Main loop

The main_loop component deals with the timing required by the main loop of the agent. It provides primitives to wait for an event and to wake up the agent when it is required.

The mechanism of this component is based in a call to poll(2) where it waits for events in different file descriptors. Some operations are not based in file descriptors (for example, data available in some queue) and for that reason it has to provide another mechanism to awaken the main loop. While a common approach would be to send a signal it doesn't work well with poll(2) and the agent uses the well-known self-pipe trick, but as the agent is executed as a Nuttx task in some configurations and Nuttx tasks don't share file descriptors (see Nuttx documentation) then a named pipe is required. By default it tries to create a temporary pipe but this location can be overridden using the EVP_AGENT_FIFO environment variable.

void main_loop_init(void)

This function initializes the pipe required by the self-pipe trick, using the one already present in the filesystem or by creating a new one if it doesn't exist. By default it uses a temporary named pipe but it can be overridden using the EVP_AGENT_FIFO environment variable. If it cannot create this, it instead prints an error message and tries to continue, but it would work in degraded mode and unexpected behaviours can happen.

void main_loop_block(void)

This function blocks the main thread waiting for an event, and after the event arrives the pending part of the main loop is executed. It uses a call to poll(2) to block until some file descriptor is ready for some operation or until the self-pipe trick is used.

void main_loop_wakeup(const char *name)

This function uses the self-pipe trick to force an exit from the call to poll(2) done in main_loop_block(). A string is received as parameter for logging purpose recording why the main loop was awoken.

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