基于linuxthreads2.0.1线程源码如何分析线程库的初始化和线程的管理
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初步分析一下线程的初始化和管理。
线程库的初始化代码如下。
// 在main函数之前执行该函数
void __pthread_initialize(void) __attribute__((constructor));
void __pthread_initialize(void)
{
struct sigaction sa;
sigset_t mask;
/* We may be called by others. This may happen if the constructors
are not called in the order we need. */
if (__pthread_initial_thread_bos != NULL)
return;
/* 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 =
// 按STACK_SIZE大小对齐
(char *)(((long)CURRENT_STACK_FRAME - 2 * STACK_SIZE) & ~(STACK_SIZE - 1));
/* Update the descriptor for the initial thread. */
// 即main函数代表的主进程id
__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);
#endif
/* Setup signal handlers for the initial thread.
Since signal handlers are shared between threads, these settings
will be inherited by all other threads. */
// 为两个信号注册处理函数
sa.sa_handler = __pthread_sighandler;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART; /* does not matter for regular threads, but
better for the thread manager */
sigaction(PTHREAD_SIG_RESTART, &sa, NULL);
sa.sa_handler = pthread_handle_sigcancel;
sa.sa_flags = 0;
sigaction(PTHREAD_SIG_CANCEL, &sa, NULL);
/* Initially, block PTHREAD_SIG_RESTART. Will be unblocked on demand. */
// 屏蔽restart信号
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. */
// 注册退出时执行的函数
__on_exit(pthread_exit_process, NULL);
}
在执行main函数之前会先执行__pthread_initialize函数，该函数做的事情主要有
1 在栈上分配一块内存。
2 保存当前进程，进main函数对应的进程的pid。
3 注册两个信号处理函数。
4 注册退出时执行的函数
接下来我们会调用pthread_create进行线程的创建。我们来看看该函数做了什么。
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
void * (*start_routine)(void *), void *arg)
{
pthread_t self = thread_self();
struct pthread_request request;
// 还没执行过pthread_initialize_manager则执行,用于初始化manager线程
if (__pthread_manager_request < 0) {
if (pthread_initialize_manager() < 0) return EAGAIN;
}
// 给manager发一下请求
request.req_thread = self;
request.req_kind = REQ_CREATE;
request.req_args.create.attr = attr;
request.req_args.create.fn = start_routine;
request.req_args.create.arg = arg;
// 获取当前线程的信号掩码
sigprocmask(SIG_SETMASK, (const sigset_t *) NULL,
&request.req_args.create.mask);
// 通过管道写入，通知manager线程，新建一个线程
__libc_write(__pthread_manager_request, (char *) &request, sizeof(request));
// 挂起,等待manager唤醒
suspend(self);
// 等于0说明创建成功，否则返回失败的错误码,p_retval在pthread_handle_create中设置
if (self->p_retcode == 0) *thread = (pthread_t) self->p_retval;
return self->p_retcode;
}
我们发现，该函数没有做实际的事情，他通过往管道写了一些数据。这时候就要先看pthread_initialize_manager函数了。
static int pthread_initialize_manager(void)
{
int manager_pipe[2];
/* Setup stack for thread manager */
// 在堆上分配一块内存用于manager线程的栈
__pthread_manager_thread_bos = malloc(THREAD_MANAGER_STACK_SIZE);
if (__pthread_manager_thread_bos == NULL) return -1;
// limit
__pthread_manager_thread_tos =
__pthread_manager_thread_bos + THREAD_MANAGER_STACK_SIZE;
/* Setup pipe to communicate with thread manager */
if (pipe(manager_pipe) == -1) {
free(__pthread_manager_thread_bos);
return -1;
}
__pthread_manager_request = manager_pipe[1]; /* writing end */
__pthread_manager_reader = manager_pipe[0]; /* reading end */
/* Start the thread manager */
// 新建一个manager线程,manager_pipe是__thread_manager函数的入参
if (__clone(__pthread_manager,
__pthread_manager_thread_tos,
CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND,
(void *)(long)manager_pipe[0]) == -1) {
free(__pthread_manager_thread_bos);
__libc_close(manager_pipe[0]);
__libc_close(manager_pipe[1]);
__pthread_manager_request = -1;
return -1;
}
return 0;
}
该函数做了几件事情
1 在堆上申请一块内存用作manager线程的栈
2 创建了一个管道，用于manager线程和其他线程通信。
3 然后新建了一个进程，然后执行__pthread_manager函数。（具体可参考http://www.man7.org/linux/man-pages/man2/clone.2.html）
manager线程是linuxthreads线程库比较重要的存在，他是管理其他线程的线程。我们接着看_pthread_manager函数的代码。
/* The server thread managing requests for thread creation and termination */
int __pthread_manager(void *arg)
{
// 管道的读端
int reqfd = (long)arg;
sigset_t mask;
fd_set readfds;
struct timeval timeout;
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);
#endif
/* Block all signals except PTHREAD_SIG_RESTART */
// 初始化为全1
sigfillset(&mask);
// 设置某一位为0,这里设置可以处理restart信号
sigdelset(&mask, PTHREAD_SIG_RESTART);
// 设置进程的信号掩码
sigprocmask(SIG_SETMASK, &mask, NULL);
/* Enter server loop */
while(1) {
// 清0
FD_ZERO(&readfds);
// 置某位为1，位数由reqfd算得,这里是管道读端的文件描述符
FD_SET(reqfd, &readfds);
// 阻塞的超时时间
timeout.tv_sec = 2;
timeout.tv_usec = 0;
// 定时阻塞等待管道有数据可读
n = __select(FD_SETSIZE, &readfds, NULL, NULL, &timeout);
/* Check for termination of the main thread */
// 父进程id为1说明主进程（线程）已经退出，子进程被init（pid=1）进程接管了，
if (getppid() == 1) {
// 0说明不需要给主线程发，因为他已经退出了
pthread_kill_all_threads(SIGKILL, 0);
return 0;
}
/* Check for dead children */
if (terminated_children) {
terminated_children = 0;
pthread_reap_children();
}
/* Read and execute request */
// 管道有数据可读
if (n == 1 && FD_ISSET(reqfd, &readfds)) {
// 读出来放到request
n = __libc_read(reqfd, (char *)&request, sizeof(request));
ASSERT(n == sizeof(request));
switch(request.req_kind) {
// 创建线程
case REQ_CREATE:
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);
// 唤醒父线程
restart(request.req_thread);
break;
case REQ_FREE:
pthread_handle_free(request.req_args.free.thread);
break;
case REQ_PROCESS_EXIT:
pthread_handle_exit(request.req_thread,
request.req_args.exit.code);
break;
case REQ_MAIN_THREAD_EXIT:
// 标记主线程退出
main_thread_exiting = 1;
// 其他线程已经退出了，只有主线程了，唤醒主线程，主线程也退出，见pthread_exit，如果还有子线程没退出则主线程不能退出
if (__pthread_main_thread->p_nextlive == __pthread_main_thread) {
restart(__pthread_main_thread);
return 0;
}
break;
}
}
}
}
该函数是manager线程的主要代码。他类似一个服务器一起。接收其他线程发过来的信息，然后处理。在switch那里可以看到具体的处理。这里我们只看线程创建的逻辑。函数是pthread_handle_create。
// pthread_create发送信号给manager，manager调该函数创建线程
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 sseg;
int pid;
pthread_t new_thread;
int i;
/* Find a free stack segment for the current stack */
sseg = 0;
while (1) {
while (1) {
if (sseg >= num_stack_segments) {
if (pthread_grow_stack_segments() == -1) return EAGAIN;
}
if (stack_segments[sseg] == 0) break;
sseg++;
}
// 标记已使用
stack_segments[sseg] = 1;
// 存储线程元数据的地方
new_thread = THREAD_SEG(sseg);
/* Allocate space for stack and thread descriptor. */
// 给线程分配栈
if (mmap((caddr_t)((char *)(new_thread+1) - INITIAL_STACK_SIZE),
INITIAL_STACK_SIZE, PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED | MAP_GROWSDOWN, -1, 0)
!= (caddr_t) -1) break;
/* It seems part of this segment is already mapped. Leave it marked
as reserved (to speed up future scans) and try the next. */
sseg++;
}
/* Initialize the thread descriptor */
new_thread->p_nextwaiting = NULL;
new_thread->p_spinlock = 0;
new_thread->p_signal = 0;
new_thread->p_signal_jmp = NULL;
new_thread->p_cancel_jmp = NULL;
new_thread->p_terminated = 0;
new_thread->p_detached = attr == NULL ? 0 : attr->detachstate;
new_thread->p_exited = 0;
new_thread->p_retval = NULL;
new_thread->p_joining = NULL;
new_thread->p_cleanup = NULL;
new_thread->p_cancelstate = PTHREAD_CANCEL_ENABLE;
new_thread->p_canceltype = PTHREAD_CANCEL_DEFERRED;
new_thread->p_canceled = 0;
new_thread->p_errno = 0;
new_thread->p_h_errno = 0;
new_thread->p_initial_fn = start_routine;
new_thread->p_initial_fn_arg = arg;
new_thread->p_initial_mask = mask;
for (i = 0; i < PTHREAD_KEYS_MAX; i++) new_thread->p_specific[i] = NULL;
/* Do the cloning */
pid = __clone(pthread_start_thread, new_thread,
(CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND
| PTHREAD_SIG_RESTART),
new_thread);
/* Check if cloning succeeded */
if (pid == -1) {
/* Free the stack */
munmap((caddr_t)((char *)(new_thread+1) - INITIAL_STACK_SIZE),
INITIAL_STACK_SIZE);
stack_segments[sseg] = 0;
return EAGAIN;
}
/* Set the priority and policy for the new thread, if available. */
if (attr != NULL && attr->schedpolicy != SCHED_OTHER) {
switch(attr->inheritsched) {
case PTHREAD_EXPLICIT_SCHED:
sched_setscheduler(pid, attr->schedpolicy, &attr->schedparam);
break;
case PTHREAD_INHERIT_SCHED:
{ struct sched_param father_param;
int father_policy;
father_policy = sched_getscheduler(father_pid);
sched_getparam(father_pid, &father_param);
sched_setscheduler(pid, father_policy, &father_param);
}
break;
}
}
/* Insert new thread in doubly linked list of active threads */
// 头插法，插入主线程和其他线程之间，
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;
/* We're all set */
*thread = new_thread;
return 0;
}
该函数分配一个tcb结构体表示新的线程。然后分配一个线程栈，调用clone新建一个进程。最后链接到线程链表中。最后执行pthread_start_thread函数。该函数代码如下。
// 传给clone函数的参数
static int pthread_start_thread(void *arg)
{
// 新建的线程
pthread_t self = (pthread_t) arg;
void * outcome;
/* Initialize special thread_self processing, if any. */
#ifdef INIT_THREAD_SELF
INIT_THREAD_SELF(self);
#endif
/* Make sure our pid field is initialized, just in case we get there
before our father has initialized it. */
// 记录线程对应进程的id
self->p_pid = getpid();
/* Initial signal mask is that of the creating thread. (Otherwise,
we'd just inherit the mask of the thread manager.) */
// 设置线程的信号掩码，值继承于父线程
sigprocmask(SIG_SETMASK, &self->p_initial_mask, NULL);
/* Run the thread code */
// 开始执行线程的主函数
outcome = self->p_initial_fn(self->p_initial_fn_arg);
/* Exit with the given return value */
// 执行完退出
pthread_exit(outcome);
return 0;
}
没有太多逻辑，执行用户传进来的函数。执行完后退出。
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