【正文】 一般程序在用戶空間執(zhí)行。（3） TASK_UNINTERRUPTIBLE（不可中斷）——除了不會(huì)因?yàn)榻邮艿叫盘?hào)而被喚醒從而投入運(yùn)行外，這個(gè)狀態(tài)與可打斷的狀態(tài)相同。在x86體系上，current把棧指針的后13個(gè)有效位屏蔽掉，用來(lái)計(jì)算出thread_info的偏移。}每個(gè)任務(wù)的thread_info 結(jié)構(gòu)在它的內(nèi)核棧的尾端分配。進(jìn)程描述符中包含的數(shù)據(jù)能完整的描述一個(gè)正在執(zhí)行的程序：它打開(kāi)的文件，進(jìn)程的地址空間，掛起的信號(hào)，進(jìn)程的狀態(tài)，還有其他更多的信息。在調(diào)用結(jié)束的時(shí)，在返回這個(gè)相同位置上，父進(jìn)程恢復(fù)執(zhí)行，子進(jìn)程開(kāi)始執(zhí)行。 執(zhí)行線程，簡(jiǎn)稱線程（thread），是在進(jìn)程中活動(dòng)的對(duì)象。s actual task_struct.3 Storing the Process DescriptorThe system identifies processes by a unique process identification value or PID. The PID is a numerical value that is represented by the opaque type pid_t, which is typically an int. Because of backward patibility with earlier Unix and Linux versions, however, the default maximum value is only 32,768 although the value can optionally be increased to the full range afforded the type. The kernel stores this value as pid inside each process descriptor.This maximum value is important because it is essentially the maximum number of processes that may exist concurrently on the system. Although 32,768 might be sufficient for a desktop system, large servers may require many more processes. The lower the value, the sooner the values will wrap around, destroying the useful notion that higher values indicate later run processes than lower values. If the system is willing to break patibility with old applications, the administrator may increase the maximum value via /proc/sys/kernel/pid_max.Inside the kernel, tasks are typically referenced directly by a pointer to their task_struct structure. In fact, most kernel code that deals with processes works directly with struct task_struct. Consequently, it is very useful to be able to quickly look up the process descriptor of the currently executing task, which is done via the current macro. This macro must be separately implemented by each architecture. Some architectures save a pointer to the task_struct structure of the currently running process in a register, allowing for efficient access. Other architectures, such as x86 (which has few registers to waste), make use of the fact that struct thread_info is stored on the kernel stack to calculate the location of thread_info and subsequently the task_struct. On x86, current is calculated by masking out the 13 least significant bits of the stack pointer to obtain the thread_info structure. This is done by the current_thread_info() function. The assembly is shown here:movl $8192, %eaxandl %esp, %eaxThis assumes that the stack size is 8KB. When 4KB stacks are enabled, 4096 is used in lieu of 8192.Finally, current dereferences the task member of thread_info to return the task_struct:current_thread_info()task。 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 chil