【正文】 rk（）實際上是由clone（）系統(tǒng)調(diào)用實現(xiàn)的，后者將在后面討論。1 進程描述符及任務(wù)結(jié)構(gòu)內(nèi)核把進程存放在叫做任務(wù)隊列（task list）的雙向循環(huán)鏈表中。這樣做的目的是為了讓那些像x86這樣寄存器較的硬件體系結(jié)構(gòu)只要通過棧指針就能算出它的位置，從而避免使用額外的寄存器專門記錄。 __s32 preempt_count。PID 是一個數(shù)，表示為pid_t隱含類型，實際上就是一個int類型。實際上，內(nèi)核中大部分處理進程的代碼都是直接通過task_struct進行的。當(dāng)4KB的棧啟用時，就用4096，而不是8192。該域的值也必為下列五種狀態(tài)標(biāo)志之一：（1） TASK_RUNNING（運行）——進程是可執(zhí)行的，它或者正在執(zhí)行，或者在運行隊列中等待執(zhí)行。（4） TASK_ZOMBIE（僵死）——該進程已經(jīng)結(jié)束了，但是其父進程還沒有調(diào)用wait()系統(tǒng)調(diào)用，為了父進程能夠獲知它的消息，子進程的進程描述符仍然被保留著。該函數(shù)將制定的進程設(shè)置為給定的狀態(tài)。在此上下文中current宏是有效的。系統(tǒng)調(diào)用和異常處理程序是對內(nèi)核明確定義的接口。方法set_current_state(state),和set_task_state(current, state)含義是等同的。（5） TASK_STOPPED（停止）——進程停止執(zhí)行，進程沒有投入運行也不能投入欲行。（2） TASK_INTERRUPTIBLE（可中斷）——進程正在睡眠（也就是說它被阻塞），等待某些條件的達成。 對比一下這部分在PowerPC上的實現(xiàn)（IBM基于RISC的現(xiàn)代微處理器），我們可以發(fā)現(xiàn)當(dāng)前task_struct的地址是保存在一個寄存器中的。硬件體系結(jié)構(gòu)不同，該宏的實現(xiàn)也就不同，它必須針對專門的硬件體系結(jié)構(gòu)作處理。內(nèi)核把每個進程PID存放在他們各自的進策劃那個描述符中。 Struct restart_block restart_block。這個新的結(jié)構(gòu)能使在匯編代碼中計算器偏移變得相當(dāng)?shù)娜菀住＿M程描述符中包含一個具體進程的所有信息。這個函數(shù)會終結(jié)進程并將其占有的資源釋放掉。無疑，進程在它被創(chuàng)建的時刻開始存活。對Linux而言，線程只不過是一種特殊的進程罷了。但進程并不僅僅局限于一段可執(zhí)行程序（Unix稱其為代碼段（text section））。s state to TASK_RUNNING. The process also awakes prematurely and bees runnable if it receives a signal.(3) TASK_UNINTERRUPTIBLE This state is identical to TASK_INTERRUPTIBLE except that it does not wake up and bee runnable if it receives a signal. This is used in situations where the process must wait without interruption or when the event is expected to occur quite quickly. Because the task does not respond to signals in this state, this state is less often used than TASK_INTERRUPTIBLE(4)TASK_ZOMBIE The task has terminated, but its parent has not yet issued a wait4() system call. The task39。}。unsigned long flags。 a process is an active program and related resources. Indeed, two or more processes can exist that are executing the same program. In fact, two or more processes can exist that share various resources, such as open files or an address space.A process begins its life when, not surprisingly, it is created. In Linux, this occurs by means of the fork()system call, which creates a new process by duplicating an existing one. The process that calls fork() is the parent, whereas the new process is the child. The parent resumes execution and the child starts execution at the same place, where the call returns. The fork() system call returns from the kernel twice:once in the parent process and again in the newborn child.Often, immediately after a fork it is desirable to execute a new, different, program. The exec*() family of function calls is used to create a new address space and load a new program into it. In modern Linux kernels, fork() is actually implemented via the clone() system call, which is discussed in a followingsection.Finally, a program exits via the exit() system call. This function terminates the process and frees all its resources. A parent process can inquire about the status of a terminated child via the wait4() system call, which enables a process to wait for the termination of a specific process. When a process exits, it is placed into a special zombie state that is used to represent terminated processes until the parent calls wait