《Linux/UNIX系统编程手册》 英文版读书笔记第29章

errnois defined as amacro that expands into a function call returning a modifiable lvalue that is distinct for each thread. (Since
the lvalue is modifiable, it is still possible to write assignment statements of the formerrno = valuein threaded programs.)

On Linux, programs that use the Pthreads API must be compiled with thecc –pthread option. The effects of this option include the following: The_REENTRANTpreprocessor macro is defined. This causes the declarations of a few reentrant functions to be exposed.
z The program is linked with the libpthread library (the equivalent of–lpthread).

When a program is started, the resulting process consists of a single thread, called the initial or mainthread

#include <pthread.h>
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
 void *(*start)(void *), void *arg);

The thread that callspthread_create()continues execution with the next statement that follows the call

Typically,argpoints to a global or heap variable, but it can also be specified asNULL

PTHREAD_CANCELED, the value returned when a thread is canceled (see Chapter 32), is usually some implementationdefined integer value cast tovoid *。

A portable application would need to ensure that normally terminating threads don’t return integer values that match PTHREAD_CANCELED on any of the implementations on which the application is to run

SUSv3 explicitly notes that the implementation need not initialize the buffer pointed to bythreadbeforethe new thread starts executing; that is, the new thread may start running beforepthread_create()returns to its caller. 

If the new thread needs to obtain its own ID, then it must do so usingpthread_self()

线程的终止有四种方式

:
z The thread’s start function performs a return specifying a return value for the thread.
z The thread calls pthread_exit() (described below).
z The thread is canceled using pthread_cancel() (described in Section 32.1).
z Any of the threads calls exit(), or themain thread performs areturn(in the main()function), which causes all threads in the process to terminate immediately.

include <pthread.h>
void pthread_exit(void *retval);

pthread_exit()can be called from any function that has been called by the thread’sstart function.
pthread_exit specifies a return value that can be obtained in another thread by callingpthread_join

The value pointed to by retval should not be located on the thread’s stack, since the contents of that stack becomeundefinedon thread termination。 The same statement applies to the value given to areturnstatement in
the thread’s start function.


If the main thread callspthread_exit()instead of callingexit()or performing a return, then the other threadscontinue to execute

获取线程ID

include <pthread.h>
pthread_t pthread_self(void);

检测线程ID是否一样

include <pthread.h>
int pthread_equal(pthread_t t1, pthread_t t2);

SUSv3 also notes that an implementation is permitted to reusea thread ID after aterminatedthread has been joined withpthread_join()or after adetachedthread has terminated

POSIX thread IDs are assigned and maintained by the threading implementation
The thread ID returned bygettid()is a number (similar to a process ID) that is assigned by thekernel

pthread_join() 函数等待指定的线程thread终止，thread终止后pthread_join立即返回

include <pthread.h>
int pthread_join(pthread_t thread, void **retval);
Returns 0 on success, or a positive error number on error

Calling pthread_join() for a thread ID that has been previously joined can lead to unpredictable behavior

重复对同一个thread调用两次pthread_join会造成无法预测的行为，

it might instead join with a thread created later that happened to reuse the same thread ID

可能join一个thread，是在之后创建的，只不过重用了之前的thread ID.

If a thread is not detached (see Section 29.7), then we must join with it using pthread_join(). If we fail to do this, then, when the thread terminates, it produces the thread equivalent of a zombie process
如果线程没有被detach,我们必用对它使用pthread_join，否则该线程终止后变为僵尸线程。

The task that pthread_join()performs for threads is similar to that performed bywaitpid()for processes. However, there are some notable differences
pthread_join和waitpid两个函数很相似，但是也有一些显著的不同。

Threads are peers. Any thread in a process can usepthread_join()to join with any other thread in the process. For example, if thread A creates thread B, which creates thread C, then it is possible for thread A to join with thread C, or vice versa. This differs from the hierarchical relationship between processes. When a parent process creates a child usingfork(), it is the only process that canwait()on that child. There is no such relationship between the thread that callspthread_create()and the resulting new thread.

线程是平等的，比如线程A创建了线程B,线程B创建了线程C，那么线程A可以join线程C，反之也可以。在进程中，只有父进程通过fork创建了子进程，只有这个父进程可以wait这个子进程。
z There is no way of saying “join with any thread” (for processes, we can do this using the callwaitpid(–1, &status, options)); nor is there a way to do a nonblocking join (analogous to thewaitpid()WNOHANGflag). There are ways to achieve similar functionality using condition variables;
线程不像进程，进程wait可以等待任意进程终止，线程也没有nonblocking 等待，不过线程可以通过使用条件值来实现相同功能。

The limitation that pthread_join() can join only with a specific thread ID is intentional. The idea is that a program should join only with the threads that it“knows” about. 

pthread_join的缺点就是只能join特定的threadID.这样设计的其中的原因就是一个程序只能Join它知道的线程。

The problem with a “join with any thread” operation stems from the fact that there is no hierarchy of threads, so such an operation could indeed join withany thread, including one that was privately created by a library function. (The condition-variable technique that we show in Section30.2.4allows a
thread to join only with any other thread that it knows about.) As a consequence, the library would no longer be able to join with that thread in order to
obtain its status, and it would erroneously try to join with a thread ID that had already been joined. In other words, a “join with any thread” operation is incompatible with modular program design.


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