Concurrent Programming. Concurrent Programming with Processes and Threads

Transcription

1 Concurrent Programming Concurrent Programming with Processes and Threads

2 Review The fork system call creates a process Memory and resources are allocated The exec system call allow a program to execute within the address space of the calling process

3 Interprocess Communications with Pipe A pipe is used for one-way communication of a stream of bytes. Creating a pipe int pfd[2]; pipe(pfd);

4 I/O with Pipe Use read and write system calls write(pfd[1], buf, size); read(pfd[0], buf, SIZE); A pipe opened before the fork becomes shared between the two processes. Before fork After fork

5 One Way Communication with Pipes Each process closes one of the ends of the pipe. Example #include <stdio.h> /* standard I/O routines. */ #include <unistd.h> /*defines pipe(),amongst other things.*/ /* this routine handles the work of the child process. */ void do_child(int data_pipe[]) { int c; /* data received from the parent. */ int rc; /* return status of read(). */ /* first, close the un-needed write-part of the pipe. */ close(data_pipe[1]); /* now enter a loop of reading data from the pipe, and printing it*/ while ((rc = read(data_pipe[0], &c, 1)) > 0) {putchar(c); } /* probably pipe was broken, or got EOF via the pipe. */ exit(0);}

6 Example (cont.) /* this routine handles the work of the parent process. */ void do_parent(int data_pipe[]) { int c; /* data received from the user. */ int rc; /* return status of getchar(). */ /* first, close the un-needed read-part of the pipe. */ close(data_pipe[0]); /* now enter a loop of read user input, and writing it to the pipe. */ while ((c = getchar()) > 0) { /* write the character to the pipe. */ rc = write(data_pipe[1], &c, 1); if (rc == -1) { /* write failed - notify the user and exit */ perror("parent: write"); close(data_pipe[1]); exit(1); } } /* probably got EOF from the user. */ close(data_pipe[1]);/*close the pipe,to let the child know we're done. */ exit(0); }

7 Example (Cont.) int main(int argc, char* argv[]) {/* Now the main function. */ int data_pipe[2]; /* an array to store the file descriptors of the pipe. */ int pid; /* pid of child process, or 0, as returned via fork. */ int rc; /* stores return values of various routines. */ /* first, create a pipe. */ rc = pipe(data_pipe); if (rc == -1) { perror("pipe"); exit(1); } /*now fork off a child process,and set their handling routines.*/ pid = fork(); switch (pid) { case -1: /* fork failed. */ perror("fork"); exit(1); case 0: /* inside child process. */ do_child(data_pipe); /* do_child exits - NOT REACHED */ default: /* inside parent process. */ do_parent(data_pipe); /* do_parent exits - NOT REACHED */ } return 0; /* NOT REACHED */ }

8 Threads (Posix) Creating threads pthread_create(); Destroying threads pthread_exit Wait for another thread exit pthread_join Determine my ID pthread_self

9 Thread Programming Example #include <stdio.h> /* standard I/O routines */ #include <sched.h> #include <pthread.h> /*pthread functions and structures */ /* function to be executed by the new thread */ void* do_loop(void* data) { int i; /* counter, to print numbers */ int j; /* counter, for delay */ int me = *((int*)data); /* thread identifying number */ for (i=0; i<10; i++) { for (j=0; j<500000; j++) /* delay loop */ ; printf("'%d' - Got '%d'\n", me, i); } /* terminate the thread */ pthread_exit(null); }

10 Thread Programming Example (Cont.) /* Program's execution begins in main */ int main(int argc, char* argv[]) { int thr_id; /* thread ID for the newly created thread */ pthread_t p_thread; /* thread's structure */ int a = 1; /* thread 1 identifying number */ int b = 2; /* thread 2 identifying number */ /* create a new thread that will execute 'do_loop()' */ thr_id = pthread_create(&p_thread, NULL, do_loop,(void*)&a); /* run 'do_loop()' in the main thread as well */ do_loop((void*)&b); /* NOT REACHED */ return 0; }

11 Thread Programming Notes: pthread_create has 4 parameters A pointer to a structure of type pthread_t A set of thread attributes A function to execute Arguments to pass to the function Example: pthread_create(&p_thread, NULL, do_loop,(void*)&a); The delay loop is just to allow us to observe the concurrent execution The main program itself is a thread

12 Thread Programming Compiling On Davinci cc -D_REENTRANT -mt -lpthread threads.c gcc threads.c -lpthread On SGI cc -D_POSIX_C_SOURCE=199506L threads.c -lpthread

13 Thread Programming Running '2' - Got '0' '1' - Got '0' '2' - Got '1' '1' - Got '1' '1' - Got '2' '2' - Got '2' '1' - Got '3' '2' - Got '3' '1' - Got '4' '2' - Got '4' '1' - Got '5' 2' - Got '5' '1' - Got '6' '2' - Got '6' '1' - Got '7' '2' - Got '7' '1' - Got '8' '2' - Got '8' '1' - Got '9' '2' - Got '9'

14 Threads versus Processes A Programming Perspective Processes Easy synchronization A process blocks on a read from a pipe Sharing data is demanding Threads Need to create pipes Easy to share data Synchronization can be tricky