int __pthread_create_2_1(pthread_t *thread, const pthread_attr_t *attr,
void * (*start_routine)(void *), void *arg)
// thread 為pthread_t，實則是pthread指針。
// typedef struct pthread *pthread_t; ->unsigned long int
// attr 創建線程時的附加屬性
// start_routine 執行的線程函數
// arg參數首地址指針
{
pthread_descr self = thread_self();
// 向管理線程的請求
struct pthread_request request;
// 在初始化時 全局 int __pthread_manager_request = -1;
if (__pthread_manager_request < 0) {
// 首先Linuxthreads需要有__pthread_manager_thread，這個線程是個很特殊的線程
// 初始化Linuxthreads系統線程
if (__pthread_initialize_manager() < 0) return EAGAIN;
}
request.req_thread = self;
// 請求類型 CREATE ，並將新線程（輕量進程）的參數都打包到request結構中。
request.req_kind = REQ_CREATE;
request.req_args.create.attr = attr;
request.req_args.create.fn = start_routine;
request.req_args.create.arg = arg;
//sigprocmask用於改變進程的當前阻塞信號集
sigprocmask(SIG_SETMASK, (const sigset_t *) NULL,
&request.req_args.create.mask);
// 向__pthread_manager_request寫入request請求。 request請求將被_pthread_manager_thread讀取並處理。
__libc_write(__pthread_manager_request, (char *) &request, sizeof(request));
// 掛起一下
suspend(self);
if (THREAD_GETMEM(self, p_retcode) == 0)
*thread = (pthread_t) THREAD_GETMEM(self, p_retval);
return THREAD_GETMEM(self, p_retcode);
}
#if defined HAVE_ELF && defined PIC && defined DO_VERSIONING
default_symbol_version (__pthread_create_2_1, pthread_create, GLIBC_2.1);
int __pthread_create_2_0(pthread_t *thread, const pthread_attr_t *attr,
void * (*start_routine)(void *), void *arg)
{
/* The ATTR attribute is not really of type `pthread_attr_t *'. It has
the old size and access to the new members might crash the program.
We convert the struct now. */
pthread_attr_t new_attr;
if (attr != NULL)
{
size_t ps = __getpagesize ();
memcpy (&new_attr, attr,
(size_t) &(((pthread_attr_t*)NULL)->__guardsize));
new_attr.__guardsize = ps;
new_attr.__stackaddr_set = 0;
new_attr.__stackaddr = NULL;
new_attr.__stacksize = STACK_SIZE - ps;
attr = &new_attr;
}
return __pthread_create_2_1 (thread, attr, start_routine, arg);
}
symbol_version (__pthread_create_2_0, pthread_create, GLIBC_2.0); // 由此可知pthread_create入口函數為__pthread_create_2_1
#else
strong_alias (__pthread_create_2_1, pthread_create)
#endif
由上面的函數可知，pthread_create對線程的創建實際上是向pthread_manager_thread發送個request請求。
下面我們就看pthread_manager_thread是怎樣初始化的：
首先要進行基本系統的初始化工作：
即填充__pthread_initial_thread（算是模板吧？）其他屬性：
/* Descriptor of the initial thread */
struct _pthread_descr_struct __pthread_initial_thread = {
&__pthread_initial_thread, /* pthread_descr p_nextlive */ // 將進程中所有用戶線程串在了一起
&__pthread_initial_thread, /* pthread_descr p_prevlive */
NULL, /* pthread_descr p_nextwaiting */
NULL, /* pthread_descr p_nextlock */
PTHREAD_THREADS_MAX, /* pthread_t p_tid */
0, /* int p_pid */
0, /* int p_priority */
&__pthread_handles[0].h_lock, /* struct _pthread_fastlock * p_lock */
0, /* int p_signal */
NULL, /* sigjmp_buf * p_signal_buf */
NULL, /* sigjmp_buf * p_cancel_buf */
0, /* char p_terminated */
0, /* char p_detached */
0, /* char p_exited */
NULL, /* void * p_retval */
0, /* int p_retval */
NULL, /* pthread_descr p_joining */
NULL, /* struct _pthread_cleanup_buffer * p_cleanup */
0, /* char p_cancelstate */
0, /* char p_canceltype */
0, /* char p_canceled */
NULL, /* int *p_errnop */
0, /* int p_errno */
NULL, /* int *p_h_errnop */
0, /* int p_h_errno */
NULL, /* char * p_in_sighandler */
0, /* char p_sigwaiting */
PTHREAD_START_ARGS_INITIALIZER, /* struct pthread_start_args p_start_args */
{NULL}, /* void ** p_specific[PTHREAD_KEY_1STLEVEL_SIZE] */
{NULL}, /* void * p_libc_specific[_LIBC_TSD_KEY_N] */
0, /* int p_userstack */
NULL, /* void * p_guardaddr */
0, /* size_t p_guardsize */
&__pthread_initial_thread, /* pthread_descr p_self */
0, /* Always index 0 */
0, /* int p_report_events */
{{{0, }}, 0, NULL}, /* td_eventbuf_t p_eventbuf */
ATOMIC_INITIALIZER, /* struct pthread_atomic p_resume_count */
0, /* char p_woken_by_cancel */
NULL /* struct pthread_extricate_if *p_extricate */
};
static void pthread_initialize(void)
{
struct sigaction sa;
sigset_t mask;
struct rlimit limit;
int max_stack;
//__pthread_initial_thread_bos: Limit between the stack of the initial thread (above) and the
//stacks of other threads (below). Aligned on a STACK_SIZE boundary.
/* 如果初始化工作已完成，則退出 */
if (__pthread_initial_thread_bos != NULL) return;
#ifdef TEST_FOR_COMPARE_AND_SWAP
/* Test if compare-and-swap is available */
__pthread_has_cas = compare_and_swap_is_available();
#endif
/* For the initial stack, reserve at least STACK_SIZE bytes of stack
below the current stack address, and align that on a
STACK_SIZE boundary. */
__pthread_initial_thread_bos =
(char *)(((long)CURRENT_STACK_FRAME. - 2 * STACK_SIZE) & ~(STACK_SIZE - 1));
/* 對initial_thread.pid賦值：主進程pid */
__pthread_initial_thread.p_pid = __getpid();
/* If we have special thread_self processing, initialize that for the
main thread now. */
#ifdef INIT_THREAD_SELF
INIT_THREAD_SELF(&__pthread_initial_thread, 0);
#endif
/* 共享主進程errno/h_errno.*/
__pthread_initial_thread.p_errnop = &_errno;
__pthread_initial_thread.p_h_errnop = &_h_errno;
/* Play with the stack size limit to make sure that no stack ever grows
beyond STACK_SIZE minus two pages (one page for the thread descriptor
immediately beyond, and one page to act as a guard page). */
getrlimit(RLIMIT_STACK, &limit); // 獲取STACK limit
max_stack = STACK_SIZE - 2 * __getpagesize();
if (limit.rlim_cur > max_stack) {
limit.rlim_cur = max_stack;
setrlimit(RLIMIT_STACK, &limit);
}
#ifdef __SIGRTMIN
/* Initialize real-time signals. */
init_rtsigs ();
#endif
/* Setup signal handlers for the initial thread.
Since signal handlers are shared between threads, these settings
will be inherited by all other threads. */
// 設置initial thread信號處理函數 其他線程都將繼承
#ifndef __i386__
sa.sa_handler = pthread_handle_sigrestart;
#else
sa.sa_handler = (__sighandler_t) pthread_handle_sigrestart;
#endif
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
__sigaction(__pthread_sig_restart, &sa, NULL);
#ifndef __i386__
sa.sa_handler = pthread_handle_sigcancel;
#else
sa.sa_handler = (__sighandler_t) pthread_handle_sigcancel;
#endif
sa.sa_flags = 0;
__sigaction(__pthread_sig_cancel, &sa, NULL);
if (__pthread_sig_debug > 0) {
sa.sa_handler = pthread_handle_sigdebug;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
__sigaction(__pthread_sig_debug, &sa, NULL);
}
/* Initially, block __pthread_sig_restart. Will be unblocked on demand. */
sigemptyset(&mask);
sigaddset(&mask, __pthread_sig_restart);
sigprocmask(SIG_BLOCK, &mask, NULL);
/* Register an exit function to kill all other threads. */
/* Do it early so that user-registered atexit functions are called
before pthread_exit_process. */
__on_exit(pthread_exit_process, NULL);
}
// 下面是__pthread_manager_thread 的初始化
/* Descriptor of the manager thread; none of this is used but the error
variables, the p_pid and p_priority fields,
and the address for identification. */
struct _pthread_descr_struct __pthread_manager_thread = {
NULL, /* pthread_descr p_nextlive */ // 這兩個值為空！
NULL, /* pthread_descr p_prevlive */
NULL, /* pthread_descr p_nextwaiting */
NULL, /* pthread_descr p_nextlock */
0, /* int p_tid */
0, /* int p_pid */
0, /* int p_priority */
&__pthread_handles[1].h_lock, /* struct _pthread_fastlock * p_lock */
0, /* int p_signal */
NULL, /* sigjmp_buf * p_signal_buf */
NULL, /* sigjmp_buf * p_cancel_buf */
0, /* char p_terminated */
0, /* char p_detached */
0, /* char p_exited */
NULL, /* void * p_retval */
0, /* int p_retval */
NULL, /* pthread_descr p_joining */
NULL, /* struct _pthread_cleanup_buffer * p_cleanup */
0, /* char p_cancelstate */
0, /* char p_canceltype */
0, /* char p_canceled */
&__pthread_manager_thread.p_errno, /* int *p_errnop */
0, /* int p_errno */
NULL, /* int *p_h_errnop */
0, /* int p_h_errno */
NULL, /* char * p_in_sighandler */
0, /* char p_sigwaiting */
PTHREAD_START_ARGS_INITIALIZER, /* struct pthread_start_args p_start_args */
{NULL}, /* void ** p_specific[PTHREAD_KEY_1STLEVEL_SIZE] */
{NULL}, /* void * p_libc_specific[_LIBC_TSD_KEY_N] */
0, /* int p_userstack */
NULL, /* void * p_guardaddr */
0, /* size_t p_guardsize */
&__pthread_manager_thread, /* pthread_descr p_self */
1, /* Always index 1 */
0, /* int p_report_events */
{{{0, }}, 0, NULL}, /* td_eventbuf_t p_eventbuf */
ATOMIC_INITIALIZER, /* struct pthread_atomic p_resume_count */
0, /* char p_woken_by_cancel */
NULL /* struct pthread_extricate_if *p_extricate */
};
int __pthread_initialize_manager(void)
{
// 管理線程與其他線程之間的通信是通過管道完成的。
/*在一個進程空間內，管理線程與其他線程之間通過一對"管理管道（manager_pipe[2]）"來通訊，
該管道在創建管理線程之前創建，在成功啟動了管理線程之後，管理管道的讀端和寫端分別賦給兩個全局變量
__pthread_manager_reader和__pthread_manager_request，之後，每個用戶線程都通過
__pthread_manager_request向管理線程發請求，但管理線程本身並沒有直接使用__pthread_manager_reader，
管道的讀端（manager_pipe[0]）是作為__clone()的參數之一傳給管理線程的，
管理線程的工作主要就是監聽管道讀端，並對從中取出的請求作出反應。*/
int manager_pipe[2];
int pid;
struct pthread_request request;
/* 如果基本的初始化未進行，那麼進行初始化*/
if (__pthread_initial_thread_bos == NULL) pthread_initialize();
/* 設置stack */
__pthread_manager_thread_bos = malloc(THREAD_MANAGER_STACK_SIZE);
if (__pthread_manager_thread_bos == NULL) return -1;
// top值
__pthread_manager_thread_tos =
__pthread_manager_thread_bos + THREAD_MANAGER_STACK_SIZE;
/* 設置通信管道 */
if (pipe(manager_pipe) == -1) {
free(__pthread_manager_thread_bos);
return -1;
}
/* 在創建線程時 pid一定不會為0 */
pid = 0;
if (__pthread_initial_thread.p_report_events)
{
/* It's a bit more complicated. We have to report the creation of
the manager thread. */
int idx = __td_eventword (TD_CREATE);
uint32_t mask = __td_eventmask (TD_CREATE);
if ((mask & (__pthread_threads_events.event_bits[idx]
| __pthread_initial_thread.p_eventbuf.eventmask.event_bits[idx]))
!= 0)
{
// sys_fork：創建 pthread_manager線程 ：共享VM FS FILES等
pid = __clone(__pthread_manager_event,
(void **) __pthread_manager_thread_tos,
CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND,
(void *)(long)manager_pipe[0]); // 讀管道
if (pid != -1)
{
/* Now fill in the information about the new thread in
the newly created thread's data structure. We cannot let
the new thread do this since we don't know whether it was
already scheduled when we send the event. */
// 對新線程添加屬性
__pthread_manager_thread.p_eventbuf.eventdata =
&__pthread_manager_thread;
__pthread_manager_thread.p_eventbuf.eventnum = TD_CREATE;
__pthread_last_event = &__pthread_manager_thread;
// _pthread_manager_thread的是2*PTHREAD_THREADS_MAX+1
__pthread_manager_thread.p_tid = 2* PTHREAD_THREADS_MAX + 1;
__pthread_manager_thread.p_pid = pid; // clone進程pid
/* Now call the function which signals the event. */
__linuxthreads_create_event ();
/* Now restart the thread. */
__pthread_unlock(__pthread_manager_thread.p_lock);
}
}
}
// 如果__pthread_initial_thread.p_report_events標記了，自然不再創建新線程了
if (pid == 0)
pid = __clone(__pthread_manager, (void **) __pthread_manager_thread_tos,
CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND,
(void *)(long)manager_pipe[0]);
if (pid == -1) {
free(__pthread_manager_thread_bos);
__libc_close(manager_pipe[0]);
__libc_close(manager_pipe[1]);
return -1;
}
// __pthread_manager_request fd = manager_pipe[1]; 交給其他線程寫請求
__pthread_manager_request = manager_pipe[1]; /* 全局變量writing end */
__pthread_manager_reader = manager_pipe[0]; /* 全局變量reading end */
__pthread_manager_thread.p_tid = 2* PTHREAD_THREADS_MAX + 1;
__pthread_manager_thread.p_pid = pid;
/* Make gdb aware of new thread manager */
if (__pthread_threads_debug && __pthread_sig_debug > 0)
{
raise(__pthread_sig_debug);
/* We suspend ourself and gdb will wake us up when it is
ready to handle us. */
__pthread_wait_for_restart_signal(thread_self());
}
/* Synchronize debugging of the thread manager */
request.req_kind = REQ_DEBUG;
__libc_write(__pthread_manager_request, (char *) &request, sizeof(request));
return 0;
}
至此，__pthread_manager_thread創建成功。
下面看看它是怎樣管理的：__pthread_manager函數
/* The server thread managing requests for thread creation and termination */
int __pthread_manager(void *arg)
{
int reqfd = (int) (long int) arg; // 管道的讀口fd
struct pollfd ufd;
sigset_t mask;
int n;
struct pthread_request request;
/* If we have special thread_self processing, initialize it. */
#ifdef INIT_THREAD_SELF
INIT_THREAD_SELF(&__pthread_manager_thread, 1);
#endif
/* Set the error variable. */
__pthread_manager_thread.p_errnop = &__pthread_manager_thread.p_errno;
__pthread_manager_thread.p_h_errnop = &__pthread_manager_thread.p_h_errno;
/* Block all signals except __pthread_sig_cancel and SIGTRAP */
sigfillset(&mask);
sigdelset(&mask, __pthread_sig_cancel); /* for thread termination */
sigdelset(&mask, SIGTRAP); /* for debugging purposes */
sigprocmask(SIG_SETMASK, &mask, NULL);
/* 提高優先級 等於主線程的優先級 */
__pthread_manager_adjust_prio(__pthread_main_thread->p_priority);
/* Synchronize debugging of the thread manager */
// 從管道中讀取請求
n = __libc_read(reqfd, (char *)&request, sizeof(request));
ASSERT(n == sizeof(request) && request.req_kind == REQ_DEBUG);
ufd.fd = reqfd;
ufd.events = POLLIN;
/* Enter server loop */
while(1) {
n = __poll(&ufd, 1, 2000); // 2秒超時
/* main thread的終止 將導致所有thread終止 */
if (getppid() == 1) {
pthread_kill_all_threads(SIGKILL, 0);
_exit(0);
}
/* Check for dead children */
if (terminated_children) {
terminated_children = 0;
pthread_reap_children();
}
/* 讀取請求 */
if (n == 1 && (ufd.revents & POLLIN)) {
n = __libc_read(reqfd, (char *)&request, sizeof(request));
ASSERT(n == sizeof(request));
switch(request.req_kind) {
case REQ_CREATE: //創建
request.req_thread->p_retcode =
pthread_handle_create((pthread_t *) &request.req_thread->p_retval,
request.req_args.create.attr,
request.req_args.create.fn,
request.req_args.create.arg,
&request.req_args.create.mask,
request.req_thread->p_pid,
request.req_thread->p_report_events,
&request.req_thread->p_eventbuf.eventmask);
restart(request.req_thread);
break;
case REQ_FREE:
pthread_handle_free(request.req_args.free.thread_id);
break;
case REQ_PROCESS_EXIT:
pthread_handle_exit(request.req_thread,
request.req_args.exit.code);
break;
case REQ_MAIN_THREAD_EXIT:
main_thread_exiting = 1;
if (__pthread_main_thread->p_nextlive == __pthread_main_thread) {
restart(__pthread_main_thread);
return 0;
}
break;
case REQ_POST:
__new_sem_post(request.req_args.post);
break;
case REQ_DEBUG:
/* Make gdb aware of new thread and gdb will restart the
new thread when it is ready to handle the new thread. */
if (__pthread_threads_debug && __pthread_sig_debug > 0)
raise(__pthread_sig_debug);
break;
}
}
}
}
下面我們看看新的線程是怎樣創建的：
static int pthread_handle_create(pthread_t *thread, const pthread_attr_t *attr,
void * (*start_routine)(void *), void *arg,
sigset_t * mask, int father_pid,
int report_events,
td_thr_events_t *event_maskp)
{
size_t sseg;
int pid;
pthread_descr new_thread;
char * new_thread_bottom;
pthread_t new_thread_id;
char *guardaddr = NULL;
size_t guardsize = 0;
int pagesize = __getpagesize();
/* First check whether we have to change the policy and if yes, whether
we can do this. Normally this should be done by examining the
return value of the __sched_setscheduler call in pthread_start_thread
but this is hard to implement. FIXME */
if (attr != NULL && attr->__schedpolicy != SCHED_OTHER && geteuid () != 0)
return EPERM;
/* Find a free segment for the thread, and allocate a stack if needed */
for (sseg = 2; ; sseg++)
{
if (sseg >= PTHREAD_THREADS_MAX)
return EAGAIN;
if (__pthread_handles[sseg].h_descr != NULL)
continue;
if (pthread_allocate_stack(attr, thread_segment(sseg), pagesize,
&new_thread, &new_thread_bottom,
&guardaddr, &guardsize) == 0)
break;
}
__pthread_handles_num++;
/* Allocate new thread identifier */
pthread_threads_counter += PTHREAD_THREADS_MAX;
new_thread_id = sseg + pthread_threads_counter;
/* Initialize the thread descriptor. Elements which have to be
initialized to zero already have this value. */
// 對新的線程進行屬性賦值
new_thread->p_tid = new_thread_id; //tid=n*PTHREAD_THREADS_MAX+n+1
new_thread->p_lock = &(__pthread_handles[sseg].h_lock);
new_thread->p_cancelstate = PTHREAD_CANCEL_ENABLE;
new_thread->p_canceltype = PTHREAD_CANCEL_DEFERRED;
new_thread->p_errnop = &new_thread->p_errno;
new_thread->p_h_errnop = &new_thread->p_h_errno;
new_thread->p_guardaddr = guardaddr;
new_thread->p_guardsize = guardsize;
new_thread->p_self = new_thread;
new_thread->p_nr = sseg;
/* Initialize the thread handle */
__pthread_init_lock(&__pthread_handles[sseg].h_lock);
__pthread_handles[sseg].h_descr = new_thread;
__pthread_handles[sseg].h_bottom = new_thread_bottom;
/* Determine scheduling parameters for the thread */
new_thread->p_start_args.schedpolicy = -1;
if (attr != NULL) {
new_thread->p_detached = attr->__detachstate;
new_thread->p_userstack = attr->__stackaddr_set;
switch(attr->__inheritsched) {
case PTHREAD_EXPLICIT_SCHED:
new_thread->p_start_args.schedpolicy = attr->__schedpolicy;
memcpy (&new_thread->p_start_args.schedparam, &attr->__schedparam,
sizeof (struct sched_param));
break;
case PTHREAD_INHERIT_SCHED:
new_thread->p_start_args.schedpolicy = __sched_getscheduler(father_pid);
__sched_getparam(father_pid, &new_thread->p_start_args.schedparam);
break;
}
new_thread->p_priority =
new_thread->p_start_args.schedparam.sched_priority;
}
/* Finish setting up arguments to pthread_start_thread */
// 新線程 函數 參數賦值
new_thread->p_start_args.start_routine = start_routine;
new_thread->p_start_args.arg = arg;
new_thread->p_start_args.mask = *mask;
/* Raise priority of thread manager if needed */
__pthread_manager_adjust_prio(new_thread->p_priority);
/* Do the cloning. We have to use two different functions depending
on whether we are debugging or not. */
pid = 0; /* Note that the thread never can have PID zero. */
if (report_events)
{
/* See whether the TD_CREATE event bit is set in any of the
masks. */
int idx = __td_eventword (TD_CREATE);
uint32_t mask = __td_eventmask (TD_CREATE);
if ((mask & (__pthread_threads_events.event_bits[idx]
| event_maskp->event_bits[idx])) != 0)
{
/* Lock the mutex the child will use now so that it will stop. */
__pthread_lock(new_thread->p_lock, NULL);
/* We have to report this event. */
pid = __clone(pthread_start_thread_event, (void **) new_thread,
CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND |
__pthread_sig_cancel, new_thread);
if (pid != -1)
{
/* Now fill in the information about the new thread in
the newly created thread's data structure. We cannot let
the new thread do this since we don't know whether it was
already scheduled when we send the event. */
new_thread->p_eventbuf.eventdata = new_thread;
new_thread->p_eventbuf.eventnum = TD_CREATE;
__pthread_last_event = new_thread;
/* We have to set the PID here since the callback function
in the debug library will need it and we cannot guarantee
the child got scheduled before the debugger. */
new_thread->p_pid = pid;
/* Now call the function which signals the event. */
__linuxthreads_create_event ();
/* Now restart the thread. */
__pthread_unlock(new_thread->p_lock);
}
}
}
if (pid == 0)
pid = __clone(pthread_start_thread, (void **) new_thread,
CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND |
__pthread_sig_cancel, new_thread);
/* Check if cloning succeeded */
if (pid == -1) {
/* Free the stack if we allocated it */
if (attr == NULL || !attr->__stackaddr_set)
{
if (new_thread->p_guardsize != 0)
munmap(new_thread->p_guardaddr, new_thread->p_guardsize);
munmap((caddr_t)((char *)(new_thread+1) - INITIAL_STACK_SIZE),
INITIAL_STACK_SIZE);
}
__pthread_handles[sseg].h_descr = NULL;
__pthread_handles[sseg].h_bottom = NULL;
__pthread_handles_num--;
return errno;
}
/* 將線程添加到__pthread_main_thread 雙向鏈表中 由管理線程管理*/
new_thread->p_prevlive = __pthread_main_thread;
new_thread->p_nextlive = __pthread_main_thread->p_nextlive;
__pthread_main_thread->p_nextlive->p_prevlive = new_thread;
__pthread_main_thread->p_nextlive = new_thread;
/* Set pid field of the new thread, in case we get there before the
child starts. */
new_thread->p_pid = pid; // 記錄了clone 返回的pid
/* We're all set */
*thread = new_thread_id;
return 0;
}
總結：
1）Linuxthread 是用sys_clone系統調用完成的。本質上說，每個線程其實就是一個進程，只不過線程共享部分資源。
2）操作系統對線程的管理，實則就是對進程的管理。在/proc目錄下我們可以找到這些進程
3）Linuxthread 是通過 __pthread_manager_thread 管理其他線程的。
4）線程有各自的線程id，由LinuxThreads分配和維護。其實也有各自的真實pid，但由結構隱藏了，對外的pid為主進程的pid
5）線程對操作系統來說就是進程，因此對信號的處理是以進程為單位分發的
6）線程總數受限於系統進程數
7）系統管理線程的瓶頸：所有線程由管理線程管理
8）線程之間的調度是由內核調度器來處理的
9）由於計算線程本地數據的方法是基於堆棧地址的位置。
