apue2 阅读笔记--第11章
线程基础
tips :对前面习题的一点回顾
1） fgets 最多读取MAX - 1个字符，结尾自动加\0;
2）Standard input and standard output are both line buffered when a program is run interactively. When fgets is called, standard output is flushed automatically.
3）获取当前时间，并以一定格式存储到字符串：
#include "apue.h"
#include <time.h>
int
main(void)
{
time_t caltime;
struct tm *tm;
char line[MAXLINE];
if ((caltime = time(NULL)) == -1)
err_sys("time error");
if ((tm = localtime(&caltime)) == NULL)
err_sys("localtime error");
if (strftime(line, MAXLINE, "%a %b %d %X %Z %Y\n", tm) == 0)
err_sys("strftime error");
fputs(line, stdout);
exit(0);
}
4）
Some UNIX system implementations purposely arrange that, when a program is executed, location 0 in the data segment is not accessible. Why?
This provides a way to terminate the process when it tries to dereference a null pointer, a common C programming error.
哦，原来零地址不可引用是这样实现的。
1. 线程概念
A thread consists of the information necessary to represent an execution context within a process. This includes a thread ID that identifies the thread within a process, a set of register values, a stack, a scheduling priority and policy, a signal mask, an errno variable (recall Section 1.7), and thread-specific data (Section 12.6). Everything within a process is sharable among the threads in a process, including the text of the executable program, the program's global and heap memory, the stacks, and the file descriptors.
注意多个线程共享进程数据，包括堆和栈。
线程池模型：
2. 线程的创建
#include <pthread.h>
int pthread_create(pthread_t *restrict tidp, const pthread_attr_t *restrict attr,
void *(*start_rtn)(void), void *restrict arg);
Returns: 0 if OK, error number on failure
不能保证pthread_t 是unsigned int 类型，所以提供下面两个函数
#include <pthread.h>
pthread_t pthread_self(void);
Returns: the thread ID of the calling thread
#include <pthread.h> int pthread_equal(pthread_t tid1, pthread_t tid2); Returns: nonzero if equal, 0 otherwise
3. 线程的终止
A single thread can exit in three ways, thereby stopping its flow of control, without terminating the entire process.
-
The thread can simply return from the start routine. The return value is the thread's exit code.
-
The thread can be canceled by another thread in the same process.
-
The thread can call pthread_exit.
#include <pthread.h> void pthread_exit(void *rval_ptr); The rval_ptr is a typeless pointer, similar to the single argument passed to the start routine. This pointer is available to other threads in the process by calling the pthread_join function.
#include <pthread.h> int pthread_join(pthread_t thread, void **rval_ptr); Returns: 0 if OK, error number on failure
The calling thread will block until the specified thread calls pthread_exit, returns from its start routine, or is canceled. If the thread simply returned from its start routine, rval_ptr will contain the return code. If the thread was canceled, the memory location specified by rval_ptr is set to PTHREAD_CANCELED.
By calling pthread_join, we automatically place a thread in the detached state (discussed shortly) so that its resources can be recovered. If the thread was already in the detached state, calling pthread_join fails, returning EINVAL.
If we're not interested in a thread's return value, we can set rval_ptr to NULL. In this case, calling pthread_join allows us to wait for the specified thread, but does not retrieve the thread's termination status.
上面是线程的自杀，以及主线程进行的入殓工作。
下面是他杀：
One thread can request that another in the same process be canceled by calling the pthread_cancel function.
#include <pthread.h>
int pthread_cancel(pthread_t tid);
Returns: 0 if OK, error number on failure
4. 线程控制
1） By default, a thread's termination status is retained until pthread_join is called for that thread. A thread's underlying storage can be reclaimed immediately on termination if that thread has been detached. When a thread is detached, the pthread_join function can't be used to wait for its termination status. A call to pthread_join for a detached thread will fail, returning EINVAL. We can detach a thread by calling pthread_detach.
注意： 如果一个线程是detach的，那就不用pthread_join去阻塞地等待它结束了。
2） 同步
通过互斥量加锁：
We can protect our data and ensure access by only one thread at a time by using the pthreads mutual-exclusion interfaces. A mutex is basically a lock that we set (lock) before accessing a shared resource and release (unlock) when we're done. While it is set, any other thread that tries to set it will block until we release it. If more than one thread is blocked when we unlock the mutex, then all threads blocked on the lock will be made runnable, and the first one to run will be able to set the lock. The others will see that the mutex is still locked and go back to waiting for it to become available again. In this way, only one thread will proceed at a time.
This mutual-exclusion mechanism works only if we design our threads to follow the same data-access rules. The operating system doesn't serialize access to data for us. If we allow one thread to access a shared resource without first acquiring a lock, then inconsistencies can occur even though the rest of our threads do acquire the lock before attempting to access the shared resource.
A mutex variable is represented by the pthread_mutex_t data type. Before we can use a mutex variable, we must first initialize it by either setting it to the constant PTHREAD_MUTEX_INITIALIZER (for statically-allocated mutexes only) or calling pthread_mutex_init. If we allocate the mutex dynamically (by calling malloc, for example), then we need to call pthread_mutex_destroy before freeing the memory.
#include <pthread.h>
int pthread_mutex_init(pthread_mutex_t *restrict mutex, const pthread_mutexattr_t *restrict attr);
int pthread_mutex_destroy(pthread_mutex_t *mutex);
Both return: 0 if OK, error number on failure
To initialize a mutex with the default attributes, we set attr to NULL. We will discuss nondefault mutex attributes in Section 12.4.
To lock a mutex, we call pthread_mutex_lock. If the mutex is already locked, the calling thread will block until the mutex is unlocked. To unlock a mutex, we call pthread_mutex_unlock.
#include <pthread.h>
int pthread_mutex_lock(pthread_mutex_t *mutex);
int pthread_mutex_trylock(pthread_mutex_t *mutex); //不阻塞
int pthread_mutex_unlock(pthread_mutex_t *mutex);
All return: 0 if OK, error number on failure
If a thread can't afford to block, it can use pthread_mutex_trylock to lock the mutex conditionally. If the mutex is unlocked at the time pthread_mutex_trylock is called, then pthread_mutex_trylock will lock the mutex without blocking and return 0. Otherwise, pthread_mutex_trylock will fail, returning EBUSY without locking the mutex.
来个小例子：
Figure 11.10. Using a mutex to protect a data structure
#include <stdlib.h>
#include <pthread.h>
struct foo {
int f_count;
pthread_mutex_t f_lock;
/* ... more stuff here ... */
};
struct foo *
foo_alloc(void) /* allocate the object */
{
struct foo *fp;
if ((fp = malloc(sizeof(struct foo))) != NULL) {
fp->f_count = 1;
if (pthread_mutex_init(&fp->f_lock, NULL) != 0) {
free(fp);
return(NULL);
}
/* ... continue initialization ... */
}
return(fp);
}
void
foo_hold(struct foo *fp) /* add a reference to the object */
{
pthread_mutex_lock(&fp->f_lock);
fp->f_count++;
pthread_mutex_unlock(&fp->f_lock);
}
void
foo_rele(struct foo *fp) /* release a reference to the object */
{
pthread_mutex_lock(&fp->f_lock);
if (--fp->f_count == 0) { /* last reference */
pthread_mutex_unlock(&fp->f_lock);
pthread_mutex_destroy(&fp->f_lock);
free(fp);
} else {
pthread_mutex_unlock(&fp->f_lock);
}
}
这段关于死锁的描述不错
Deadlock Avoidance
A thread will deadlock itself if it tries to lock the same mutex twice, but there are less obvious ways to create deadlocks with mutexes. For example, when we use more than one mutex in our programs, a deadlock can occur if we allow one thread to hold a mutex and block while trying to lock a second mutex at the same time that another thread holding the second mutex tries to lock the first mutex. Neither thread can proceed, because each needs a resource that is held by the other, so we have a deadlock.
Deadlocks can be avoided by carefully controlling the order in which mutexes are locked. For example, assume that you have two mutexes, A and B, that you need to lock at the same time. If all threads always lock mutex A before mutex B, no deadlock can occur from the use of the two mutexes (but you can still deadlock on other resources). Similarly, if all threads always lock mutex B before mutex A, no deadlock will occur. You'll have the potential for a deadlock only when one thread attempts to lock the mutexes in the opposite order from another thread.
Sometimes, an application's architecture makes it difficult to apply a lock ordering. If enough locks and data structures are involved that the functions you have available can't be molded to fit a simple hierarchy, then you'll have to try some other approach. In this case, you might be able to release your locks and try again at a later time. You can use the pthread_mutex_trylock interface to avoid deadlocking in this case. If you are already holding locks and pthread_mutex_trylock is successful, then you can proceed. If it can't acquire the lock, however, you can release the locks you already hold, clean up, and try again later.
另外，还可以通过线程信号量、读写锁和条件变量来进行线程间的同步。
