GDatagramBased: GIO Reference Manual

## GDatagramBased GDatagramBased — Low-level datagram communications interface |

## Functions

gboolean | (*GDatagramBasedSourceFunc) ()
gint | g_datagram_based_receive_messages ()
gint | g_datagram_based_send_messages ()
GSource * | g_datagram_based_create_source ()
GIOCondition | g_datagram_based_condition_check ()
gboolean | g_datagram_based_condition_wait ()

## Types and Values

| GDatagramBased
struct | GDatagramBasedInterface

## Object Hierarchy

        GInterface
        ╰── GDatagramBased

## Prerequisites

GDatagramBased requires GObject.

## Known Derived Interfaces

GDatagramBased is required by GDtlsClientConnection, GDtlsConnection and GDtlsServerConnection.

## Known Implementations

GDatagramBased is implemented by GSocket.

## Includes

    #include <gio/gio.h>

## Description

A GDatagramBased is a networking interface for representing datagram-based communications. It is a more or less direct mapping of the core parts of the BSD socket API in a portable GObject interface. It is implemented by GSocket, which wraps the UNIX socket API on UNIX and winsock2 on Windows.

GDatagramBased is entirely platform independent, and is intended to be used alongside higher-level networking APIs such as GIOStream.

It uses vectored scatter/gather I/O by default, allowing for many messages to be sent or received in a single call. Where possible, implementations of the interface should take advantage of vectored I/O to minimise processing or system calls. For example, GSocket uses `recvmmsg()` and `sendmmsg()` where possible. Callers should take advantage of scatter/gather I/O (the use of multiple buffers per message) to avoid unnecessary copying of data to assemble or disassemble a message.

Each GDatagramBased operation has a timeout parameter which may be negative for blocking behaviour, zero for non-blocking behaviour, or positive for timeout behaviour. A blocking operation blocks until finished or there is an error. A non-blocking operation will return immediately with a `G_IO_ERROR_WOULD_BLOCK` error if it cannot make progress. A timeout operation will block until the operation is complete or the timeout expires; if the timeout expires it will return what progress it made, or `G_IO_ERROR_TIMED_OUT` if no progress was made. To know when a call would successfully run you can call `g_datagram_based_condition_check()` or `g_datagram_based_condition_wait()`. You can also use `g_datagram_based_create_source()` and attach it to a GMainContext to get callbacks when I/O is possible.

When running a non-blocking operation applications should always be able to handle getting a `G_IO_ERROR_WOULD_BLOCK` error even when some other function said that I/O was possible. This can easily happen in case of a race condition in the application, but it can also happen for other reasons. For instance, on Windows a socket is always seen as writable until a write returns `G_IO_ERROR_WOULD_BLOCK`.

As with GSocket, GDatagramBaseds can be either connection oriented (for example, SCTP) or connectionless (for example, UDP). GDatagramBaseds must be datagram-based, not stream-based. The interface does not cover connection establishment — use methods on the underlying type to establish a connection before sending and receiving data through the GDatagramBased API. For connectionless socket types the target/source address is specified or received in each I/O operation.

Like most other APIs in GLib, GDatagramBased is not inherently thread safe. To use a GDatagramBased concurrently from multiple threads, you must implement your own locking.

## Functions

### GDatagramBasedSourceFunc ()

    gboolean
    (*GDatagramBasedSourceFunc) (GDatagramBased *datagram_based,
                                 GIOCondition condition,
                                 gpointer user_data);

This is the function type of the callback used for the GSource returned by `g_datagram_based_create_source()`.

#### Parameters

datagram_based | the GDatagramBased |
condition | the current condition at the source fired |
user_data | data passed in by the user |

#### Returns

`G_SOURCE_REMOVE` if the source should be removed, `G_SOURCE_CONTINUE` otherwise

Since: 2.48

### g_datagram_based_receive_messages ()

    gint
    g_datagram_based_receive_messages (GDatagramBased *datagram_based,
                                       GInputMessage *messages,
                                       guint num_messages,
                                       gint flags,
                                       gint64 timeout,
                                       GCancellable *cancellable,
                                       GError **error);

Receive one or more data messages from `datagram_based` in one go.

`messages` must point to an array of GInputMessage structs and `num_messages` must be the length of this array. Each GInputMessage contains a pointer to an array of GInputVector structs describing the buffers that the data received in each message will be written to.

`flags` modify how all messages are received. The commonly available arguments for this are available in the GSocketMsgFlags enum, but the values there are the same as the system values, and the flags are passed in as-is, so you can pass in system-specific flags too. These flags affect the overall receive operation. Flags affecting individual messages are returned in GInputMessage.flags.

The other members of GInputMessage are treated as described in its documentation.

If `timeout` is negative the call will block until `num_messages` have been received, the connection is closed remotely (EOS), `cancellable` is cancelled, or an error occurs.

If `timeout` is 0 the call will return up to `num_messages` without blocking, or `G_IO_ERROR_WOULD_BLOCK` if no messages are queued in the operating system to be received.

If `timeout` is positive the call will block on the same conditions as if `timeout` were negative. If the timeout is reached before any messages are received, `G_IO_ERROR_TIMED_OUT` is returned, otherwise it will return the number of messages received before timing out. (Note: This is effectively the behaviour of `MSG_WAITFORONE` with `recvmmsg()`.)

To be notified when messages are available, wait for the `G_IO_IN` condition. Note though that you may still receive `G_IO_ERROR_WOULD_BLOCK` from `g_datagram_based_receive_messages()` even if you were previously notified of a `G_IO_IN` condition.

If the remote peer closes the connection, any messages queued in the underlying receive buffer will be returned, and subsequent calls to `g_datagram_based_receive_messages()` will return 0 (with no error set).

If the connection is shut down or closed (by calling `g_socket_close()` or `g_socket_shutdown()` with `shutdown_read` set, if it’s a GSocket, for example), all calls to this function will return `G_IO_ERROR_CLOSED`.

On error -1 is returned and `error` is set accordingly. An error will only be returned if zero messages could be received; otherwise the number of messages successfully received before the error will be returned. If `cancellable` is cancelled, `G_IO_ERROR_CANCELLED` is returned as with any other error.

#### Parameters

datagram_based | a GDatagramBased |
messages | an array of GInputMessage structs. | [array length=num_messages]
num_messages | the number of elements in `messages` |
flags | an int containing GSocketMsgFlags flags for the overall operation |
timeout | the maximum time (in microseconds) to wait, 0 to not block, or -1 to block indefinitely |
cancellable | a `GCancellable`. | [nullable]
error | return location for a GError |

#### Returns

number of messages received, or -1 on error. Note that the number of messages received may be smaller than `num_messages` if `timeout` is zero or positive, if the peer closed the connection, or if `num_messages` was larger than `UIO_MAXIOV` (1024), in which case the caller may re-try to receive the remaining messages.

Since: 2.48

### g_datagram_based_send_messages ()

    gint
    g_datagram_based_send_messages (GDatagramBased *datagram_based,
                                    GOutputMessage *messages,
                                    guint num_messages,
                                    gint flags,
                                    gint64 timeout,
                                    GCancellable *cancellable,
                                    GError **error);

Send one or more data messages from `datagram_based` in one go.

`messages` must point to an array of GOutputMessage structs and `num_messages` must be the length of this array. Each GOutputMessage contains an address to send the data to, and a pointer to an array of GOutputVector structs to describe the buffers that the data to be sent for each message will be gathered from.

`flags` modify how the message is sent. The commonly available arguments for this are available in the GSocketMsgFlags enum, but the values there are the same as the system values, and the flags are passed in as-is, so you can pass in system-specific flags too.

The other members of GOutputMessage are treated as described in its documentation.

If `timeout` is negative the call will block until `num_messages` have been sent, `cancellable` is cancelled, or an error occurs.

If `timeout` is 0 the call will send up to `num_messages` without blocking, or will return `G_IO_ERROR_WOULD_BLOCK` if there is no space to send messages.

If `timeout` is positive the call will block on the same conditions as if `timeout` were negative. If the timeout is reached before any messages are sent, `G_IO_ERROR_TIMED_OUT` is returned, otherwise it will return the number of messages sent before timing out.

To be notified when messages can be sent, wait for the `G_IO_OUT` condition. Note though that you may still receive `G_IO_ERROR_WOULD_BLOCK` from `g_datagram_based_send_messages()` even if you were previously notified of a `G_IO_OUT` condition. (On Windows in particular, this is very common due to the way the underlying APIs work.)

If the connection is shut down or closed (by calling `g_socket_close()` or `g_socket_shutdown()` with `shutdown_write` set, if it’s a GSocket, for example), all calls to this function will return `G_IO_ERROR_CLOSED`.

On error -1 is returned and `error` is set accordingly. An error will only be returned if zero messages could be sent; otherwise the number of messages successfully sent before the error will be returned. If `cancellable` is cancelled, `G_IO_ERROR_CANCELLED` is returned as with any other error.

#### Parameters

datagram_based | a GDatagramBased |
messages | an array of GOutputMessage structs. | [array length=num_messages]
num_messages | the number of elements in `messages` |
flags | an int containing GSocketMsgFlags flags |
timeout | the maximum time (in microseconds) to wait, 0 to not block, or -1 to block indefinitely |
cancellable | a `GCancellable`. | [nullable]
error | return location for a GError |

#### Returns

number of messages sent, or -1 on error. Note that the number of messages sent may be smaller than `num_messages` if `timeout` is zero or positive, or if `num_messages` was larger than `UIO_MAXIOV` (1024), in which case the caller may re-try to send the remaining messages.

Since: 2.48

### g_datagram_based_create_source ()

    GSource *
    g_datagram_based_create_source (GDatagramBased *datagram_based,
                                    GIOCondition condition,
                                    GCancellable *cancellable);

Creates a GSource that can be attached to a GMainContext to monitor for the availability of the specified `condition` on the GDatagramBased. The GSource keeps a reference to the `datagram_based` .

The callback on the source is of the GDatagramBasedSourceFunc type.

It is meaningless to specify `G_IO_ERR` or `G_IO_HUP` in `condition` ; these conditions will always be reported in the callback if they are true.

If non-`NULL`, `cancellable` can be used to cancel the source, which will cause the source to trigger, reporting the current condition (which is likely 0 unless cancellation happened at the same time as a condition change). You can check for this in the callback using `g_cancellable_is_cancelled()`.

#### Parameters

datagram_based | a GDatagramBased |
condition | a GIOCondition mask to monitor |
cancellable | a GCancellable. | [nullable]

#### Returns

a newly allocated GSource.

[transfer full]

Since: 2.48

### g_datagram_based_condition_check ()

    GIOCondition
    g_datagram_based_condition_check (GDatagramBased *datagram_based,
                                      GIOCondition condition);

Checks on the readiness of `datagram_based` to perform operations. The operations specified in `condition` are checked for and masked against the currently-satisfied conditions on `datagram_based` . The result is returned.

`G_IO_IN` will be set in the return value if data is available to read with `g_datagram_based_receive_messages()`, or if the connection is closed remotely (EOS); and if the datagram_based has not been closed locally using some implementation-specific method (such as `g_socket_close()` or `g_socket_shutdown()` with `shutdown_read` set, if it’s a GSocket).

If the connection is shut down or closed (by calling `g_socket_close()` or `g_socket_shutdown()` with `shutdown_read` set, if it’s a GSocket, for example), all calls to this function will return `G_IO_ERROR_CLOSED`.

`G_IO_OUT` will be set if it is expected that at least one byte can be sent using `g_datagram_based_send_messages()` without blocking. It will not be set if the datagram_based has been closed locally.

`G_IO_HUP` will be set if the connection has been closed locally.

`G_IO_ERR` will be set if there was an asynchronous error in transmitting data previously enqueued using `g_datagram_based_send_messages()`.

Note that on Windows, it is possible for an operation to return `G_IO_ERROR_WOULD_BLOCK` even immediately after `g_datagram_based_condition_check()` has claimed that the GDatagramBased is ready for writing. Rather than calling `g_datagram_based_condition_check()` and then writing to the GDatagramBased if it succeeds, it is generally better to simply try writing right away, and try again later if the initial attempt returns `G_IO_ERROR_WOULD_BLOCK`.

It is meaningless to specify `G_IO_ERR` or `G_IO_HUP` in `condition` ; these conditions will always be set in the output if they are true. Apart from these flags, the output is guaranteed to be masked by `condition` .

This call never blocks.

#### Parameters

datagram_based | a GDatagramBased |
condition | a GIOCondition mask to check |

#### Returns

the GIOCondition mask of the current state

Since: 2.48

### g_datagram_based_condition_wait ()

    gboolean
    g_datagram_based_condition_wait (GDatagramBased *datagram_based,
                                     GIOCondition condition,
                                     gint64 timeout,
                                     GCancellable *cancellable,
                                     GError **error);

Waits for up to `timeout` microseconds for condition to become true on `datagram_based` . If the condition is met, `TRUE` is returned.

If `cancellable` is cancelled before the condition is met, or if `timeout` is reached before the condition is met, then `FALSE` is returned and `error` is set appropriately (`G_IO_ERROR_CANCELLED` or `G_IO_ERROR_TIMED_OUT`).

#### Parameters

datagram_based | a GDatagramBased |
condition | a GIOCondition mask to wait for |
timeout | the maximum time (in microseconds) to wait, 0 to not block, or -1 to block indefinitely |
cancellable | a GCancellable. | [nullable]
error | return location for a GError |

#### Returns

`TRUE` if the condition was met, `FALSE` otherwise

Since: 2.48

## Types and Values

### GDatagramBased

    typedef struct _GDatagramBased GDatagramBased;

Interface for socket-like objects with datagram semantics.

Since: 2.48

### struct GDatagramBasedInterface

    struct GDatagramBasedInterface {
      GTypeInterface g_iface;

      /* Virtual table */
      gint          (*receive_messages)     (GDatagramBased       *datagram_based,
                                             GInputMessage        *messages,
                                             guint                 num_messages,
                                             gint                  flags,
                                             gint64                timeout,
                                             GCancellable         *cancellable,
                                             GError              **error);
      gint          (*send_messages)        (GDatagramBased       *datagram_based,
                                             GOutputMessage       *messages,
                                             guint                 num_messages,
                                             gint                  flags,
                                             gint64                timeout,
                                             GCancellable         *cancellable,
                                             GError              **error);

      GSource      *(*create_source)        (GDatagramBased       *datagram_based,
                                             GIOCondition          condition,
                                             GCancellable         *cancellable);
      GIOCondition  (*condition_check)      (GDatagramBased       *datagram_based,
                                             GIOCondition          condition);
      gboolean      (*condition_wait)       (GDatagramBased       *datagram_based,
                                             GIOCondition          condition,
                                             gint64                timeout,
                                             GCancellable         *cancellable,
                                             GError              **error);
    };

Provides an interface for socket-like objects which have datagram semantics, following the Berkeley sockets API. The interface methods are thin wrappers around the corresponding virtual methods, and no pre-processing of inputs is implemented — so implementations of this API must handle all functionality documented in the interface methods.

#### Members

`receive_messages` () | Virtual method for `g_datagram_based_receive_messages()`. |
`send_messages` () | Virtual method for `g_datagram_based_send_messages()`. |
`create_source` () | Virtual method for `g_datagram_based_create_source()`. |
`condition_check` () | Virtual method for `g_datagram_based_condition_check()`. |
`condition_wait` () | Virtual method for `g_datagram_based_condition_wait()`. |

Since: 2.48

## See Also

GSocket, <gnetworking.h>