# 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 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. 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

     1void on_open(uv_fs_t *req) {
     2    // The request passed to the callback is the same as the one the call setup
     3    // function was passed.
     4    assert(req == &open_req);
     5    if (req->result >= 0) {
     6        iov = uv_buf_init(buffer, sizeof(buffer));
     7        uv_fs_read(uv_default_loop(), &read_req, req->result,
     8                   &iov, 1, -1, on_read);
     9    }
    10    else {
    11        fprintf(stderr, "error opening file: %s\n", uv_strerror((int)req->result));
    12    }
    13}

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

     1void on_read(uv_fs_t *req) {
     2    if (req->result < 0) {
     3        fprintf(stderr, "Read error: %s\n", uv_strerror(req->result));
     4    }
     5    else if (req->result == 0) {
     6        uv_fs_t close_req;
     7        // synchronous
     8        uv_fs_close(uv_default_loop(), &close_req, open_req.result, NULL);
     9    }
    10    else if (req->result > 0) {
    11        iov.len = req->result;
    12        uv_fs_write(uv_default_loop(), &write_req, 1, &iov, 1, -1, on_write);
    13    }
    14}
    15

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

    1
    2void on_write(uv_fs_t *req) {
    3    if (req->result < 0) {
    4        fprintf(stderr, "Write error: %s\n", uv_strerror((int)req->result));
    5    }
    6    else {
    7        uv_fs_read(uv_default_loop(), &read_req, open_req.result, &iov, 1, -1, on_read);
    8    }
    9}

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

    1int main(int argc, char **argv) {
    2    uv_fs_open(uv_default_loop(), &open_req, argv[1], O_RDONLY, 0, on_open);
    3    uv_run(uv_default_loop(), UV_RUN_DEFAULT);
    4
    5    uv_fs_req_cleanup(&open_req);
    6    uv_fs_req_cleanup(&read_req);
    7    uv_fs_req_cleanup(&write_req);
    8    return 0;
    9}

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

    int uv_fs_close(uv_loop_t* loop, uv_fs_t* req, uv_file file, uv_fs_cb cb);
    int uv_fs_open(uv_loop_t* loop, uv_fs_t* req, const char* path, int flags, int mode, uv_fs_cb cb);
    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);
    int uv_fs_unlink(uv_loop_t* loop, uv_fs_t* req, const char* path, uv_fs_cb cb);
    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);
    int uv_fs_copyfile(uv_loop_t* loop, uv_fs_t* req, const char* path, const char* new_path, int flags, uv_fs_cb cb);
    int uv_fs_mkdir(uv_loop_t* loop, uv_fs_t* req, const char* path, int mode, uv_fs_cb cb);
    int uv_fs_mkdtemp(uv_loop_t* loop, uv_fs_t* req, const char* tpl, uv_fs_cb cb);
    int uv_fs_rmdir(uv_loop_t* loop, uv_fs_t* req, const char* path, uv_fs_cb cb);
    int uv_fs_scandir(uv_loop_t* loop, uv_fs_t* req, const char* path, int flags, uv_fs_cb cb);
    int uv_fs_scandir_next(uv_fs_t* req, uv_dirent_t* ent);
    int uv_fs_opendir(uv_loop_t* loop, uv_fs_t* req, const char* path, uv_fs_cb cb);
    int uv_fs_readdir(uv_loop_t* loop, uv_fs_t* req, uv_dir_t* dir, uv_fs_cb cb);
    int uv_fs_closedir(uv_loop_t* loop, uv_fs_t* req, uv_dir_t* dir, uv_fs_cb cb);
    int uv_fs_stat(uv_loop_t* loop, uv_fs_t* req, const char* path, uv_fs_cb cb);
    int uv_fs_fstat(uv_loop_t* loop, uv_fs_t* req, uv_file file, uv_fs_cb cb);
    int uv_fs_rename(uv_loop_t* loop, uv_fs_t* req, const char* path, const char* new_path, uv_fs_cb cb);
    int uv_fs_fsync(uv_loop_t* loop, uv_fs_t* req, uv_file file, uv_fs_cb cb);
    int uv_fs_fdatasync(uv_loop_t* loop, uv_fs_t* req, uv_file file, uv_fs_cb cb);
    int uv_fs_ftruncate(uv_loop_t* loop, uv_fs_t* req, uv_file file, int64_t offset, uv_fs_cb cb);
    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);
    int uv_fs_access(uv_loop_t* loop, uv_fs_t* req, const char* path, int mode, uv_fs_cb cb);
    int uv_fs_chmod(uv_loop_t* loop, uv_fs_t* req, const char* path, int mode, uv_fs_cb cb);
    int uv_fs_utime(uv_loop_t* loop, uv_fs_t* req, const char* path, double atime, double mtime, uv_fs_cb cb);
    int uv_fs_futime(uv_loop_t* loop, uv_fs_t* req, uv_file file, double atime, double mtime, uv_fs_cb cb);
    int uv_fs_lstat(uv_loop_t* loop, uv_fs_t* req, const char* path, uv_fs_cb cb);
    int uv_fs_link(uv_loop_t* loop, uv_fs_t* req, const char* path, const char* new_path, uv_fs_cb cb);
    int uv_fs_symlink(uv_loop_t* loop, uv_fs_t* req, const char* path, const char* new_path, int flags, uv_fs_cb cb);
    int uv_fs_readlink(uv_loop_t* loop, uv_fs_t* req, const char* path, uv_fs_cb cb);
    int uv_fs_realpath(uv_loop_t* loop, uv_fs_t* req, const char* path, uv_fs_cb cb);
    int uv_fs_fchmod(uv_loop_t* loop, uv_fs_t* req, uv_file file, int mode, uv_fs_cb cb);
    int uv_fs_chown(uv_loop_t* loop, uv_fs_t* req, const char* path, uv_uid_t uid, uv_gid_t gid, uv_fs_cb cb);
    int uv_fs_fchown(uv_loop_t* loop, uv_fs_t* req, uv_file file, uv_uid_t uid, uv_gid_t gid, uv_fs_cb cb);
    int uv_fs_lchown(uv_loop_t* loop, uv_fs_t* req, const char* path, uv_uid_t uid, uv_gid_t gid, 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

     1
     2int main(int argc, char **argv) {
     3    loop = uv_default_loop();
     4
     5    uv_pipe_init(loop, &stdin_pipe, 0);
     6    uv_pipe_open(&stdin_pipe, 0);
     7
     8    uv_pipe_init(loop, &stdout_pipe, 0);
     9    uv_pipe_open(&stdout_pipe, 1);
    10
    11    uv_fs_t file_req;
    12    int fd = uv_fs_open(loop, &file_req, argv[1], O_CREAT | O_RDWR, 0644, NULL);
    13    uv_pipe_init(loop, &file_pipe, 0);
    14    uv_pipe_open(&file_pipe, fd);
    15
    16    uv_read_start((uv_stream_t*)&stdin_pipe, alloc_buffer, read_stdin);
    17
    18    uv_run(loop, UV_RUN_DEFAULT);
    19    return 0;
    20}

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

     1void alloc_buffer(uv_handle_t *handle, size_t suggested_size, uv_buf_t *buf) {
     2    *buf = uv_buf_init((char*) malloc(suggested_size), suggested_size);
     3}
     4
     5void read_stdin(uv_stream_t *stream, ssize_t nread, const uv_buf_t *buf) {
     6    if (nread < 0){
     7        if (nread == UV_EOF){
     8            // end of file
     9            uv_close((uv_handle_t *)&stdin_pipe, NULL);
    10            uv_close((uv_handle_t *)&stdout_pipe, NULL);
    11            uv_close((uv_handle_t *)&file_pipe, NULL);
    12        }
    13    } else if (nread > 0) {
    14        write_data((uv_stream_t *)&stdout_pipe, nread, *buf, on_stdout_write);
    15        write_data((uv_stream_t *)&file_pipe, nread, *buf, on_file_write);
    16    }
    17
    18    // OK to free buffer as write_data copies it.
    19    if (buf->base)
    20        free(buf->base);
    21}

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

     1typedef struct {
     2    uv_write_t req;
     3    uv_buf_t buf;
     4} write_req_t;
     5
     6void free_write_req(uv_write_t *req) {
     7    write_req_t *wr = (write_req_t*) req;
     8    free(wr->buf.base);
     9    free(wr);
    10}
    11
    12void on_stdout_write(uv_write_t *req, int status) {
    13    free_write_req(req);
    14}
    15
    16void on_file_write(uv_write_t *req, int status) {
    17    free_write_req(req);
    18}
    19
    20void write_data(uv_stream_t *dest, size_t size, uv_buf_t buf, uv_write_cb cb) {
    21    write_req_t *req = (write_req_t*) malloc(sizeof(write_req_t));
    22    req->buf = uv_buf_init((char*) malloc(size), size);
    23    memcpy(req->buf.base, buf.base, size);
    24    uv_write((uv_write_t*) req, (uv_stream_t*)dest, &req->buf, 1, cb);
    25}

`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. 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

     1int main(int argc, char **argv) {
     2    if (argc <= 2) {
     3        fprintf(stderr, "Usage: %s <command> <file1> [file2 ...]\n", argv[0]);
     4        return 1;
     5    }
     6
     7    loop = uv_default_loop();
     8    command = argv[1];
     9
    10    while (argc-- > 2) {
    11        fprintf(stderr, "Adding watch on %s\n", argv[argc]);
    12        uv_fs_event_t *fs_event_req = malloc(sizeof(uv_fs_event_t));
    13        uv_fs_event_init(loop, fs_event_req);
    14        // The recursive flag watches subdirectories too.
    15        uv_fs_event_start(fs_event_req, run_command, argv[argc], UV_FS_EVENT_RECURSIVE);
    16    }
    17
    18    return uv_run(loop, UV_RUN_DEFAULT);
    19}

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

    /*
    * Flags to be passed to uv_fs_event_start().
    */
    enum uv_fs_event_flags {
        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

     1void run_command(uv_fs_event_t *handle, const char *filename, int events, int status) {
     2    char path[1024];
     3    size_t size = 1023;
     4    // Does not handle error if path is longer than 1023.
     5    uv_fs_event_getpath(handle, path, &size);
     6    path[size] = '\0';
     7
     8    fprintf(stderr, "Change detected in %s: ", path);
     9    if (events & UV_RENAME)
    10        fprintf(stderr, "renamed");
    11    if (events & UV_CHANGE)
    12        fprintf(stderr, "changed");
    13
    14    fprintf(stderr, " %s\n", filename ? filename : "");
    15    system(command);
    16}

1

  inotify on Linux, FSEvents on Darwin, kqueue on BSDs, ReadDirectoryChangesW on Windows, event ports on Solaris, unsupported on Cygwin

2

  see Parent-child IPC