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69 lines
2.4 KiB
69 lines
2.4 KiB
4 months ago
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# LWS System Helpers
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Lws now has a little collection of helper utilities for common network-based
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functions necessary for normal device operation, eg, async DNS, ntpclient
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(necessary for tls validation), and DHCP client.
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## Conventions
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If any system helper is enabled for build, lws creates an additional vhost
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"system" at Context Creation time. Wsi that are created for the system
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features are bound to this. In the context object, this is available as
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`.vhost_system`.
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# Attaching to an existing context from other threads
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To simplify the case different pieces of code want to attach to a single
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lws_context at runtime, from different thread contexts, lws_system has an api
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via an lws_system operation function pointer where the other threads can use
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platform-specific locking to request callbacks to their own code from the
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lws event loop thread context safely.
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For convenience, the callback can be delayed until the system has entered or
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passed a specified system state, eg, LWS_SYSTATE_OPERATIONAL so the code will
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only get called back after the network, ntpclient and auth have been done.
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Additionally an opaque pointer can be passed to the callback when it is called
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from the lws event loop context.
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## Implementing the system-specific locking
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`lws_system_ops_t` struct has a member `.attach`
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```
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int (*attach)(struct lws_context *context, int tsi, lws_attach_cb_t *cb,
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lws_system_states_t state, void *opaque,
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struct lws_attach_item **get);
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```
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This should be defined in user code as setting locking, then passing the
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arguments through to a non-threadsafe helper
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```
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int
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__lws_system_attach(struct lws_context *context, int tsi, lws_attach_cb_t *cb,
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lws_system_states_t state, void *opaque,
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struct lws_attach_item **get);
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```
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that does the actual attach work. When it returns, the locking should be
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unlocked and the return passed back.
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## Attaching the callback request
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User code should call the lws_system_ops_t `.attach` function like
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```
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lws_system_get_ops(context)->attach(...);
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```
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The callback function which will be called from the lws event loop context
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should look like this
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```
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void my_callback(struct lws_context *context, int tsi, void *opaque);
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```
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with the callback function name passed into the (*attach)() call above. When
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the callback happens, the opaque user pointer set at the (*attach)() call is
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passed back to it as an argument.
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