1、Chapter 5 Linux Multi-thread Programming李素科北京大學軟件與微電子學院2007,2,What is a thread?,A thread is an independent sequence of execution of program code inside a UNIX process A thread is often called a lightweight process but it is NOT a process (something smaller than a process),process space,thread,3,Why

2、Threads?,Do concurrent control more efficiently Use fork() to create a process: is expensive, usually the child process need copy the whole context from the parent process. Inter-process communication is expensive and difficult. OS does more effort to switch process contexts. Creating a thread Threa

3、ds use and exist within the process resources, so creating a thread is faster than creating a process. Inter-thread communication is easy, usually we can use global variable or struct to share data between threads. OS switches thread contexts easier than process contexts.,4,Shared or Not Shared betw

4、een thread and process,Shared process instruction most data(A thread can have its own private data) open files signal handlers and signal dispositions current working directory user and group ID Not shared thread ID set of registers (including pc and sp) stack signal mask priority,5,What are Pthread

5、s?,IEEE POSIX standard p1003.1c (Pthreads) specifies the APIs and the way other POSIX interfaces deal with threads Pthreads are defined as a set of C language programming types and procedure calls i.e. Create a new thread: #include int pthread_create(pthread_t * thread, pthread_attr_t * attr, void *

6、 (*start_routine)(void *), void * arg); We want to link a pthread application in Linux , we need libpthread library. so, you can compile file with -lpthread flag # gcc xxxx.c -o xxxx -lpthread,6,Creating Threads,#include int pthread_create(pthread_t * thread, pthread_attr_t * attr, void *(*start_rou

7、tine)(void *), void * arg);,thread identifier,thread attribute pthread_attr_init() sets the attribute of a thread,point to function, What does the thread do?,Sometime, you need pass some arguments to the thread function,Return value: On success, the identifier of the newly created thread is stored i

8、n the location pointed by the thread argument, and a 0 is returned. On error, a non-zero error code is returned.,7,Waiting For the Termination of Threads,#include int pthread_join(pthread_t th, void *thread_return); pthread_join suspends the execution of the calling thread until the thread identifie

9、d by th terminates If thread_return is not NULL, the return value of th is stored in the location pointed to by thread_return The joined thread th must be in the joinable state: it must not have been detached using pthread_detach or the PTHREAD_CREATE_DETACHED attribute to pthread_create pthread_joi

10、n must be called once for each joinable thread created to avoid memory leaks，because memory resources (thread descriptor and stack) of joinable threads are not deallocated if the caller do not call pthread_join.,Waiting for th to terminate,8,Destroying Threads,#include void pthread_exit(void *retval

11、); Ways for threads to terminate: The thread returns from its starting routine; The thread calls pthread_exit (); The thread is canceled by another thread via pthread_cancel(); The thread receives a signal that terminates it; The entire process is terminated.,return value of the thread,can not point

12、 to local thread object,9,pthread_detach,#include int pthread_detach(pthread_t th); This guarantees that the memory resources consumed by th will be freed immediately when th terminates. But this prevents other threads from synchronizing on the termination of th using pthread_join. A thread can be c

13、reated initially in the detached state, using the detachstate attribute to pthread_create. After pthread_detach completes, subsequent attempts to perform pthread_join on th will fail. If another thread is already joining the thread th at the time pthread_detach is called, pthread_detach does nothing

14、 and leaves th in the joinable state. On success, 0 is returned. On error, a non-zero error code is returned.,10,pthread_self,#include pthread_t pthread_self(void); return identifier of current thread,11,Mutex-protecting your critical sections, pthread_mutex_t mylock; mylock = PTHREAD_MUTEX_INITIALI

15、ZER; pthread_mutex_lock( ,A mutex will be owned by only one thread at one time If another thread call pthread_mutex_lock() on a unlocked mutex, another thread which calls pthread_mutex_lock too is suspended and waits for the owner of locked mutex thread to unlock the mutex first.,12,Avoiding Deadloc

16、ks,/* Thread A */ pthread_mutex_lock(,/* Thread B */ pthread_mutex_lock(,time,13,Avoiding Deadlocks Cont.,pthread_mutex_lock(,time,pthread_mutex_lock(,14,Operations on Mutexes,#include pthread_mutex_t fastmutex = PTHREAD_MUTEX_INITIALIZER; int pthread_mutex_init(pthread_mutex_t *mutex, const pthread

17、_mutex- attr_t *mutexattr); int pthread_mutex_lock(pthread_mutex_t *mutex); int pthread_mutex_trylock(pthread_mutex_t *mutex); int pthread_mutex_unlock(pthread_mutex_t *mutex); int pthread_mutex_destroy(pthread_mutex_t *mutex); In the LinuxThreads implementation, no resources are associated with mut

18、ex objects, thus pthread_mutex_destroy actually does nothing except checking that the mutex is unlocked.,15,It is time to write a simple program,#include #include #include #include #define THREAD_NUMBER 10 static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; int sum =0; void* inc(void *arg) int

19、 i =(*(int *)arg); pthread_mutex_lock( ,16,Cont,int main(int argc, char *argv) pthread_t ptTHREAD_NUMBER; int i; int argTHREAD_NUMBER; for(i=0; iTHREAD_NUMBER; i+) argi=i; if(pthread_create( ,Compile: #gcc counter.c o counter -lpthread Run it:# ./counter sum is 45,17,Multi-thread Web Client,This exa

20、mple is modified from the book UNIX Network Programming, Volume 1, by Richard Stevens #include #include #include #include #include #include #include #include #include #include #include #define MAXLINE 1024 #define MAXFILES20 #define SERV80 /* port number or service name */,18,Cont,struct file char *

21、f_name; /* filename */ char *f_host; /* hostname or IP address */ int f_fd; /* descriptor */ int f_flags; /* F_xxx below */ unsigned long int f_tid; /* thread ID */ fileMAXFILES; #define F_CONNECTING 1 /* connect() in progress */ #define F_READING 2 /* connect() complete; now reading */ #define F_DO

22、NE 4 /* all done */ #define F_JOINED 8 /* main has pthread_joined */ #define GET_CMD GET %s HTTP/1.0rnrn int nconn, nfiles, nlefttoconn, nlefttoread; int ndone; /* number of terminated threads*/ pthread_mutex_t ndone_mutex = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t ndone_cond = PTHREAD_COND_INITIAL

23、IZER;,19,Cont,main(int argc, char *argv) inti, maxnconn; pthread_t tid; struct file *fptr; if (argc file1 .n); exit(0); maxnconn = atoi(argv1); if(argc-4) MAXFILES) nfiles = MAXFILES; else nfiles = argc -4; for (i = 0; i nfiles; i+) filei.f_name = argvi + 4; filei.f_host = argv2; filei.f_flags = 0;

24、,20,Cont,printf(nfiles = %dn, nfiles); home_page(argv2, argv3); nlefttoread = nlefttoconn = nfiles; nconn = 0; while (nlefttoread 0) while (nconn 0) /* 4find a file to read */ for (i = 0 ; i nfiles; i+) if (filei.f_flags = 0) break; if (i = nfiles) printf(nlefttoconn = %d but nothing found, nlefttoc

25、onn); exit(1); pthread_create( ,21,Cont,pthread_mutex_lock( ,思考，如果不用這個API， 該如何改這個程序？,22,Cont- do_get_read,void *do_get_read(void *vptr) intfd, n; charlineMAXLINE; struct file*fptr; fptr = (struct file *) vptr; fd = tcp_connect(fptr-f_host, SERV); fptr-f_fd = fd; printf(do_get_read for %s, fd %d, thr

26、ead %dn, fptr-f_name, fd, fptr-f_tid); write_get_cmd(fptr); /* write() the GET command */ /* 4Read servers reply */ for ( ; ; ) if ( (n = read(fd, line, MAXLINE) = 0) break;/* server closed connection */,23,Cont- do_get_read,printf(read %d bytes from %sn, n, fptr-f_name); printf(end-of-file on %sn,

27、fptr-f_name); close(fd); fptr-f_flags = F_DONE; /* clears F_READING */ pthread_mutex_lock( /* terminate thread */ ,24,Result,rootlocalhost multithreadwebclient# g+ multith_web.c -o multith_web -lpthread rootlocalhost multithreadwebclient# ./multith_web 3 / /templates/sspku_gate_index/images/index_04

28、.gif /images/banners/logo_xiaoli.png /images/banners/logo_COS2.jpg nfiles = 3 read 1024 bytes of home page . end-of-file on home page do_get_read for /templates/sspku_gate_index/images/index_04.gif, fd 3, thread 3086826400 wrote 64 bytes for /templates/sspku_gate_index/images/index_04.gif do_get_rea

29、d for /images/banners/logo_COS2.jpg, fd 6, thread 3065846688 wrote 46 bytes for /images/banners/logo_COS2.jpg do_get_read for /images/banners/logo_xiaoli.png, fd 5, thread 3076336544 wrote 48 bytes for /images/banners/logo_xiaoli.png .,25,Pthread-operations on conditions,#include pthread_cond_t cond

30、 = PTHREAD_COND_INITIALIZER; int pthread_cond_init(pthread_cond_t *cond, pthread_condattr_t *cond_attr); int pthread_cond_signal(pthread_cond_t *cond); int pthread_cond_broadcast(pthread_cond_t *cond); int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex); int pthread_cond_timedwait(pt

31、hread_cond_t *cond, pthread_mutex_t *mutex, const struct timespec *abstime); int pthread_cond_destroy(pthread_cond_t *cond);,26,pthread_cond_t,A condition is a synchronization device that allows threads to suspend execution and relinquish the processors until some predicate on shared data is satisfi

32、ed. The basic operations on conditions are: signal the condition (when the predicate becomes true), and wait for the condition, suspending the thread execution until another thread signals the condition.,27,pthread_cond_t,A condition variable must always be associated with a mutex, to avoid the race

33、 condition where a thread prepares to wait on a condition variable and another thread signals the condition just before the first thread actually waits on it.,28,pthread_cond_init,#include int pthread_cond_init (pthread_cond_t *cond, pthread_condattr_t*cond_attr); pthread_cond_init initializes the c

34、ondition variable cond, using the condition attributes specified in cond_attr, or default attributes cond_attr is NULL. The LinuxThreads implementation supports no attributes for conditions, hence the cond_attr parameter is actually ignored. Variables of type pthread_cond_t can also be initialized s

35、tatically, using the constant PTHREAD_COND_INITIALIZER. return 0 on success and a non-zeroerror code on error; not return error code.,29,Example,pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t cond = PTHREAD_COND_INITIALIZER; Or : / initialize the condition variable if(pthread_cond_i

36、nit ( pthread_cond_signal restarts one of the threads that are waiting on the condition variable cond. If no threads are waiting on cond, nothing happens. If several threads are waiting on cond, exactly one is restarted, but it is not specified which. return 0 on success and a non-zeroerror code on

37、error; not return error code.,31,Example,pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t cond = PTHREAD_COND_INITIALIZER; Or : / initialize the condition variable if(pthread_cond_init ( ,32,pthread_cond_broadcast,#include int pthread_cond_broadcast(pthread_cond_t *cond); pthread_cond

38、_broadcast restarts all the threads that are waiting on the condition variable cond. Nothing happens if no threads are waiting on cond. return 0 on success and a non-zeroerror code on error; not return error code.,33,Example,pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t cond = PTHR

39、EAD_COND_INITIALIZER; Or : / initialize the condition variable if(pthread_cond_init ( ,34,pthread_cond_wait,#include int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex); pthread_cond_wait atomically unlocks the mutex (as per pthread_unlock_mutex) and waits for the condition variable

40、cond to be signaled. The thread execution is suspended and does not consume any CPU time until the condition variable is signaled.,35,Cont,The mutex must be locked by the calling thread on entrance to pthread_cond_wait. Before returning to the calling thread, pthread_cond_wait re-acquires mutex (as

41、per pthread_lock_mutex). For example while (ndone = 0) pthread_cond_wait( not return error code.,37,Example,pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t cond = PTHREAD_COND_INITIALIZER; Or : / initialize the condition variable if(pthread_cond_init ( ,38,pthread_cond_timedwait,#inc

42、lude int pthread_cond_timedwait(pthread_cond_t *cond, pthread_mutex_t *mutex, const struct timespec *abstime); pthread_cond_timedwait atomically unlocks mutex and waits on cond, as pthread_cond_wait does, but it also bounds the duration of the wait.,39,Cont,If cond has not been signaled within the a

43、mount of time specified by abstime, the mutex is reacquired and pthread_cond_timedwait returns the error ETIMEDOUT. The abstime parameter specifies an absolute time, with the same origin as time(2) and gettimeofday(2): from 00:00:00 GMT, January 1, 1970.,40,pthread_cond_destroy,#include int pthread_

44、cond_destroy(pthread_cond_t *cond); pthread_cond_destroy destroys a condition variable, freeing the resources it might hold. No threads must be waiting on the condition variable on entrance to pthread_cond_destroy. In the LinuxThreads implementation, no resources are associated with condition variab

45、les, thus pthread_cond_destroy actually does nothing except checking that the condition has no waiting threads.,41,An Example from Linux Manual,pthread_mutex_lock(,pthread_mutex_lock(,int x,y; pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t cond = PTHREAD_COND_INITIALIZER;,Thread A,Thread B,42,Another Example from Linux Manual,struct timeval now; struct timespec timeout; int retcode; pthread_mutex_lock(,43,Workshop,Modify the program counter.c then le