dispatch_create_channel()

Updated: October 26, 2022

Allocate a dispatch structure, specifying a channel ID

Synopsis:

#include <sys/iofunc.h>
#include <sys/dispatch.h>

dispatch_t *dispatch_create_channel( int chid,
                                     unsigned flags );

Arguments:

chid
The ID of the channel to use for the dispatch layer, or -1 to create the channel.
flags
Zero or more of the following bits:
  • DISPATCH_FLAG_NOLOCK — (QNX Neutrino 7.1 or later) enable the lockless handling of messages (see below).

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The dispatch_create_channel() function allocates and initializes a dispatch structure, and returns a handle to this structure. This function is similar to dispatch_create() but lets you specify a channel and flags. This is useful if you want to control the channel for name_attach() or resmgr_attach(). For example, if you wanted two or more names to the same channel, or if you wanted non-default channel flags.

After you've called this function, you need to call one or more of the following functions to attach handlers for the things you want the dispatch layer to deal with:

Then you call dispatch_context_alloc() to allocate the context for dispatch_block() and dispatch_handler().

Note: Once you've called dispatch_context_alloc(), don't call message_attach() or resmgr_attach() specifying a larger maximum message size or a larger number of message parts for the same dispatch structure. In QNX Neutrino 7.0 or later, these functions indicate an error of EINVAL if this happens.

If the dispatch structure is for use in a resource manager, meaning you will later call resmgr_attach() with the associated dispatch handle, then at least the flags _NTO_CHF_UNBLOCK and _NTO_CHF_DISCONNECT should be set for the supplied channel to ensure proper resource manager behavior.

If you wish, you can do a resmgr_attach() with a NULL path. This has the effect of initializing the dispatch structure to receive messages, among other things.

Note: In order to create a public channel (i.e., without _NTO_CHF_PRIVATE set), your process must have the PROCMGR_AID_PUBLIC_CHANNEL ability enabled. For more information, see procmgr_ability().

In most resource managers, the mapping from message types to message handlers is static for the life of the resource manager, but the lookup must be done on every message. The framework locks and unlocks the mapping data on every message, but if a resource manager is going to have a static mapping, this overhead can be eliminated. Setting the DISPATCH_FLAG_NOLOCK flag tells the dispatch framework that it can safely eliminate those protections. This means that once the dispatch framework becomes active (that is, once at least one call to dispatch_context_alloc() has been made with the dispatch framework), the mapping can't be changed. This has the restrictions that for an active dispatch framework (dpp) you may not call:

The dispatch_create_channel() function is part of the dispatch layer of a resource manager. For more information, see Layers in a resource manager in the “The Bones of a Resource Manager” chapter of Writing a Resource Manager.

Returns:

A handle to a dispatch structure, or NULL if an error occurred (errno is set). The dispatch_t structure is an opaque data type; you can't access its contents directly.

Errors:

EINVAL
(QNX Neutrino 7.0 or later) The flags argument is invalid.
ENOMEM
Insufficient memory to allocate context.

Classification:

QNX Neutrino

Safety:  
Cancellation point No
Interrupt handler No
Signal handler No
Thread Yes