# `uv_handle_t` — Base handle

uv_handle_t is the base type for all libuv handle types.

Structures are aligned so that any libuv handle can be cast to uv_handle_t. All API functions defined here work with any handle type.

Libuv handles are not movable. Pointers to handle structures passed to functions must remain valid for the duration of the requested operation. Take care when using stack allocated handles.

## Data types

type uv_handle_t

  The base libuv handle type.

enum uv_handle_type

  The kind of the libuv handle.

      typedef enum {
        UV_UNKNOWN_HANDLE = 0,
        UV_ASYNC,
        UV_CHECK,
        UV_FS_EVENT,
        UV_FS_POLL,
        UV_HANDLE,
        UV_IDLE,
        UV_NAMED_PIPE,
        UV_POLL,
        UV_PREPARE,
        UV_PROCESS,
        UV_STREAM,
        UV_TCP,
        UV_TIMER,
        UV_TTY,
        UV_UDP,
        UV_SIGNAL,
        UV_FILE,
        UV_HANDLE_TYPE_MAX
      } uv_handle_type;

type uv_any_handle

  Union of all handle types.

typedef void (*uv_alloc_cb)(uv_handle_t *handle, size_t suggested_size, uv_buf_t *buf)

  Type definition for callback passed to `uv_read_start()` and `uv_udp_recv_start()`. The user must allocate memory and fill the supplied `uv_buf_t` structure. If NULL is assigned as the buffer’s base or 0 as its length, a `UV_ENOBUFS` error will be triggered in the `uv_udp_recv_cb` or the `uv_read_cb` callback.

  Each buffer is used only once and the user is responsible for freeing it in the `uv_udp_recv_cb` or the `uv_read_cb` callback.

  A suggested size (65536 at the moment in most cases) is provided, but it’s just an indication, not related in any way to the pending data to be read. The user is free to allocate the amount of memory they decide.

  As an example, applications with custom allocation schemes such as using freelists, allocation pools or slab based allocators may decide to use a different size which matches the memory chunks they already have.

  Example:

      static void my_alloc_cb(uv_handle_t* handle, size_t suggested_size, uv_buf_t* buf) {
        buf->base = malloc(suggested_size);
        buf->len = suggested_size;
      }

typedef void (*uv_close_cb)(uv_handle_t *handle)

  Type definition for callback passed to `uv_close()`.

### Public members

uv_loop_t *uv_handle_t.loop

  Pointer to the `uv_loop_t` the handle is running on. Readonly.

uv_handle_type uv_handle_t.type

  The `uv_handle_type`, indicating the type of the underlying handle. Readonly.

void *uv_handle_t.data

  Space for user-defined arbitrary data. libuv does not use this field.

## API

UV_HANDLE_TYPE_MAP(iter_macro)

  Macro that expands to a series of invocations of iter_macro for each of the handle types. iter_macro is invoked with two arguments: the name of the uv_handle_type element without the UV_ prefix, and the name of the corresponding structure type without the uv_ prefix and _t suffix.

int uv_is_active(const uv_handle_t *handle)

  Returns non-zero if the handle is active, zero if it’s inactive. What “active” means depends on the type of handle:

  - A uv_async_t handle is always active and cannot be deactivated, except by closing it with uv_close().

  - A uv_pipe_t, uv_tcp_t, uv_udp_t, etc. handle - basically any handle that deals with i/o - is active when it is doing something that involves i/o, like reading, writing, connecting, accepting new connections, etc.

  - A uv_check_t, uv_idle_t, uv_timer_t, etc. handle is active when it has been started with a call to uv_check_start(), uv_idle_start(), etc.

  Rule of thumb: if a handle of type uv_foo_t has a uv_foo_start() function, then it’s active from the moment that function is called. Likewise, uv_foo_stop() deactivates the handle again.

int uv_is_closing(const uv_handle_t *handle)

  Returns non-zero if the handle is closing or closed, zero otherwise.

  Note

  This function should only be used between the initialization of the handle and the arrival of the close callback.

void uv_close(uv_handle_t *handle, uv_close_cb close_cb)

  Request handle to be closed. close_cb will be called asynchronously after this call. This MUST be called on each handle before memory is released. Moreover, the memory can only be released in close_cb or after it has returned.

  Handles that wrap file descriptors are closed immediately but close_cb will still be deferred to the next iteration of the event loop. It gives you a chance to free up any resources associated with the handle.

  In-progress requests, like uv_connect_t or uv_write_t, are cancelled and have their callbacks called asynchronously with status=UV_ECANCELED.

void uv_ref(uv_handle_t *handle)

  Reference the given handle. References are idempotent, that is, if a handle is already referenced calling this function again will have no effect.

  See Reference counting.

void uv_unref(uv_handle_t *handle)

  Un-reference the given handle. References are idempotent, that is, if a handle is not referenced calling this function again will have no effect.

  See Reference counting.

int uv_has_ref(const uv_handle_t *handle)

  Returns non-zero if the handle referenced, zero otherwise.

  See Reference counting.

size_t uv_handle_size(uv_handle_type type)

  Returns the size of the given handle type. Useful for FFI binding writers who don’t want to know the structure layout.

## Miscellaneous API functions

The following API functions take a `uv_handle_t` argument but they work just for some handle types.

int uv_send_buffer_size(uv_handle_t *handle, int *value)

  Gets or sets the size of the send buffer that the operating system uses for the socket.

  If *value == 0, then it will set *value to the current send buffer size. If *value > 0 then it will use *value to set the new send buffer size.

  On success, zero is returned. On error, a negative result is returned.

  This function works for TCP, pipe and UDP handles on Unix and for TCP and UDP handles on Windows.

  Note

  Linux will set double the size and return double the size of the original set value.

int uv_recv_buffer_size(uv_handle_t *handle, int *value)

  Gets or sets the size of the receive buffer that the operating system uses for the socket.

  If *value == 0, then it will set *value to the current receive buffer size. If *value > 0 then it will use *value to set the new receive buffer size.

  On success, zero is returned. On error, a negative result is returned.

  This function works for TCP, pipe and UDP handles on Unix and for TCP and UDP handles on Windows.

  Note

  Linux will set double the size and return double the size of the original set value.

int uv_fileno(const uv_handle_t *handle, uv_os_fd_t *fd)

  Gets the platform dependent file descriptor equivalent.

  The following handles are supported: TCP, pipes, TTY, UDP and poll. Passing any other handle type will fail with UV_EINVAL.

  If a handle doesn’t have an attached file descriptor yet or the handle itself has been closed, this function will return UV_EBADF.

  Warning

  Be very careful when using this function. libuv assumes it’s in control of the file descriptor so any change to it may lead to malfunction.

uv_loop_t *uv_handle_get_loop(const uv_handle_t *handle)

  Returns handle->loop.

  New in version 1.19.0.

void *uv_handle_get_data(const uv_handle_t *handle)

  Returns handle->data.

  New in version 1.19.0.

void *uv_handle_set_data(uv_handle_t *handle, void *data)

  Sets handle->data to data.

  New in version 1.19.0.

uv_handle_type uv_handle_get_type(const uv_handle_t *handle)

  Returns handle->type.

  New in version 1.19.0.

const char *uv_handle_type_name(uv_handle_type type)

  Returns the name for the equivalent struct for a given handle type, e.g. “pipe” (as in `uv_pipe_t`) for UV_NAMED_PIPE.

  If no such handle type exists, this returns NULL.

  New in version 1.19.0.

## Reference counting

The libuv event loop (if run in the default mode) will run until there are no active and referenced handles left. The user can force the loop to exit early by unreferencing handles which are active, for example by calling `uv_unref()` after calling `uv_timer_start()`.

A handle can be referenced or unreferenced, the refcounting scheme doesn’t use a counter, so both operations are idempotent.

All handles are referenced when active by default, see `uv_is_active()` for a more detailed explanation on what being active involves.