_check_s uv_check_t;
typedef struct uv_idle_s uv_idle_t;
typedef struct uv_async_s uv_async_t;
typedef struct uv_getaddrinfo_s uv_getaddrinfo_t;
typedef struct uv_process_s uv_process_t;
typedef struct uv_counters_s uv_counters_t;
typedef struct uv_cpu_info_s uv_cpu_info_t;

Watchers are setup by a corresponding:

uv_TYPE_init(uv_TYPE_t*)

function.

A watcher is set to actually listen for events by invoking:

uv_TYPE_start(uv_TYPE_t*, callback)

and stopped by calling the corresponding:

uv_TYPE_stop(uv_TYPE_t*)

Callbacks are functions which are called by libuv whenever an event the watcher is interested in has taken place. Application specific logic will usually be implemented in the callback. For example, an IO watcher’s callback will receive the data read from a file, a timer callback will be triggered on timeout and so on.

#include <stdio.h>
#include <uv.h>

int64_t counter = 0;

void wait_for_a_while(uv_idle_t* handle, int status) {
    counter++;

    if (counter >= 10e6)
        uv_idle_stop(handle);
}

int main() {
    uv_idle_t idler;

    uv_idle_init(uv_default_loop(), &idler);
    uv_idle_start(&idler, wait_for_a_while);

    printf("Idling...\n");
    uv_run(uv_default_loop());

    return 0;
}