_LWP_PARK(2) | System Calls Manual | _LWP_PARK(2)

# NAME

`_lwp_park` —
wait interruptably in the kernel

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <lwp.h>`

int
`_lwp_park`(clockid_t clock_id, int flags, struct timespec *ts, lwpid_t unpark, const void *hint, const void *unparkhint);

# DESCRIPTION

`_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:

- The ts argument is non-`NULL`, and the time it specifies has passed. The ts time can be an relative interval to wait if the flags argument does not contain `TIMER_ABSTIME` or it can be an absolute time. The clock_id argument contains the clock to be used; it can be: `CLOCK_REALTIME` or `CLOCK_MONOTONIC`.
- The LWP receives a directed signal posted using `_lwp_kill`(), or is elected to handle a signal on behalf of its containing process.
- The LWP is awoken by another LWP in the same process that has made a call to `_lwp_wakeup`().
- The LWP is awoken by another LWP in the same process that has made a call to `_lwp_unpark`() or `_lwp_unpark_all`().

If the ts argument contains a relative time interval, it will be modified to contain the remaining time to sleep when `_lwp_park`() returns.

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 system call. If unpark is non-zero, the system will behave as if the following call had been made before the calling thread begins to wait:

    	_lwp_unpark(unpark, unparkhint);

# RETURN VALUES

`_lwp_park`() may return a value of 0. Otherwise, -1 is returned and errno is set to provide more information.

# ERRORS

[`EALREADY`]
  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`(), `_lwp_unpark_all`(), `_lwp_wakeup`().
[`EINVAL`]
  The time value specified by ts is invalid.
[`ESRCH`]
  No LWP can be found in the current process corresponding to unpark.
[`ETIMEDOUT`]
  The UTC time specified by ts has passed.

# SEE ALSO

_lwp_unpark(2), _lwp_unpark_all(2), _lwp_wakeup(2)

# HISTORY

The `_lwp_park`() system call first appeared in NetBSD 5.0.

December 7, 2017 | NetBSD 9.5