SLEEP(9)

名称
sleep, tsleep, wakeup - プロセスコンテクストの sleep と wakeup

書式
int tsleep(void *ident, int priority, char *wmesg, int timo) void sleep(void *ident, int priority) void wakeup(void *ident)

解説
これらの関数はコンテクストスイッチにより(@)実装されています (These functions implement voluntary context switching)。 tsleep() と sleep() は、カレントコンテクストが処理を続けることができない以下のような場合に、 カーネルの至る所で使われています:

•   カレントプロセスが、未処理の I/O 処理の結果を待つ必要がある場合

•   カレントプロセスが 必要とするリソース (例えばメモリなど) を一時的に利用できない場合

•   カレントプロセスが、他のプロセスがロックしているデータ構造をアクセスしようとした場合

関数 wakeup() は is used to notify sleeping processes of possible changes to the condition that caused them to go to sleep. Typically, an awakened process will -- after it has acquired a context again -- retry the action that blocked its operation to see if the “blocking” condition has cleared.

The tsleep() function takes the following arguments:

ident
    An identifier of the “wait channel” representing the resource for which the current process needs to wait. This typically is the virtual address of some kernel data-structure related to the resource for which the process is contending. The same identifier must be used in a call to wakeup() to get the process going again. ident should not be NULL.

priority
    The process priority to be used when the process is awakened and put on the queue of runnable processes. This mechanism is used to optimize “throughput” of processes executing in kernel mode. If the flag PCATCH is OR'ed into priority the process checks for posted signals before and after sleeping.

wmesg
    A pointer to a character string indicating the reason a process is sleeping. The kernel does not use the string, but makes it available (through the process structure field p_wmesg) for user level utilities such as ps(1).

timo
    If non-zero, the process will sleep for at most timo/hz seconds. If this amount of time elapses and no wakeup(ident) has occurred, and no signal (if PCATCH was set) was posted, tsleep() will return EWOULDBLOCK.

The sleep() function puts the process in an uninterruptable sleep. It is functionally equivalent to:

    tsleep(ident, priority & PRIMASK, 0, 0)

The wakeup() function will mark all processes which are currently sleeping on the identifier ident as runnable. Eventually, each of the processes will resume execution in the kernel context, causing a return from [t]sleep(). Note that processes returning from sleep should always re-evaluate the conditions that blocked them, since a call to wakeup() merely signals a possible change to the blocking conditions. For example, when two or more processes are waiting for an exclusive lock, only one of them will succeed in acquiring the lock when it is released. All others will have to go back to sleep and wait for the next opportunity.

戻り値
tsleep() returns 0 if it returns as a result of a wakeup(). If a tsleep() returns as a result of a signal, the return value is ERESTART if the signal has the SA_RESTART property (see sigaction(2)), and EINTR otherwise. If tsleep() returns because of a timeout it returns EWOULDBLOCK.