_lwp_park() can be used to synchronize access to resources among multiple light-weight processes. It causes the
calling LWP to wait interruptably in the kernel, until one of the following conditions is met:
o The abstime argument is non-NULL, and the absolute UTC time it specifies has passed.
o The LWP receives a directed signal posted using _lwp_kill(), or is elected to handle a signal on behalf of
its containing process.
o The LWP is awoken by another LWP in the same process that has made a call to _lwp_wakeup().
o The LWP is awoken by another LWP in the same process that has made a call to _lwp_unpark() or
The preferred method to awaken an LWP sleeping as a result of a call to _lwp_park() is to make a call to
_lwp_unpark(), or _lwp_unpark_all(). The _lwp_wakeup() system call is a more general facility, and requires
more resources to execute.
The optional hint argument specifies the address of object upon which the LWP is synchronizing. When the hint
value is matched between calls to _lwp_park() and _lwp_unpark() or _lwp_unpark_all(), it may reduce the time
necessary for the system to resume execution of waiting LWPs.
The unpark and unparkhint arguments can be used to fold a park operation and unpark operation into a single sys-
tem call. If unpark is non-zero, the system will behave as if the following call had been made before the call-
ing thread begins to wait:
_lwp_park() may return a value of 0. Otherwise, -1 is returned and errno is set to provide more information.
A request was made to wake the LWP before it began to wait in the kernel.
[EINTR] The LWP has been awoken by a signal or by a call to one of the following functions: _lwp_unpark(),
[EINVAL] The time value specified by abstime is invalid.
[ESRCH] No LWP can be found in the current process corresponding to unpark.
The UTC time specified by abstime has passed.