---
name: "wakeup_one"
section: "9"
platform: "linux"
package: "freebsd-manpages"
version: "6.0-5"
release: "Debian 4.0 (Etch)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/wakeup_one"
---

# wakeup\_one(9)

wait for events

## NAME

**sleep**, **msleep**, **tsleep**, **wakeup** – wait for events

## SYNOPSIS

**#include \<sys/param.h\>**

**#include \<sys/systm.h\>**

**#include \<sys/proc.h\>**

*int*\

**tsleep**(*void \*ident*, *int priority*, *const char \*wmesg*, *int timo*);

*int*\

**msleep**(*void \*ident*, *struct mtx \*mtx*, *int priority*, *const char \*wmesg*, *int timo*);

*void*\

**wakeup**(*void \*ident*);

*void*\

**wakeup\_one**(*void \*ident*);

## DESCRIPTION

The functions **tsleep**() and **wakeup**() handle event-based process blocking. If a process must wait for an external event, it is put on sleep by **tsleep**(). The parameter *ident* is an arbitrary address that uniquely identifies the event on which the process is being asleep. All processes sleeping on a single *ident* are woken up later by **wakeup**(), often called from inside an interrupt routine, to indicate that the resource the process was blocking on is available now.

The parameter *wmesg* is a string describing the sleep condition for tools like [ps(1)](/ps/1.md). Due to the limited space of those programs to display arbitrary strings, this message should not be longer than 6 characters.

The **wakeup\_one**() function is used to make the first process in the queue that is sleeping on the parameter *ident* runnable. This can prevent the system from becoming saturated when a large number of processes are sleeping on the same address, but only one of them can actually do any useful work when made runnable.

The **tsleep**() function is the general sleep call. Suspends the current process until a wakeup is performed on the specified identifier. The process will then be made runnable with the specified *priority*. Sleeps at most *timo* / hz seconds (0 means no timeout). If the *Giant* lock is not held, then *timo* must be non-zero. If *priority* includes the PCATCH flag, signals are checked before and after sleeping, else signals are not checked. Returns 0 if awakened, EWOULDBLOCK if the timeout expires. If PCATCH is set and a signal needs to be delivered, ERESTART is returned if the current system call should be restarted if possible, and EINTR is returned if the system call should be interrupted by the signal (return EINTR).

The **msleep**() function is a variation on tsleep. The parameter *mtx* is a mutex which will be released before sleeping and reacquired before **msleep**() returns. If *priority* includes the PDROP flag, the *mtx* parameter will not be reacquired before returning. The mutex is used to ensure that a condition can be checked atomically, and that the current process can be suspended without missing a change to the condition, or an associated wakeup.

## RETURN VALUES

See above.

## SEE ALSO

[ps(1)](/ps/1.md), [malloc(9)](/malloc/9.md), [mi\_switch(9)](/mi_switch/9.md)

## HISTORY

The sleep/wakeup process synchronization mechanism is very old. It appeared in a very early version of UNIX.

The **tsleep**() function appeared in 4.4BSD.

The **sleep**() function used to be the traditional form. It did not let you specify a timeout or a *wmesg*, hence it was discontinued.

## AUTHORS

This manual page was written by Jörg Wunsch ⟨joerg@FreeBSD.org⟩.