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Table of Contents

- Introduction

- Who this book is for

- Background

- Code

- Basics of libuv

- Event loops

- Hello World

- Error handling

- Handles and Requests

- Filesystem

- Reading/Writing files

- Filesystem operations

- Buffers and Streams

- File change events

- Networking

- TCP

- UDP

- Querying DNS

- Network interfaces

- Threads

- Core thread operations

- Synchronization Primitives

- libuv work queue

- Inter-thread communication

- Processes

- Spawning child processes

- Changing process parameters

- Detaching processes

- Sending signals to processes

- Signals

- Child Process I/O

- Pipes

- Advanced event loops

- Stopping an event loop

- Utilities

- Timers

- Event loop reference count

- Idler pattern

- Passing data to worker thread

- External I/O with polling

- Check & Prepare watchers

- Loading libraries

- TTY

- About

- Licensing

Alternate formats

The book is also available in:

- PDF

- ePub

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Introduction

This ‘book’ is a small set of tutorials about using libuv [https://github.com/libuv/libuv] as a high performance evented I/O library which offers the same API on Windows and Unix.

It is meant to cover the main areas of libuv, but is not a comprehensive reference discussing every function and data structure. The official libuv documentation [http://docs.libuv.org/en/v1.x/] may be consulted for full details.

This book is still a work in progress, so sections may be incomplete, but I hope you will enjoy it as it grows.

Who this book is for

If you are reading this book, you are either:

1. a systems programmer, creating low-level programs such as daemons or network services and clients. You have found that the event loop approach is well suited for your application and decided to use libuv.

2. a node.js module writer, who wants to wrap platform APIs written in C or C++ with a set of (a)synchronous APIs that are exposed to JavaScript. You will use libuv purely in the context of node.js. For this you will require some other resources as the book does not cover parts specific to v8/node.js.

This book assumes that you are comfortable with the C programming language.

Background

The node.js [http://www.nodejs.org] project began in 2009 as a JavaScript environment decoupled from the browser. Using Google’s V8 [http://code.google.com/p/v8/] and Marc Lehmann’s libev [http://software.schmorp.de/pkg/libev.html], node.js combined a model of I/O – evented – with a language that was well suited to the style of programming; due to the way it had been shaped by browsers. As node.js grew in popularity, it was important to make it work on Windows, but libev ran only on Unix. The Windows equivalent of kernel event notification mechanisms like kqueue or (e)poll is IOCP. libuv was an abstraction around libev or IOCP depending on the platform, providing users an API based on libev. In the node-v0.9.0 version of libuv libev was removed [https://github.com/joyent/libuv/issues/485].

Since then libuv has continued to mature and become a high quality standalone library for system programming. Users outside of node.js include Mozilla’s Rust [http://rust-lang.org] programming language, and a variety [https://github.com/libuv/libuv/wiki/Projects-that-use-libuv] of language bindings.

This book and the code is based on libuv version v1.3.0 [https://github.com/libuv/libuv/tags].

Code

All the code from this book is included as part of the source of the book on Github. Clone [https://github.com/nikhilm/uvbook]/Download [https://github.com/nikhilm/uvbook/downloads] the book, then build libuv:

cd libuv
./autogen.sh
./configure
make

There is no need to make install. To build the examples run make in the code/ directory.

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Basics of libuv

libuv enforces an asynchronous, event-driven style of programming. Its core job is to provide an event loop and callback based notifications of I/O and other activities. libuv offers core utilities like timers, non-blocking networking support, asynchronous file system access, child processes and more.

Event loops

In event-driven programming, an application expresses 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 as they come in [1].

Note

How the I/O is run in the background is not of our concern, but due to the way our computer hardware works, with the thread as the basic unit of the processor, libuv and OSes will usually run background/worker threads and/or polling to perform tasks in a non-blocking manner.

Bert Belder, one of the libuv core developers has a small video explaining the architecture of libuv and its background. If you have no prior experience with either libuv or libev, it is a quick, useful watch.

libuv’s event loop is explained in more detail in the documentation [http://docs.libuv.org/en/v1.x/design.html#the-i-o-loop].

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

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.

Starting with libuv v1.0, users should allocate the memory for the loops before initializing it with uv_loop_init(uv_loop_t *). This allows you to plug in custom memory management. Remember to de-initialize the loop using uv_loop_close(uv_loop_t *) and then delete the storage. The examples never close loops since the program quits after the loop ends and the system will reclaim memory. Production grade projects, especially long running systems programs, should take care to release correctly.

Default loop

A default loop is provided by libuv and can be accessed using uv_default_loop(). You should use this loop if you only want a single loop.

Note

node.js uses the default loop as its main loop. If you are writing bindings you should be aware of this.

Error handling

Initialization functions or synchronous functions which may fail return a negative number on error. Async functions that may fail will pass a status parameter to their callbacks. The error messages are defined as UV_E* constants [http://docs.libuv.org/en/v1.x/errors.html#error-constants].

You can use the uv_strerror(int) and uv_err_name(int) functions to get a const char * describing the error or the error name respectively.

I/O read callbacks (such as for files and sockets) are passed a parameter nread. If nread is less than 0, there was an error (UV_EOF is the end of file error, which you may want to handle differently).

Handles and Requests

libuv works by the user expressing interest in particular events. This is usually done by creating a handle to an I/O device, timer or process. Handles are opaque structs named as uv_TYPE_t where type signifies what the handle is used for.

libuv watchers

/* Handle types. */
typedef struct uv_loop_s uv_loop_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_process_s uv_process_t;
typedef struct uv_fs_event_s uv_fs_event_t;
typedef struct uv_fs_poll_s uv_fs_poll_t;
typedef struct uv_signal_s uv_signal_t;

/* Request types. */
typedef struct uv_req_s uv_req_t;
typedef struct uv_getaddrinfo_s uv_getaddrinfo_t;
typedef struct uv_getnameinfo_s uv_getnameinfo_t;
typedef struct uv_shutdown_s uv_shutdown_t;
typedef struct uv_write_s uv_write_t;
typedef struct uv_connect_s uv_connect_t;
typedef struct uv_udp_send_s uv_udp_send_t;
typedef struct uv_fs_s uv_fs_t;
typedef struct uv_work_s uv_work_t;

/* None of the above. */
typedef struct uv_cpu_info_s uv_cpu_info_t;
typedef struct uv_interface_address_s uv_interface_address_t;
typedef struct uv_dirent_s uv_dirent_t;

Handles represent long-lived objects. Async operations on such handles are identified using requests. A request is short-lived (usually used across only one callback) and usually indicates one I/O operation on a handle. Requests are used to preserve context between the initiation and the callback of individual actions. For example, an UDP socket is represented by a uv_udp_t, while individual writes to the socket use a uv_udp_send_t structure that is passed to the callback after the write is done.

Handles are setup by a corresponding:

uv_TYPE_init(uv_loop_t *, uv_TYPE_t *)

function.

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.

Idling

Here is an example of using an idle handle. The callback is called once on every turn of the event loop. A use case for idle handles is discussed in Utilities. Let us use an idle watcher to look at the watcher life cycle and see how uv_run() will now block because a watcher is present. The idle watcher is stopped when the count is reached and uv_run() exits since no event watchers are active.

idle-basic/main.c

#include <stdio.h>
#include <uv.h>

int64_t counter = 0;

void wait_for_a_while(uv_idle_t* handle) {
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(), UV_RUN_DEFAULT);

uv_loop_close(uv_default_loop());
return 0;
}

Storing context

In callback based programming style you’ll often want to pass some ‘context’ – application specific information – between the call site and the callback. All handles and requests have a void* data member which you can set to the context and cast back in the callback. This is a common pattern used throughout the C library ecosystem. In addition uv_loop_t also has a similar data member.

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Filesystem

Simple filesystem read/write is achieved using the uv_fs_* functions and the uv_fs_t struct.

Note

The libuv filesystem operations are different from socket operations. Socket operations use the non-blocking operations provided by the operating system. Filesystem operations use blocking functions internally, but invoke these functions in a thread pool [http://docs.libuv.org/en/v1.x/threadpool.html#thread-pool-work-scheduling] and notify watchers registered with the event loop when application interaction is required.

All filesystem functions have two forms - synchronous and asynchronous.

The synchronous forms automatically get called (and block) if the callback is null. The return value of functions is a libuv error code. This is usually only useful for synchronous calls. The asynchronous form is called when a callback is passed and the return value is 0.

Reading/Writing files

A file descriptor is obtained using

int uv_fs_open(uv_loop_t* loop, uv_fs_t* req, const char* path, int flags, int mode, uv_fs_cb cb)

flags and mode are standard Unix flags [http://man7.org/linux/man-pages/man2/open.2.html]. libuv takes care of converting to the appropriate Windows flags.

File descriptors are closed using

int uv_fs_close(uv_loop_t* loop, uv_fs_t* req, uv_file file, uv_fs_cb cb)

Filesystem operation callbacks have the signature:

void callback(uv_fs_t* req);

Let’s see a simple implementation of cat. We start with registering a callback for when the file is opened:

uvcat/main.c - opening a file

The result field of a uv_fs_t is the file descriptor in case of the uv_fs_open callback. If the file is successfully opened, we start reading it.

uvcat/main.c - read callback

In the case of a read call, you should pass an initialized buffer which will be filled with data before the read callback is triggered. The uv_fs_* operations map almost directly to certain POSIX functions, so EOF is indicated in this case by result being 0. In the case of streams or pipes, the UV_EOF constant would have been passed as a status instead.

Here you see a common pattern when writing asynchronous programs. The uv_fs_close() call is performed synchronously. Usually tasks which are one-off, or are done as part of the startup or shutdown stage are performed synchronously, since we are interested in fast I/O when the program is going about its primary task and dealing with multiple I/O sources. For solo tasks the performance difference usually is negligible and may lead to simpler code.

Filesystem writing is similarly simple using uv_fs_write(). Your callback will be triggered after the write is complete. In our case the callback simply drives the next read. Thus read and write proceed in lockstep via callbacks.

uvcat/main.c - write callback

Warning

Due to the way filesystems and disk drives are configured for performance, a write that ‘succeeds’ may not be committed to disk yet.

We set the dominos rolling in main():

uvcat/main.c

Warning

The uv_fs_req_cleanup() function must always be called on filesystem requests to free internal memory allocations in libuv.

Filesystem operations

All the standard filesystem operations like unlink, rmdir, stat are supported asynchronously and have intuitive argument order. They follow the same patterns as the read/write/open calls, returning the result in the uv_fs_t.result field. The full list:

Filesystem operations

UV_EXTERN int uv_fs_close(uv_loop_t* loop,
uv_fs_t* req,
uv_file file,
uv_fs_cb cb);
UV_EXTERN int uv_fs_open(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
int flags,
int mode,
uv_fs_cb cb);
UV_EXTERN int uv_fs_read(uv_loop_t* loop,
uv_fs_t* req,
uv_file file,
const uv_buf_t bufs[],
unsigned int nbufs,
int64_t offset,
uv_fs_cb cb);
UV_EXTERN int uv_fs_unlink(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
uv_fs_cb cb);
UV_EXTERN int uv_fs_write(uv_loop_t* loop,
uv_fs_t* req,
uv_file file,
const uv_buf_t bufs[],
unsigned int nbufs,
int64_t offset,
uv_fs_cb cb);
UV_EXTERN int uv_fs_mkdir(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
int mode,
uv_fs_cb cb);
UV_EXTERN int uv_fs_mkdtemp(uv_loop_t* loop,
uv_fs_t* req,
const char* tpl,
uv_fs_cb cb);
UV_EXTERN int uv_fs_rmdir(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
uv_fs_cb cb);
UV_EXTERN int uv_fs_scandir(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
int flags,
uv_fs_cb cb);
UV_EXTERN int uv_fs_scandir_next(uv_fs_t* req,
uv_dirent_t* ent);
UV_EXTERN int uv_fs_stat(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
uv_fs_cb cb);
UV_EXTERN int uv_fs_fstat(uv_loop_t* loop,
uv_fs_t* req,
uv_file file,
uv_fs_cb cb);
UV_EXTERN int uv_fs_rename(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
const char* new_path,
uv_fs_cb cb);
UV_EXTERN int uv_fs_fsync(uv_loop_t* loop,
uv_fs_t* req,
uv_file file,
uv_fs_cb cb);
UV_EXTERN int uv_fs_fdatasync(uv_loop_t* loop,
uv_fs_t* req,
uv_file file,
uv_fs_cb cb);
UV_EXTERN int uv_fs_ftruncate(uv_loop_t* loop,
uv_fs_t* req,
uv_file file,
int64_t offset,
uv_fs_cb cb);
UV_EXTERN int uv_fs_sendfile(uv_loop_t* loop,
uv_fs_t* req,
uv_file out_fd,
uv_file in_fd,
int64_t in_offset,
size_t length,
uv_fs_cb cb);
UV_EXTERN int uv_fs_access(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
int mode,
uv_fs_cb cb);
UV_EXTERN int uv_fs_chmod(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
int mode,
uv_fs_cb cb);
UV_EXTERN int uv_fs_utime(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
double atime,
double mtime,
uv_fs_cb cb);
UV_EXTERN int uv_fs_futime(uv_loop_t* loop,
uv_fs_t* req,
uv_file file,
double atime,
double mtime,
uv_fs_cb cb);
UV_EXTERN int uv_fs_lstat(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
uv_fs_cb cb);
UV_EXTERN int uv_fs_link(uv_loop_t* loop,
uv_fs_t* req,
const char* path,
const char* new_path,
uv_fs_cb cb);

Buffers and Streams

The basic I/O handle in libuv is the stream (uv_stream_t). TCP sockets, UDP sockets, and pipes for file I/O and IPC are all treated as stream subclasses.

Streams are initialized using custom functions for each subclass, then operated upon using

int uv_read_start(uv_stream_t*, uv_alloc_cb alloc_cb, uv_read_cb read_cb);
int uv_read_stop(uv_stream_t*);
int uv_write(uv_write_t* req, uv_stream_t* handle,
const uv_buf_t bufs[], unsigned int nbufs, uv_write_cb cb);

The stream based functions are simpler to use than the filesystem ones and libuv will automatically keep reading from a stream when uv_read_start() is called once, until uv_read_stop() is called.

The discrete unit of data is the buffer – uv_buf_t. This is simply a collection of a pointer to bytes (uv_buf_t.base) and the length (uv_buf_t.len). The uv_buf_t is lightweight and passed around by value. What does require management is the actual bytes, which have to be allocated and freed by the application.

Error

THIS PROGRAM DOES NOT ALWAYS WORK, NEED SOMETHING BETTER**

To demonstrate streams we will need to use uv_pipe_t. This allows streaming local files [2]. Here is a simple tee utility using libuv. Doing all operations asynchronously shows the power of evented I/O. The two writes won’t block each other, but we have to be careful to copy over the buffer data to ensure we don’t free a buffer until it has been written.

The program is to be executed as:

./uvtee <output_file>

We start off opening pipes on the files we require. libuv pipes to a file are opened as bidirectional by default.

uvtee/main.c - read on pipes

The third argument of uv_pipe_init() should be set to 1 for IPC using named pipes. This is covered in Processes. The uv_pipe_open() call associates the pipe with the file descriptor, in this case 0 (standard input).

We start monitoring stdin. The alloc_buffer callback is invoked as new buffers are required to hold incoming data. read_stdin will be called with these buffers.

uvtee/main.c - reading buffers

The standard malloc is sufficient here, but you can use any memory allocation scheme. For example, node.js uses its own slab allocator which associates buffers with V8 objects.

The read callback nread parameter is less than 0 on any error. This error might be EOF, in which case we close all the streams, using the generic close function uv_close() which deals with the handle based on its internal type. Otherwise nread is a non-negative number and we can attempt to write that many bytes to the output streams. Finally remember that buffer allocation and deallocation is application responsibility, so we free the data.

The allocation callback may return a buffer with length zero if it fails to allocate memory. In this case, the read callback is invoked with error UV_ENOBUFS. libuv will continue to attempt to read the stream though, so you must explicitly call uv_close() if you want to stop when allocation fails.

The read callback may be called with nread = 0, indicating that at this point there is nothing to be read. Most applications will just ignore this.

uvtee/main.c - Write to pipe

write_data() makes a copy of the buffer obtained from read. This buffer does not get passed through to the write callback trigged on write completion. To get around this we wrap a write request and a buffer in write_req_t and unwrap it in the callbacks. We make a copy so we can free the two buffers from the two calls to write_data independently of each other. While acceptable for a demo program like this, you’ll probably want smarter memory management, like reference counted buffers or a pool of buffers in any major application.

Warning

If your program is meant to be used with other programs it may knowingly or unknowingly be writing to a pipe. This makes it susceptible to aborting on receiving a SIGPIPE [http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#The_special_problem_of_SIGPIPE]. It is a good idea to insert:

signal(SIGPIPE, SIG_IGN)

in the initialization stages of your application.

File change events

All modern operating systems provide APIs to put watches on individual files or directories and be informed when the files are modified. libuv wraps common file change notification libraries [1]. This is one of the more inconsistent parts of libuv. File change notification systems are themselves extremely varied across platforms so getting everything working everywhere is difficult. To demonstrate, I’m going to build a simple utility which runs a command whenever any of the watched files change:

./onchange <command> <file1> [file2] ...

The file change notification is started using uv_fs_event_init():

onchange/main.c - The setup

The third argument is the actual file or directory to monitor. The last argument, flags, can be:

UV_FS_EVENT_WATCH_ENTRY = 1,
UV_FS_EVENT_STAT = 2,
UV_FS_EVENT_RECURSIVE = 4

UV_FS_EVENT_WATCH_ENTRY and UV_FS_EVENT_STAT don’t do anything (yet). UV_FS_EVENT_RECURSIVE will start watching subdirectories as well on supported platforms.

The callback will receive the following arguments:

1. uv_fs_event_t *handle - The handle. The path field of the handle is the file on which the watch was set.

2. const char *filename - If a directory is being monitored, this is the file which was changed. Only non-null on Linux and Windows. May be null even on those platforms.

3. int flags - one of UV_RENAME or UV_CHANGE, or a bitwise OR of

both.

4. int status - Currently 0.

In our example we simply print the arguments and run the command using system().

onchange/main.c - file change notification callback

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Networking

Networking in libuv is not much different from directly using the BSD socket interface, some things are easier, all are non-blocking, but the concepts stay the same. In addition libuv offers utility functions to abstract the annoying, repetitive and low-level tasks like setting up sockets using the BSD socket structures, DNS lookup, and tweaking various socket parameters.

The uv_tcp_t and uv_udp_t structures are used for network I/O.

Note

The code samples in this chapter exist to show certain libuv APIs. They are not examples of good quality code. They leak memory and don’t always close connections properly.

TCP

TCP is a connection oriented, stream protocol and is therefore based on the libuv streams infrastructure.

Server

Server sockets proceed by:

1. uv_tcp_init the TCP handle.

2. uv_tcp_bind it.

3. Call uv_listen on the handle to have a callback invoked whenever a new connection is established by a client.

4. Use uv_accept to accept the connection.

5. Use stream operations to communicate with the client.

Here is a simple echo server

tcp-echo-server/main.c - The listen socket

You can see the utility function uv_ip4_addr being used to convert from a human readable IP address, port pair to the sockaddr_in structure required by the BSD socket APIs. The reverse can be obtained using uv_ip4_name.

Note

There are uv_ip6_* analogues for the ip4 functions.

Most of the setup functions are synchronous since they are CPU-bound. uv_listen is where we return to libuv’s callback style. The second arguments is the backlog queue – the maximum length of queued connections.

When a connection is initiated by clients, the callback is required to set up a handle for the client socket and associate the handle using uv_accept. In this case we also establish interest in reading from this stream.

tcp-echo-server/main.c - Accepting the client

The remaining set of functions is very similar to the streams example and can be found in the code. Just remember to call uv_close when the socket isn’t required. This can be done even in the uv_listen callback if you are not interested in accepting the connection.

Client

Where you do bind/listen/accept on the server, on the client side it’s simply a matter of calling uv_tcp_connect. The same uv_connect_cb style callback of uv_listen is used by uv_tcp_connect. Try:

uv_tcp_t* socket = (uv_tcp_t*)malloc(sizeof(uv_tcp_t));
uv_tcp_init(loop, socket);

uv_connect_t* connect = (uv_connect_t*)malloc(sizeof(uv_connect_t));

struct sockaddr_in dest;
uv_ip4_addr("127.0.0.1", 80, &dest);

uv_tcp_connect(connect, socket, (const struct sockaddr*)&dest, on_connect);

where on_connect will be called after the connection is established. The callback receives the uv_connect_t struct, which has a member .handle pointing to the socket.

UDP

The User Datagram Protocol [http://en.wikipedia.org/wiki/User_Datagram_Protocol] offers connectionless, unreliable network communication. Hence libuv doesn’t offer a stream. Instead libuv provides non-blocking UDP support via the uv_udp_t handle (for receiving) and uv_udp_send_t request (for sending) and related functions. That said, the actual API for reading/writing is very similar to normal stream reads. To look at how UDP can be used, the example shows the first stage of obtaining an IP address from a DHCP [http://tools.ietf.org/html/rfc2131] server – DHCP Discover.

Note

You will have to run udp-dhcp as root since it uses well known port numbers below 1024.

udp-dhcp/main.c - Setup and send UDP packets

Note

The IP address 0.0.0.0 is used to bind to all interfaces. The IP address 255.255.255.255 is a broadcast address meaning that packets will be sent to all interfaces on the subnet. port 0 means that the OS randomly assigns a port.

First we setup the receiving socket to bind on all interfaces on port 68 (DHCP client) and start a read on it. This will read back responses from any DHCP server that replies. We use the UV_UDP_REUSEADDR flag to play nice with any other system DHCP clients that are running on this computer on the same port. Then we setup a similar send socket and use uv_udp_send to send a broadcast message on port 67 (DHCP server).

It is necessary to set the broadcast flag, otherwise you will get an EACCES error [1]. The exact message being sent is not relevant to this book and you can study the code if you are interested. As usual the read and write callbacks will receive a status code of < 0 if something went wrong.

Since UDP sockets are not connected to a particular peer, the read callback receives an extra parameter about the sender of the packet.

nread may be zero if there is no more data to be read. If addr is NULL, it indicates there is nothing to read (the callback shouldn’t do anything), if not NULL, it indicates that an empty datagram was received from the host at addr. The flags parameter may be UV_UDP_PARTIAL if the buffer provided by your allocator was not large enough to hold the data. In this case the OS will discard the data that could not fit (That’s UDP for you!).

udp-dhcp/main.c - Reading packets

UDP Options

Time-to-live

The TTL of packets sent on the socket can be changed using uv_udp_set_ttl.

IPv6 stack only

IPv6 sockets can be used for both IPv4 and IPv6 communication. If you want to restrict the socket to IPv6 only, pass the UV_UDP_IPV6ONLY flag to uv_udp_bind [2].

Multicast

A socket can (un)subscribe to a multicast group using:

UV_EXTERN int uv_udp_set_membership(uv_udp_t* handle,
const char* multicast_addr,
const char* interface_addr,
uv_membership membership);

where membership is UV_JOIN_GROUP or UV_LEAVE_GROUP.

The concepts of multicasting are nicely explained in this guide [http://www.tldp.org/HOWTO/Multicast-HOWTO-2.html].

Local loopback of multicast packets is enabled by default [3], use uv_udp_set_multicast_loop to switch it off.

The packet time-to-live for multicast packets can be changed using uv_udp_set_multicast_ttl.

Querying DNS

libuv provides asynchronous DNS resolution. For this it provides its own getaddrinfo replacement [4]. In the callback you can perform normal socket operations on the retrieved addresses. Let’s connect to Freenode to see an example of DNS resolution.

dns/main.c

If uv_getaddrinfo returns non-zero, something went wrong in the setup and your callback won’t be invoked at all. All arguments can be freed immediately after uv_getaddrinfo returns. The hostname, servname and hints structures are documented in the getaddrinfo man page. The callback can be NULL in which case the function will run synchronously.

In the resolver callback, you can pick any IP from the linked list of struct addrinfo(s). This also demonstrates uv_tcp_connect. It is necessary to call uv_freeaddrinfo in the callback.

dns/main.c

libuv also provides the inverse uv_getnameinfo [http://docs.libuv.org/en/v1.x/dns.html#c.uv_getnameinfo].

Network interfaces

Information about the system’s network interfaces can be obtained through libuv using uv_interface_addresses. This simple program just prints out all the interface details so you get an idea of the fields that are available. This is useful to allow your service to bind to IP addresses when it starts.

interfaces/main.c

is_internal is true for loopback interfaces. Note that if a physical interface has multiple IPv4/IPv6 addresses, the name will be reported multiple times, with each address being reported once.

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Threads

Wait a minute? Why are we on threads? Aren’t event loops supposed to be the way to do web-scale programming? Well... no. Threads are still the medium in which processors do their jobs. Threads are therefore mighty useful sometimes, even though you might have to wade through various synchronization primitives.

Threads are used internally to fake the asynchronous nature of all of the system calls. libuv also uses threads to allow you, the application, to perform a task asynchronously that is actually blocking, by spawning a thread and collecting the result when it is done.

Today there are two predominant thread libraries: the Windows threads implementation and POSIX’s pthreads [http://man7.org/linux/man-pages/man7/pthreads.7.html]. libuv’s thread API is analogous to the pthreads API and often has similar semantics.

A notable aspect of libuv’s thread facilities is that it is a self contained section within libuv. Whereas other features intimately depend on the event loop and callback principles, threads are complete agnostic, they block as required, signal errors directly via return values, and, as shown in the first example, don’t even require a running event loop.

libuv’s thread API is also very limited since the semantics and syntax of threads are different on all platforms, with different levels of completeness.

This chapter makes the following assumption: There is only one event loop, running in one thread (the main thread). No other thread interacts with the event loop (except using uv_async_send).

Core thread operations

There isn’t much here, you just start a thread using uv_thread_create() and wait for it to close using uv_thread_join().

thread-create/main.c

Tip

uv_thread_t is just an alias for pthread_t on Unix, but this is an implementation detail, avoid depending on it to always be true.

The second parameter is the function which will serve as the entry point for the thread, the last parameter is a void * argument which can be used to pass custom parameters to the thread. The function hare will now run in a separate thread, scheduled pre-emptively by the operating system:

thread-create/main.c

Unlike pthread_join() which allows the target thread to pass back a value to the calling thread using a second parameter, uv_thread_join() does not. To send values use Inter-thread communication.

Synchronization Primitives

This section is purposely spartan. This book is not about threads, so I only catalogue any surprises in the libuv APIs here. For the rest you can look at the pthreads man pages.

Mutexes

The mutex functions are a direct map to the pthread equivalents.

libuv mutex functions

UV_EXTERN int uv_mutex_init(uv_mutex_t* handle);
UV_EXTERN void uv_mutex_destroy(uv_mutex_t* handle);
UV_EXTERN void uv_mutex_lock(uv_mutex_t* handle);
UV_EXTERN int uv_mutex_trylock(uv_mutex_t* handle);
UV_EXTERN void uv_mutex_unlock(uv_mutex_t* handle);

The uv_mutex_init() and uv_mutex_trylock() functions will return 0 on success, and an error code otherwise.

If libuv has been compiled with debugging enabled, uv_mutex_destroy(), uv_mutex_lock() and uv_mutex_unlock() will abort() on error. Similarly uv_mutex_trylock() will abort if the error is anything other than EAGAIN or EBUSY.

Recursive mutexes are supported by some platforms, but you should not rely on them. The BSD mutex implementation will raise an error if a thread which has locked a mutex attempts to lock it again. For example, a construct like:

uv_mutex_lock(a_mutex);
uv_thread_create(thread_id, entry, (void *)a_mutex);
uv_mutex_lock(a_mutex);
// more things here

can be used to wait until another thread initializes some stuff and then unlocks a_mutex but will lead to your program crashing if in debug mode, or return an error in the second call to uv_mutex_lock().

Note

Mutexes on linux support attributes for a recursive mutex, but the API is not exposed via libuv.

Locks

Read-write locks are a more granular access mechanism. Two readers can access shared memory at the same time. A writer may not acquire the lock when it is held by a reader. A reader or writer may not acquire a lock when a writer is holding it. Read-write locks are frequently used in databases. Here is a toy example.

locks/main.c - simple rwlocks

Run this and observe how the readers will sometimes overlap. In case of multiple writers, schedulers will usually give them higher priority, so if you add two writers, you’ll see that both writers tend to finish first before the readers get a chance again.

We also use barriers in the above example so that the main thread can wait for all readers and writers to indicate they have ended.

Others

libuv also supports semaphores [http://en.wikipedia.org/wiki/Semaphore_(programming)], condition variables [http://en.wikipedia.org/wiki/Condition_variable#Waiting_and_signaling] and barriers [http://en.wikipedia.org/wiki/Barrier_(computer_science)] with APIs very similar to their pthread counterparts.

In addition, libuv provides a convenience function uv_once(). Multiple threads can attempt to call uv_once() with a given guard and a function pointer, only the first one will win, the function will be called once and only once:

/* Initialize guard */
static uv_once_t once_only = UV_ONCE_INIT;

int i = 0;

void increment() {
i++;
}

void thread1() {
/* ... work */
uv_once(once_only, increment);
}

void thread2() {
/* ... work */
uv_once(once_only, increment);
}

int main() {
/* ... spawn threads */
}

After all threads are done, i == 1.

libuv v0.11.11 onwards also added a uv_key_t struct and api [http://docs.libuv.org/en/v1.x/threading.html#thread-local-storage] for thread-local storage.

libuv work queue

uv_queue_work() is a convenience function that allows an application to run a task in a separate thread, and have a callback that is triggered when the task is done. A seemingly simple function, what makes uv_queue_work() tempting is that it allows potentially any third-party libraries to be used with the event-loop paradigm. When you use event loops, it is imperative to make sure that no function which runs periodically in the loop thread blocks when performing I/O or is a serious CPU hog, because this means that the loop slows down and events are not being handled at full capacity.

However, a lot of existing code out there features blocking functions (for example a routine which performs I/O under the hood) to be used with threads if you want responsiveness (the classic ‘one thread per client’ server model), and getting them to play with an event loop library generally involves rolling your own system of running the task in a separate thread. libuv just provides a convenient abstraction for this.

Here is a simple example inspired by node.js is cancer [http://teddziuba.github.io/2011/10/node-js-is-cancer.html]. We are going to calculate fibonacci numbers, sleeping a bit along the way, but run it in a separate thread so that the blocking and CPU bound task does not prevent the event loop from performing other activities.

queue-work/main.c - lazy fibonacci

The actual task function is simple, nothing to show that it is going to be run in a separate thread. The uv_work_t structure is the clue. You can pass arbitrary data through it using the void* data field and use it to communicate to and from the thread. But be sure you are using proper locks if you are changing things while both threads may be running.

The trigger is uv_queue_work:

queue-work/main.c

The thread function will be launched in a separate thread, passed the uv_work_t structure and once the function returns, the after function will be called on the thread the event loop is running in. It will be passed the same structure.

For writing wrappers to blocking libraries, a common pattern is to use a baton to exchange data.

Since libuv version 0.9.4 an additional function, uv_cancel(), is available. This allows you to cancel tasks on the libuv work queue. Only tasks that are yet to be started can be cancelled. If a task has already started executing, or it has finished executing, uv_cancel() will fail.

uv_cancel() is useful to cleanup pending tasks if the user requests termination. For example, a music player may queue up multiple directories to be scanned for audio files. If the user terminates the program, it should quit quickly and not wait until all pending requests are run.

Let’s modify the fibonacci example to demonstrate uv_cancel(). We first set up a signal handler for termination.

queue-cancel/main.c

When the user triggers the signal by pressing Ctrl+C we send uv_cancel() to all the workers. uv_cancel() will return 0 for those that are already executing or finished.

queue-cancel/main.c

For tasks that do get cancelled successfully, the after function is called with status set to UV_ECANCELED.

queue-cancel/main.c

uv_cancel() can also be used with uv_fs_t and uv_getaddrinfo_t requests. For the filesystem family of functions, uv_fs_t.errorno will be set to UV_ECANCELED.

Tip

A well designed program would have a way to terminate long running workers that have already started executing. Such a worker could periodically check for a variable that only the main process sets to signal termination.

Inter-thread communication

Sometimes you want various threads to actually send each other messages while they are running. For example you might be running some long duration task in a separate thread (perhaps using uv_queue_work) but want to notify progress to the main thread. This is a simple example of having a download manager informing the user of the status of running downloads.

progress/main.c

The async thread communication works on loops so although any thread can be the message sender, only threads with libuv loops can be receivers (or rather the loop is the receiver). libuv will invoke the callback (print_progress) with the async watcher whenever it receives a message.

Warning

It is important to realize that since the message send is async, the callback may be invoked immediately after uv_async_send is called in another thread, or it may be invoked after some time. libuv may also combine multiple calls to uv_async_send and invoke your callback only once. The only guarantee that libuv makes is – The callback function is called at least once after the call to uv_async_send. If you have no pending calls to uv_async_send, the callback won’t be called. If you make two or more calls, and libuv hasn’t had a chance to run the callback yet, it may invoke your callback only once for the multiple invocations of uv_async_send. Your callback will never be called twice for just one event.

progress/main.c

In the download function, we modify the progress indicator and queue the message for delivery with uv_async_send. Remember: uv_async_send is also non-blocking and will return immediately.

progress/main.c

The callback is a standard libuv pattern, extracting the data from the watcher.

Finally it is important to remember to clean up the watcher.

progress/main.c

After this example, which showed the abuse of the data field, bnoordhuis [https://github.com/bnoordhuis] pointed out that using the data field is not thread safe, and uv_async_send() is actually only meant to wake up the event loop. Use a mutex or rwlock to ensure accesses are performed in the right order.

Note

mutexes and rwlocks DO NOT work inside a signal handler, whereas uv_async_send does.

One use case where uv_async_send is required is when interoperating with libraries that require thread affinity for their functionality. For example in node.js, a v8 engine instance, contexts and its objects are bound to the thread that the v8 instance was started in. Interacting with v8 data structures from another thread can lead to undefined results. Now consider some node.js module which binds a third party library. It may go something like this:

1. In node, the third party library is set up with a JavaScript callback to be invoked for more information:

var lib = require('lib');
lib.on_progress(function() {
console.log("Progress");
});

lib.do();

// do other stuff

2. lib.do is supposed to be non-blocking but the third party lib is blocking, so the binding uses uv_queue_work.

3. The actual work being done in a separate thread wants to invoke the progress callback, but cannot directly call into v8 to interact with JavaScript. So it uses uv_async_send.

4. The async callback, invoked in the main loop thread, which is the v8 thread, then interacts with v8 to invoke the JavaScript callback.

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Processes

libuv offers considerable child process management, abstracting the platform differences and allowing communication with the child process using streams or named pipes.

A common idiom in Unix is for every process to do one thing and do it well. In such a case, a process often uses multiple child processes to achieve tasks (similar to using pipes in shells). A multi-process model with messages may also be easier to reason about compared to one with threads and shared memory.

A common refrain against event-based programs is that they cannot take advantage of multiple cores in modern computers. In a multi-threaded program the kernel can perform scheduling and assign different threads to different cores, improving performance. But an event loop has only one thread. The workaround can be to launch multiple processes instead, with each process running an event loop, and each process getting assigned to a separate CPU core.

Spawning child processes

The simplest case is when you simply want to launch a process and know when it exits. This is achieved using uv_spawn.

spawn/main.c

Note

options is implicitly initialized with zeros since it is a global variable. If you change options to a local variable, remember to initialize it to null out all unused fields:

uv_process_options_t options = {0};

The uv_process_t struct only acts as the handle, all options are set via uv_process_options_t. To simply launch a process, you need to set only the file and args fields. file is the program to execute. Since uv_spawn uses execvp [http://www.kernel.org/doc/man-pages/online/pages/man3/exec.3.html] internally, there is no need to supply the full path. Finally as per underlying conventions, the arguments array has to be one larger than the number of arguments, with the last element being NULL.

After the call to uv_spawn, uv_process_t.pid will contain the process ID of the child process.

The exit callback will be invoked with the exit status and the type of signal which caused the exit.

spawn/main.c

It is required to close the process watcher after the process exits.

Changing process parameters

Before the child process is launched you can control the execution environment using fields in uv_process_options_t.

Change execution directory

Set uv_process_options_t.cwd to the corresponding directory.

Set environment variables

uv_process_options_t.env is a null-terminated array of strings, each of the form VAR=VALUE used to set up the environment variables for the process. Set this to NULL to inherit the environment from the parent (this) process.

Option flags

Setting uv_process_options_t.flags to a bitwise OR of the following flags, modifies the child process behaviour:

- UV_PROCESS_SETUID - sets the child’s execution user ID to uv_process_options_t.uid.

- UV_PROCESS_SETGID - sets the child’s execution group ID to uv_process_options_t.gid.

Changing the UID/GID is only supported on Unix, uv_spawn will fail on Windows with UV_ENOTSUP.

- UV_PROCESS_WINDOWS_VERBATIM_ARGUMENTS - No quoting or escaping of uv_process_options_t.args is done on Windows. Ignored on Unix.

- UV_PROCESS_DETACHED - Starts the child process in a new session, which will keep running after the parent process exits. See example below.

Detaching processes

Passing the flag UV_PROCESS_DETACHED can be used to launch daemons, or child processes which are independent of the parent so that the parent exiting does not affect it.

detach/main.c

Just remember that the handle is still monitoring the child, so your program won’t exit. Use uv_unref() if you want to be more fire-and-forget.

Sending signals to processes

libuv wraps the standard kill(2) system call on Unix and implements one with similar semantics on Windows, with one caveat: all of SIGTERM, SIGINT and SIGKILL, lead to termination of the process. The signature of uv_kill is:

uv_err_t uv_kill(int pid, int signum);

For processes started using libuv, you may use uv_process_kill instead, which accepts the uv_process_t watcher as the first argument, rather than the pid. In this case, remember to call uv_close on the watcher.

Signals

libuv provides wrappers around Unix signals with some Windows support [http://docs.libuv.org/en/v1.x/signal.html#signal] as well.

Use uv_signal_init() to initialize a handle and associate it with a loop. To listen for particular signals on that handler, use uv_signal_start() with the handler function. Each handler can only be associated with one signal number, with subsequent calls to uv_signal_start() overwriting earlier associations. Use uv_signal_stop() to stop watching. Here is a small example demonstrating the various possibilities:

signal/main.c

Note

uv_run(loop, UV_RUN_NOWAIT) is similar to uv_run(loop, UV_RUN_ONCE) in that it will process only one event. UV_RUN_ONCE blocks if there are no pending events, while UV_RUN_NOWAIT will return immediately. We use NOWAIT so that one of the loops isn’t starved because the other one has no pending activity.

Send SIGUSR1 to the process, and you’ll find the handler being invoked 4 times, one for each uv_signal_t. The handler just stops each handle, so that the program exits. This sort of dispatch to all handlers is very useful. A server using multiple event loops could ensure that all data was safely saved before termination, simply by every loop adding a watcher for SIGINT.

Child Process I/O

A normal, newly spawned process has its own set of file descriptors, with 0, 1 and 2 being stdin, stdout and stderr respectively. Sometimes you may want to share file descriptors with the child. For example, perhaps your applications launches a sub-command and you want any errors to go in the log file, but ignore stdout. For this you’d like to have stderr of the child be the same as the stderr of the parent. In this case, libuv supports inheriting file descriptors. In this sample, we invoke the test program, which is:

proc-streams/test.c

#include <stdio.h>

int main()
{
fprintf(stderr, "This is stderr\n");
printf("This is stdout\n");
return 0;
}

The actual program proc-streams runs this while sharing only stderr. The file descriptors of the child process are set using the stdio field in uv_process_options_t. First set the stdio_count field to the number of file descriptors being set. uv_process_options_t.stdio is an array of uv_stdio_container_t, which is:

typedef struct uv_stdio_container_s {
uv_stdio_flags flags;

union {
uv_stream_t* stream;
int fd;
} data;
} uv_stdio_container_t;

where flags can have several values. Use UV_IGNORE if it isn’t going to be used. If the first three stdio fields are marked as UV_IGNORE they’ll redirect to /dev/null.

Since we want to pass on an existing descriptor, we’ll use UV_INHERIT_FD. Then we set the fd to stderr.

proc-streams/main.c

If you run proc-stream you’ll see that only the line “This is stderr” will be displayed. Try marking stdout as being inherited and see the output.

It is dead simple to apply this redirection to streams. By setting flags to UV_INHERIT_STREAM and setting data.stream to the stream in the parent process, the child process can treat that stream as standard I/O. This can be used to implement something like CGI [http://en.wikipedia.org/wiki/Common_Gateway_Interface].

A sample CGI script/executable is:

cgi/tick.c

#include <stdio.h>
#include <unistd.h>

int main() {
int i;
for (i = 0; i < 10; i++) {
printf("tick\n");
fflush(stdout);
sleep(1);
}
printf("BOOM!\n");
return 0;
}

The CGI server combines the concepts from this chapter and Networking so that every client is sent ten ticks after which that connection is closed.

cgi/main.c

Here we simply accept the TCP connection and pass on the socket (stream) to invoke_cgi_script.

cgi/main.c

The stdout of the CGI script is set to the socket so that whatever our tick script prints, gets sent to the client. By using processes, we can offload the read/write buffering to the operating system, so in terms of convenience this is great. Just be warned that creating processes is a costly task.

Pipes

libuv’s uv_pipe_t structure is slightly confusing to Unix programmers, because it immediately conjures up | and pipe(7) [http://www.kernel.org/doc/man-pages/online/pages/man7/pipe.7.html]. But uv_pipe_t is not related to anonymous pipes, rather it is an IPC mechanism. uv_pipe_t can be backed by a Unix Domain Socket [http://www.kernel.org/doc/man-pages/online/pages/man7/unix.7.html] or Windows Named Pipe [http://msdn.microsoft.com/en-us/library/windows/desktop/aa365590(v=vs.85).aspx] to allow multiple processes to communicate. This is discussed below.

Parent-child IPC

A parent and child can have one or two way communication over a pipe created by settings uv_stdio_container_t.flags to a bit-wise combination of UV_CREATE_PIPE and UV_READABLE_PIPE or UV_WRITABLE_PIPE. The read/write flag is from the perspective of the child process.

Arbitrary process IPC

Since domain sockets [1] can have a well known name and a location in the file-system they can be used for IPC between unrelated processes. The D-BUS [http://www.freedesktop.org/wiki/Software/dbus] system used by open source desktop environments uses domain sockets for event notification. Various applications can then react when a contact comes online or new hardware is detected. The MySQL server also runs a domain socket on which clients can interact with it.

When using domain sockets, a client-server pattern is usually followed with the creator/owner of the socket acting as the server. After the initial setup, messaging is no different from TCP, so we’ll re-use the echo server example.

pipe-echo-server/main.c

We name the socket echo.sock which means it will be created in the local directory. This socket now behaves no different from TCP sockets as far as the stream API is concerned. You can test this server using socat [http://www.dest-unreach.org/socat/]:

$ socat - /path/to/socket

A client which wants to connect to a domain socket will use:

void uv_pipe_connect(uv_connect_t *req, uv_pipe_t *handle, const char *name, uv_connect_cb cb);

where name will be echo.sock or similar.

Sending file descriptors over pipes

The cool thing about domain sockets is that file descriptors can be exchanged between processes by sending them over a domain socket. This allows processes to hand off their I/O to other processes. Applications include load-balancing servers, worker processes and other ways to make optimum use of CPU. libuv only supports sending TCP sockets or other pipes over pipes for now.

To demonstrate, we will look at a echo server implementation that hands of clients to worker processes in a round-robin fashion. This program is a bit involved, and while only snippets are included in the book, it is recommended to read the full code to really understand it.

The worker process is quite simple, since the file-descriptor is handed over to it by the master.

multi-echo-server/worker.c

queue is the pipe connected to the master process on the other end, along which new file descriptors get sent. It is important to set the ipc argument of uv_pipe_init to 1 to indicate this pipe will be used for inter-process communication! Since the master will write the file handle to the standard input of the worker, we connect the pipe to stdin using uv_pipe_open.

multi-echo-server/worker.c

First we call uv_pipe_pending_count() to ensure that a handle is available to read out. If your program could deal with different types of handles, uv_pipe_pending_type() can be used to determine the type. Although accept seems odd in this code, it actually makes sense. What accept traditionally does is get a file descriptor (the client) from another file descriptor (The listening socket). Which is exactly what we do here. Fetch the file descriptor (client) from queue. From this point the worker does standard echo server stuff.

Turning now to the master, let’s take a look at how the workers are launched to allow load balancing.

multi-echo-server/main.c

The child_worker structure wraps the process, and the pipe between the master and the individual process.

multi-echo-server/main.c

In setting up the workers, we use the nifty libuv function uv_cpu_info to get the number of CPUs so we can launch an equal number of workers. Again it is important to initialize the pipe acting as the IPC channel with the third argument as 1. We then indicate that the child process’ stdin is to be a readable pipe (from the point of view of the child). Everything is straightforward till here. The workers are launched and waiting for file descriptors to be written to their standard input.

It is in on_new_connection (the TCP infrastructure is initialized in main()), that we accept the client socket and pass it along to the next worker in the round-robin.

multi-echo-server/main.c

The uv_write2 call handles all the abstraction and it is simply a matter of passing in the handle (client) as the right argument. With this our multi-process echo server is operational.

Thanks to Kyle for pointing out [https://github.com/nikhilm/uvbook/issues/56] that uv_write2() requires a non-empty buffer even when sending handles.

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Advanced event loops

libuv provides considerable user control over event loops, and you can achieve interesting results by juggling multiple loops. You can also embed libuv’s event loop into another event loop based library – imagine a Qt based UI, and Qt’s event loop driving a libuv backend which does intensive system level tasks.

Stopping an event loop

uv_stop() can be used to stop an event loop. The earliest the loop will stop running is on the next iteration, possibly later. This means that events that are ready to be processed in this iteration of the loop will still be processed, so uv_stop() can’t be used as a kill switch. When uv_stop() is called, the loop won’t block for i/o on this iteration. The semantics of these things can be a bit difficult to understand, so let’s look at uv_run() where all the control flow occurs.

src/unix/core.c - uv_run

stop_flag is set by uv_stop(). Now all libuv callbacks are invoked within the event loop, which is why invoking uv_stop() in them will still lead to this iteration of the loop occurring. First libuv updates timers, then runs pending timer, idle and prepare callbacks, and invokes any pending I/O callbacks. If you were to call uv_stop() in any of them, stop_flag would be set. This causes uv_backend_timeout() to return 0, which is why the loop does not block on I/O. If on the other hand, you called uv_stop() in one of the check handlers, I/O has already finished and is not affected.

uv_stop() is useful to shutdown a loop when a result has been computed or there is an error, without having to ensure that all handlers are stopped one by one.

Here is a simple example that stops the loop and demonstrates how the current iteration of the loop still takes places.

uvstop/main.c

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Utilities

This chapter catalogues tools and techniques which are useful for common tasks. The libev man page [http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#COMMON_OR_USEFUL_IDIOMS_OR_BOTH] already covers some patterns which can be adopted to libuv through simple API changes. It also covers parts of the libuv API that don’t require entire chapters dedicated to them.

Timers

Timers invoke the callback after a certain time has elapsed since the timer was started. libuv timers can also be set to invoke at regular intervals instead of just once.

Simple use is to init a watcher and start it with a timeout, and optional repeat. Timers can be stopped at any time.

uv_timer_t timer_req;

uv_timer_init(loop, &timer_req);
uv_timer_start(&timer_req, callback, 5000, 2000);

will start a repeating timer, which first starts 5 seconds (the timeout) after the execution of uv_timer_start, then repeats every 2 seconds (the repeat). Use:

uv_timer_stop(&timer_req);

to stop the timer. This can be used safely from within the callback as well.

The repeat interval can be modified at any time with:

uv_timer_set_repeat(uv_timer_t *timer, int64_t repeat);

which will take effect when possible. If this function is called from a timer callback, it means:

- If the timer was non-repeating, the timer has already been stopped. Use uv_timer_start again.

- If the timer is repeating, the next timeout has already been scheduled, so the old repeat interval will be used once more before the timer switches to the new interval.

The utility function:

int uv_timer_again(uv_timer_t *)

applies only to repeating timers and is equivalent to stopping the timer and then starting it with both initial timeout and repeat set to the old repeat value. If the timer hasn’t been started it fails (error code UV_EINVAL) and returns -1.

An actual timer example is in the reference count section.

Event loop reference count

The event loop only runs as long as there are active handles. This system works by having every handle increase the reference count of the event loop when it is started and decreasing the reference count when stopped. It is also possible to manually change the reference count of handles using:

void uv_ref(uv_handle_t*);
void uv_unref(uv_handle_t*);

These functions can be used to allow a loop to exit even when a watcher is active or to use custom objects to keep the loop alive.

The latter can be used with interval timers. You might have a garbage collector which runs every X seconds, or your network service might send a heartbeat to others periodically, but you don’t want to have to stop them along all clean exit paths or error scenarios. Or you want the program to exit when all your other watchers are done. In that case just unref the timer immediately after creation so that if it is the only watcher running then uv_run will still exit.

This is also used in node.js where some libuv methods are being bubbled up to the JS API. A uv_handle_t (the superclass of all watchers) is created per JS object and can be ref/unrefed.

ref-timer/main.c

We initialize the garbage collector timer, then immediately unref it. Observe how after 9 seconds, when the fake job is done, the program automatically exits, even though the garbage collector is still running.

Idler pattern

The callbacks of idle handles are invoked once per event loop. The idle callback can be used to perform some very low priority activity. For example, you could dispatch a summary of the daily application performance to the developers for analysis during periods of idleness, or use the application’s CPU time to perform SETI calculations :) An idle watcher is also useful in a GUI application. Say you are using an event loop for a file download. If the TCP socket is still being established and no other events are present your event loop will pause (block), which means your progress bar will freeze and the user will face an unresponsive application. In such a case queue up and idle watcher to keep the UI operational.

idle-compute/main.c

Here we initialize the idle watcher and queue it up along with the actual events we are interested in. crunch_away will now be called repeatedly until the user types something and presses Return. Then it will be interrupted for a brief amount as the loop deals with the input data, after which it will keep calling the idle callback again.

idle-compute/main.c

Passing data to worker thread

When using uv_queue_work you’ll usually need to pass complex data through to the worker thread. The solution is to use a struct and set uv_work_t.data to point to it. A slight variation is to have the uv_work_t itself as the first member of this struct (called a baton [1]). This allows cleaning up the work request and all the data in one free call.

Here we create the baton and queue the task.

Now the task function can extract the data it needs:

We then free the baton which also frees the watcher.

External I/O with polling

Usually third-party libraries will handle their own I/O, and keep track of their sockets and other files internally. In this case it isn’t possible to use the standard stream I/O operations, but the library can still be integrated into the libuv event loop. All that is required is that the library allow you to access the underlying file descriptors and provide functions that process tasks in small increments as decided by your application. Some libraries though will not allow such access, providing only a standard blocking function which will perform the entire I/O transaction and only then return. It is unwise to use these in the event loop thread, use the libuv-work-queue instead. Of course, this will also mean losing granular control on the library.

The uv_poll section of libuv simply watches file descriptors using the operating system notification mechanism. In some sense, all the I/O operations that libuv implements itself are also backed by uv_poll like code. Whenever the OS notices a change of state in file descriptors being polled, libuv will invoke the associated callback.

Here we will walk through a simple download manager that will use libcurl [http://curl.haxx.se/libcurl/] to download files. Rather than give all control to libcurl, we’ll instead be using the libuv event loop, and use the non-blocking, async multi [http://curl.haxx.se/libcurl/c/libcurl-multi.html] interface to progress with the download whenever libuv notifies of I/O readiness.

uvwget/main.c - The setup

The way each library is integrated with libuv will vary. In the case of libcurl, we can register two callbacks. The socket callback handle_socket is invoked whenever the state of a socket changes and we have to start polling it. start_timeout is called by libcurl to notify us of the next timeout interval, after which we should drive libcurl forward regardless of I/O status. This is so that libcurl can handle errors or do whatever else is required to get the download moving.

Our downloader is to be invoked as:

$ ./uvwget [url1] [url2] ...

So we add each argument as an URL

uvwget/main.c - Adding urls

We let libcurl directly write the data to a file, but much more is possible if you so desire.

start_timeout will be called immediately the first time by libcurl, so things are set in motion. This simply starts a libuv timer which drives curl_multi_socket_action with CURL_SOCKET_TIMEOUT whenever it times out. curl_multi_socket_action is what drives libcurl, and what we call whenever sockets change state. But before we go into that, we need to poll on sockets whenever handle_socket is called.

uvwget/main.c - Setting up polling

We are interested in the socket fd s, and the action. For every socket we create a uv_poll_t handle if it doesn’t exist, and associate it with the socket using curl_multi_assign. This way socketp points to it whenever the callback is invoked.

In the case that the download is done or fails, libcurl requests removal of the poll. So we stop and free the poll handle.

Depending on what events libcurl wishes to watch for, we start polling with UV_READABLE or UV_WRITABLE. Now libuv will invoke the poll callback whenever the socket is ready for reading or writing. Calling uv_poll_start multiple times on the same handle is acceptable, it will just update the events mask with the new value. curl_perform is the crux of this program.

uvwget/main.c - Driving libcurl.

The first thing we do is to stop the timer, since there has been some progress in the interval. Then depending on what event triggered the callback, we set the correct flags. Then we call curl_multi_socket_action with the socket that progressed and the flags informing about what events happened. At this point libcurl does all of its internal tasks in small increments, and will attempt to return as fast as possible, which is exactly what an evented program wants in its main thread. libcurl keeps queueing messages into its own queue about transfer progress. In our case we are only interested in transfers that are completed. So we extract these messages, and clean up handles whose transfers are done.

uvwget/main.c - Reading transfer status.

Check & Prepare watchers

TODO

Loading libraries

libuv provides a cross platform API to dynamically load shared libraries [http://en.wikipedia.org/wiki/Shared_library#Shared_libraries]. This can be used to implement your own plugin/extension/module system and is used by node.js to implement require() support for bindings. The usage is quite simple as long as your library exports the right symbols. Be careful with sanity and security checks when loading third party code, otherwise your program will behave unpredictably. This example implements a very simple plugin system which does nothing except print the name of the plugin.

Let us first look at the interface provided to plugin authors.

plugin/plugin.h

You can similarly add more functions that plugin authors can use to do useful things in your application [2]. A sample plugin using this API is:

plugin/hello.c

Our interface defines that all plugins should have an initialize function which will be called by the application. This plugin is compiled as a shared library and can be loaded by running our application:

$ ./plugin libhello.dylib
Loading libhello.dylib
Registered plugin "Hello World!"

Note

The shared library filename will be different depending on platforms. On Linux it is libhello.so.

This is done by using uv_dlopen to first load the shared library libhello.dylib. Then we get access to the initialize function using uv_dlsym and invoke it.

plugin/main.c

uv_dlopen expects a path to the shared library and sets the opaque uv_lib_t pointer. It returns 0 on success, -1 on error. Use uv_dlerror to get the error message.

uv_dlsym stores a pointer to the symbol in the second argument in the third argument. init_plugin_function is a function pointer to the sort of function we are looking for in the application’s plugins.

TTY

Text terminals have supported basic formatting for a long time, with a pretty standardised [http://en.wikipedia.org/wiki/ANSI_escape_sequences] command set. This formatting is often used by programs to improve the readability of terminal output. For example grep --colour. libuv provides the uv_tty_t abstraction (a stream) and related functions to implement the ANSI escape codes across all platforms. By this I mean that libuv converts ANSI codes to the Windows equivalent, and provides functions to get terminal information.

The first thing to do is to initialize a uv_tty_t with the file descriptor it reads/writes from. This is achieved with:

int uv_tty_init(uv_loop_t*, uv_tty_t*, uv_file fd, int readable)

Set readable to true if you plan to use uv_read_start() on the stream.

It is then best to use uv_tty_set_mode to set the mode to normal which enables most TTY formatting, flow-control and other settings. Other [http://docs.libuv.org/en/v1.x/tty.html#c.uv_tty_mode_t] modes are also available.

Remember to call uv_tty_reset_mode when your program exits to restore the state of the terminal. Just good manners. Another set of good manners is to be aware of redirection. If the user redirects the output of your command to a file, control sequences should not be written as they impede readability and grep. To check if the file descriptor is indeed a TTY, call uv_guess_handle with the file descriptor and compare the return value with UV_TTY.

Here is a simple example which prints white text on a red background:

tty/main.c

The final TTY helper is uv_tty_get_winsize() which is used to get the width and height of the terminal and returns 0 on success. Here is a small program which does some animation using the function and character position escape codes.

tty-gravity/main.c

The escape codes are:

As you can see this is very useful to produce nicely formatted output, or even console based arcade games if that tickles your fancy. For fancier control you can try ncurses [http://www.gnu.org/software/ncurses/ncurses.html].

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About

Nikhil Marathe [http://nikhilism.com] started writing this book one afternoon (June 16, 2012) when he didn’t feel like programming. He had recently been stung by the lack of good documentation on libuv while working on node-taglib [https://github.com/nikhilm/node-taglib]. Although reference documentation was present, there were no comprehensive tutorials. This book is the output of that need and tries to be accurate. That said, the book may have mistakes. Pull requests are encouraged. You may also email him if you find an error.

Nikhil is indebted to Marc Lehmann’s comprehensive man page [http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod] about libev which describes much of the semantics of the two libraries.

This book was made using Sphinx [http://sphinx.pocoo.org/] and vim [http://www.vim.org].

Licensing

The contents of this book are licensed as Creative Commons - Attribution [http://creativecommons.org/licenses/by/3.0/]. All code is in the public domain.

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