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update docs about possibility of rcl_take no taking (#356)

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* update rcl_wait doc with respect to subs and possibility of failing takes

* add a note about possible failing takes in rcl_take docs
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William Woodall 2018-12-12 15:28:25 -08:00 • committed by GitHub
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5
rcl/include/rcl/subscription.h
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@ -217,6 +217,11 @@ rcl_subscription_get_default_options(void);
* if one is available.
* If taken is false after calling, then the ROS message will be unmodified.
*
* The taken boolean may be false even if a wait set reports that the
* subscription was ready to be taken from in some cases, e.g. when the
* state of the subscription changes it may cause the wait set to wake up
* but subsequent takes to fail to take anything.
*
* If allocation is required when taking the message, e.g. if space needs to
* be allocated for a dynamically sized array in the target message, then the
* allocator given in the subscription options is used.

4
rcl/include/rcl/wait.h
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@ -349,7 +349,9 @@ rcl_wait_set_add_service(
* this function returns.
* Items that are not `NULL` are ready, where ready means different things based
* on the type of the item.
* For subscriptions this means there are messages that can be taken.
* For subscriptions this means there may be messages that can be taken, or
* perhaps that the state of the subscriptions has changed, in which case
* rcl_take may succeed but return with taken == false.
* For guard conditions this means the guard condition was triggered.
*
* Expected usage: