openfilename iosin if file cerr Couldnt open filen return NULL while fileeof

Openfilename iosin if file cerr couldnt open filen

This preview shows page 40 - 42 out of 102 pages., ios::in); if (! file) { cerr << "Couldn't open file\n"; return NULL; } while (!file.eof()) { file.getline(buffer,bufsize); cout << filename << ":" <<buffer << endl; // Critical section ret = pthread_mutex_lock(&MUTEX); LINECOUNT++; ret = pthread_mutex_unlock(&MUTEX); // Try uncommenting this .... // Yield the process, to allow next thread to be run // sched_yield(); } file.close(); } In this version of the program, a separate thread is spawned for each file. First we call the function pthread_create() for each file we encounter. A new thread is spawned with a pointer to the function the thread should execute (in this case the same function for all threads), called ParseFile() , which reads lines from the respective files and increments the global variable LINECOUNT . Several things are important here. The main program is itself a thread. It is essential that we tell the main program to wait for the additional threads to join the main program before exiting, otherwise the main program will exit and kill all of the child threads immediately. Thread join-semantics are like wait-semantics for normal processes.
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Each of the threads updates the same global variable. Suppose now that two threads are running on different CPUs. It is possible that both threads would try to alter the value of the variable LINECOUNT simultaneously. This is called a race condition and can lead to unpredictable results. For this reason we use a mutex to lock the variable while it is being updated. We shall discuss this more in the next section. A final point to note is the commented out lines in the ParseFile() function. The call sched_yield() tells a running thread to give itself up to the scheduler, so that the next thread to be scheduled can run instead. This function can be used to switch between several threads. By calling this function after each line is read from the files, we can spread the the CPU time evenly between each thread. Actually, it is difficult to predict precisely which threads will be scheduled and when, because the threads in our program here are only a small number, compared to the total number of threads waiting to be scheduled by the system. The interaction with disk I/O can also have a complicated effect on the scheduling. On a single CPU system, threads are usually scheduled FCFS in a queue. If we yield after every instruction, it has the effect of simulating round-robin scheduling. 4.3.6 Example: LWPs in Solaris 1 Early solaris systems had user-level threads only, which were called light weight processes. Since the kernel was single threaded, only one user-level thread could run at any given time. To create a threaded process in solaris 1, one simply has to execute a LWP system call. The `lightweight processes library' then converts the normal process into a process descriptor plus a thread. Here is the simplest example /********************************************************************/ /* */ /* Creating a light weight process in SunOS 4.1.3 */ /* */ /********************************************************************/ #include <lwp/lwp.h> #include <lwp/stackdep.h> #define MINSTACKSZ 1024 #define STACKSIZE 1000 + MINSTACKSZ #define MAXPRIORITY 10 /*********************************************************************/ stkalign_t stack[STACKSIZE]; /*********************************************************************/ /* Zone 0 */ /*********************************************************************/ main () { thread_t tid; int task(); pod_setmaxpri(MAXPRIORITY);
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